By-catches of non-commercial invertebrate taxa in Skagerrak

Transcrição

By-catches of non-commercial invertebrate taxa in Skagerrak
and Kattegat, generated by demersal otter trawling
Linda Ottosson
Master Thesis in Marine Ecology, 30 points
Department of Marine Ecology, Gothenburg University, Sweden
Supervisor: Dr. Berggren Matz
Examiner: Prof. Baden Susanne
Contribution no. 374
February 2008
Sammanfattning
Trålning efter bottenlevande fiskar och skaldjur medför ofta störningar på havsbotten och
organismer som lever i, på eller i nära anslutning till botten. När trålen drar över havsbotten
plöjer den upp fåror, rör upp sedimentytan, drar upp och skadar alternativt tar död på
organismer som lever i eller på botten. Organismer som fastnar i trålens maskor eller nät
skadas och stressas när de lyfts upp ovan havsytan och hamnar i en hög på fartygsdäcket
tillsammans med andra djur. De djur som slängs överbord som dumpat material (discard)
kommer att utgöra föda för diverse predatorer och asätare såsom t ex sjöfågel, fiskar, skaldjur
etc. Mycket fokus läggs på bifångster av kommersiella fiskarter, men sällan talas det om
bifångster av icke kommersiella ryggradslösa djur (evertebrater). Hur påverkas dessa av
bottentrålning efter fisk, räka och kräfta? I denna studie ligger fokus på dessa evertebrater
som utgör en del av bifångsten och därmed utkasten vid bottentrålning efter fisk, räka och
kräfta i Skagerrak och Kattegat. Vilka arter av icke kommersiella evertebrater kommer med
upp i trålen? Är några av dem mer frekvent förekommande än andra och därmed mer utsatta?
Skiljer sig artsammansättning, den totala biomassan och individabundansen mellan olika
tråltyper, såsom kräft-, räk- och fisktrål? Denna studie inkluderade tråldrag från tre olika
fiskefartyg på 37 olika stationer i Skagerrak respektive Kattegat och trålningarna utfördes på
djup som varierade mellan 16 till 410 meter från september till november månad.
Trålningarna utfördes med hjälp av tre olika fiskefartyg varav två av dessa tillhör
Fiskeriverket och den tredje tillhör den svenska kommersiella bottentrålande fiskeflottan.
Tre olika tråltyper användes för bottentrålningen, dessa var fisktrål, kräfttrål (Nephrops
norvegicus) och räktrål (Pandalus borealis). Dessa skiljer sig bl.a. genom att näten har olika
maskstorlekar och olika placering ovan botten. Tråldragens längd varierade mellan 30 minuter
och upp till 8 timmar. Evertebraterna i bifångsten artbestämdes till taxa där så var möjligt
annars till lägsta möjliga taxonomiska nivå, organismerna räknades och deras biomassa
vägdes. För att kunna göra en jämförelse mellan de olika tråltyperna gjordes en
standardisering med hjälp av en uträknad trålad area. Individabundans, biomassa och
artdiversitet undersöktes och organismerna delades upp i olika funktionella grupper beroende
på deras mest dominanta trofiska nivå och levnadssätt. Den absolut mest förekommande
funktionella gruppen var mobila predatorer och asätare. De bestod av många olika arter och
individer. I fisk- och kräfttrålen var de sedentära suspensionsätarna de näst mest
förekommande medan den grävande depositionsätande infaunan var den tredje vanligaste
gruppen. I räktrålarna var istället dessa de näst vanligaste medan de sedentära
suspensionsätarna var de tredje vanligaste. De tre olika tråltyperna i undersökningen var
signifikant skilda åt vad gällde individabundans, biomassa respektive artdiversitet. Kräfttrålen
genererade bifångster med störst individantal och biomassa, den fick också störst andel icke
kommersiella evertebrater i förhållande till målfångstens storlek. Räktrålen däremot var mest
skonsam när det gällde bifångsten av icke kommersiella evertebrater och individabundans,
biomassa och artrikedom. Detta skulle kunna bero på att räktrålen går högre ovan botten än de
övriga gör. De vanligaste fyla i bifångsten var Arthropoda, Echinodermata och Mollusca,
vilka stod för mer än 95 % av den totala bifångsten av icke kommersiella evertebrater. Vilka
av dem som dominerade fångsten varierade mellan de olika tråltyperna. Fisktrålen
dominerades av (mollusker) Mollusca, kräfttrålen fick flest (tagghudingar) Echinodermata
medan räktrålens fångst till största delen bestod av (leddjur) Arthropoda. De olika
trålningarna som genomfördes under denna studie resulterade i en bifångst på cirka 100
stycken olika taxa av icke kommersiella evertebrater. Fisktrålningarnas bifångst var mest
divers medan räktrålen fick minst diverst antal arter i bifångsten av dessa evertebrater.
Bifångsten bestod även av hotade röd listade arter, dessa uppgick till ett antal av 11 olika
arter, varav sju av dessa tillhör fyla Arthropoda, tre Echinodermata och en av dem
2
(nässeldjur) Cnidaria. De flesta av dem återfanns i fångster fiskade på djup mellan ungefär
200 och 300 meter.
Abstract
This study has been performed to evaluate some of the effects on non-commercial
invertebrates by demersal otter trawling. When the otter trawl is being swept over the bottom
the otter boards scours the seafloor and leaves deep scars on soft sediment, with resuspension
as a result, or damages sessile erect organisms providing shelter in hard-bottom substrata. The
benthic fauna in the trawled area run the risks of a resulting deplacement, damage or high
mortality. Some of the organisms will be trapped in the trawl, and later discarded.
To investigate which species and functional groups is being part of the discarded material of
the Swedish demersal west coastal fishing industry, and to see if there is a significant
difference in species composition of the invertebrate by-catch depending on trawl type,
experiments were performed together with the Swedish Board of Fisheries and a commercial
fishing vessel.
Non-commercial invertebrates from 37 different stations in Skagerrak and Kattegat
(16-410 meters depth) were being sampled from three different fishing vessels. Three
different types of trawls were used in the experiments; Norwegian lobster- (Nephrops
norvegicus), shrimp- (Pandalus borealis) and demersal fish trawl. Trawl duration of between
30 minutes up to 8 hours were carried out using otter trawls with mesh sizes of 20, 35 and 70
millimetres. Invertebrate taxa were identified to species when possible and the numbers
counted and biomass of each species recorded as wet weight. In cases where the catches were
large, a representative sub sample was taken, and the wet weight of different taxa back
calculated to the full catch weight. For small samples the whole catch was processed. To be
able to compare the different trawling types a standardisation where the swept area of each
trawling event was taken into account. The biomass and number of individuals of
invertebrates in the by-catch was correlated to the biomass of target species catch, to examine
if the size of the total catch affects the amount of invertebrate by-catch, this gave a non
significant result. The individual abundance, diversity of species, phyla composition and
biomass per swept area were analysed using a multivariate statistical method. Each individual
was classified according to their functional groups which describes their dominant trophic
mode and their predominant foraging habit. The most dominant functional group was motile
predators or scavengers followed by sedentary suspension feeders and burrowing, dwelling
deposit feeders, except for the shrimp trawling efforts where the burrowing, dwelling deposit
feeders were more common in the by-catch, than the sedentary suspension feeders, when
considering the species diversity. The motile predators and scavengers in the by-catch
consisted of a variety of species and individuals. The trawl types included in this study
generated significantly different non-commercial invertebrate by-catch of individual
abundance, biomass or species diversity. The Nephrops trawling efforts caught by-catches
consisting of the highest individual abundance and biomass, these efforts also received the
highest proportion of non-commercial invertebrate by-catch compared to their target catch.
The shrimp trawling was most merciful of the three trawl types, by far receiving the lowest
individual abundance, biomass and species diversity. The Phyla most common in the noncommercial by-catch in this study were Arthropoda, Echinodermata and Mollusca, they were
responsible for more than 95% of this by-catch. The composition of Phyla in the by-catch was
different depending on trawl type. The by-catches of non-commercial invertebrates were
dominated by taxa belonging to Mollusca when using the fish trawl, Echinodermata with the
Nephrops trawl and Arthropoda when trawling with the shrimp trawls. Altogether almost 100
different taxa of non-commercial invertebrates were caught during the trawling efforts
3
included in this study. The by-catches from the fish trawling efforts were the most diverse
ones, while the shrimp trawling had the lowest species diversity in their by-catches, during
this study. The red listed species included in the by-catch consisted of 11 different species of
which 7 belongs to Arthropoda, 3 to Echinodermata and 1 to Cnidaria. These red listed
species were caught most frequent on a depth of 200 to 300 meter.
Keywords
By-catch, demersal otter trawling, discard, non-commercial invertebrates, red listed species
Table of content:
1
Introduction ...................................................................................................................... 5
1.1
Effects of demersal trawling ...................................................................................... 5
1.2
Demersal trawling effects on invertebrates................................................................ 6
1.3
Red Listed species...................................................................................................... 6
1.4
The aim of this study.................................................................................................. 6
2
Materials and methods.................................................................................................... 7
2.1
Area of study .............................................................................................................. 7
2.2
Materials..................................................................................................................... 7
2.2.1
Vessel and trawling equipment .......................................................................... 7
2.3
Methods...................................................................................................................... 9
2.3.1
Field sampling .................................................................................................... 9
2.3.2
Classification.................................................................................................... 10
2.3.4
Statistical methods/ Data analysis .................................................................... 11
3
Results ............................................................................................................................ 12
3.1
Functional groups..................................................................................................... 12
3.2
Abundance................................................................................................................ 15
3.3
Biomass .................................................................................................................... 20
3.4
Diversity ................................................................................................................... 26
3.5
Red Listed species.................................................................................................... 30
4
Discussion………………………………………………………………………………30
4.1
Functional groups..................................................................................................... 32
4.2
Abundance................................................................................................................ 32
4.3
Biomass .................................................................................................................... 34
4.4
Diversity ................................................................................................................... 35
4.5
Red listed species ..................................................................................................... 37
4.6
Trawling damages on invertebrates.......................................................................... 38
5
Conclusions ..................................................................................................................... 39
6
Acknowledgements......................................................................................................... 40
7
References ....................................................................................................................... 40
Appendix ................................................................................................................................. 43
4
1
Introduction
1.1
Effects of demersal trawling
Otter trawling is a widely used demersal trawling equipment world wide (Collie et al, 2000).
Trawl otter boards used for demersal fishing can dig deep furrows in the sediment while trawl
nets and weights attached can cut and disturb the sediment surface. This often leads to
physical and biological changes in the sediment environment and its associated benthic fauna
(Castro & Huber, 2005, Rosenberg el al, 2003). Intense trawling might lead to changes in the
ocean bed, such as for example reduction of structural biota, reduction in habitat complexity,
changes in seafloor structure and removal of taxa that produce structure and shelter (Auster et
al, 1996, Auster & Langton 1999, Castro & Huber, 2005, Hopkins, 2003). Also exposure to
predators and resuspension of sediment which might kill or damage suspension feeders in a
trawled area are possible outcomes (Castro & Huber, 2005). Because of decreased prey
abundance, foraging juvenile fish are also exposed to a higher risk of predation due to the
longer foraging time (Walter and Jaunes, 1993). Repeated trawling in an area result in
discernable changes of the benthic communities and reduction in the productivity of benthic
habitats and a loss of biomass. There is also potential changes in the flow of materials,
nutrients and energy through ecosystems and shift in the balance among processes of primary
production, primary consumption and secondary production (Ocean Studies Board, 2002).
The time of recovery for organisms which environments has been disturbed varies with their
sensibility to disturbance and the rate of disturbance. Fauna that live in low natural
disturbances regimes are generally more vulnerable to fishing gear disturbance, and have
often a long time of recovery. Animals that live in unconsolidated sediments in high natural
disturbance regimes are adapted to periodic sediment resuspension and smothering like that
caused by mobile bottom gear. In contrast, epifaunal communities that stabilize sediments,
reef-forming species, or fauna in habitats that experience low rates of natural disturbance have
been observed to be particularly vulnerable. Individual and species ability to survive the
trawling is directly related to its physiology, morphology and behaviour in response to the
trawling gear (Kaiser & Spencer 1995, Ocean Studies Board, 2002) Soft-bodied, erect, sessile
organisms are more vulnerable to trawling than are hard-bodied prostrate organisms (Ocean
Studies Board, 2002). Some species are more exposed than others and some are more fragile
due to slow reproduction rates, slow growth and a small amount of offspring. Species with
low turn-over rates will decrease in favour of species with high turn-over rates and also
increased scavenger and predator populations benefiting from the increased food availability
(Hopkins 2003, Ocean Studies Board, 2002).
The use of mobile fishing gear has become a source of concern because of the size of the
affected fishing grounds, the modification of the substrate, disturbance of benthic
communities, and removal of non-target species. The long-term viability of some fish
populations could be threatened if essential fish habitat is degraded. Also, because of the
decline of catches in many traditional fisheries, efforts to find under-exploited fish
populations have increased interest in exploiting less accessible, previously unused areas.
These efforts have been facilitated by the development of new gear and navigational aids.
(Ocean Studies Board, 2002) This result in increasing quantities of by-catch of noncommercial fish species and invertebrates associated with bottom substrata and since trawling
equipment have been refined to reduce the by-catch of non-target and undersized fish species,
little progress have been made in reducing the by-catch and subsequent discards of
invertebrate benthic species (Nilsen et al, 2002) the by-catches are an important issue that we
need to increase our knowledge of.
5
1.2
Demersal trawling effects on invertebrates
The invertebrates included in the discard runs a high potential risk of mortality or damage.
The risk depends on for example the characteristics of the particular species, the composition
of the catch and the duration of hauling. Discard is the part of the catch which is not retained
on board during commercial fishing, instead it is returned to the sea. It includes noncommercial and non-targeted species, but also targeted commercial species of unwanted size
or of poor condition. When quotas are reached of a certain species all of these can also be
discarded.
How non-commercial discarded invertebrate species is affected by demersal trawling is
important to highlight and a better understanding in this field is a necessity in the debate of
today regarding the problems generated by trawling. Invertebrates are an important part of the
ocean ecosystem and many invertebrate species are an essential part of the food source for
several commercially important fish species. Therefore these effects on invertebrates are
important to include when analysing the effects of demersal trawling. Previous studies that
have been performed in this field have mostly been made outside Swedish territorial waters.
Since the total annual quantity of discards of invertebrates, elasmobranches and offal into the
North Sea is estimated approximately to 240.000-380.000 tonnes (Catchpole et al, 2005), the
amount of discard is probably extensive for the fisheries in the Skagerrak and Kattegat. A part
of the discarded material also contains endangered (red listed) invertebrate species, which
makes a quantification of the discarded species of an even greater importance.
1.3
Red Listed species
The IUCN (International Union for the Conservation of Nature and Natural Resources) is a
world wide network for conservation of animals, plants and ecosystems. The Red List
Programme is a widely recognized work to ensure the conservation of all plant and animal
species by pinpointing the ones that are threaten to disappear in an area or showing a clear
decline in numbers. Thereby the species on the list are identified as species facing a particular
risk of extinction.
The following categories have been included in this study:
Critically Endangered (CR) - A taxon is Critically Endangered when it is facing an
extremely high risk of extinction in the wild in the immediate future.
Endangered (EN) - A taxon is Endangered when it is not Critically Endangered but is facing
a very high risk of extinction in the wild in the near future.
Vulnerable (VU) - A taxon is Vulnerable when it is not Critically Endangered or Endangered
but is facing a high risk of extinction in the wild in the medium-term future.
These categories above are used for species being threatened.
Nearby Threatened (NT) A taxon is Nearby Threatened when it is not belonging to either
Critically Endangered, Endangered or Vulnerable, but it is in fact close to fulfilling the
criteria of Vulnerable.
Data Deficient (DD) A taxon is Data Deficient when there is inadequate information to make
a direct, or indirect, assessment of its risk of extinction based on its distribution and/or
population status. (IUCN, 2007)
1.4
The aim of this study
The aim of this study is to reveal more about which non-commercial invertebrate species are
most frequently included in the discard when fishing demersal in Skagerrak and Kattegat.
Investigation of which of these species are being affected most negatively and does
endangered (red-listed) invertebrate species make up a part of the by-catch, and if so to what
extent? The Norwegian lobster- (Nephrops norvegicus), shrimp- (Pandalus borealis) and
6
demersal fish trawl have different mesh sizes and placement on or above the sediment surface.
Are the different trawling equipment affecting the demersal fauna differently, when regarding
species composition and the total amount of individuals and biomass? The macrofauna in the
by-catch was compared in respect to individual abundance, biomass and species composition
as well as their functional groups.
2
Materials and methods
2.1
Area of study
The Swedish west coastal waters consist of two water masses,
Skagerrak in the north and Kattegat in the south (Figure 1).
The Skagerrak boarders the North Sea and is situated in
between Lindesnes in Norway and Hanstholm in Denmark.
The border between Skagerrak and Kattegat runs from Skagen
in Denmark and the lighthouse of Tistlarna towards the
Swedish coastline at the west edge of Vallda Sandö (FIFS,
2004). The Skagerrak is a continental margin sea with a weak
influence from tidal currents. The mean depth is 218 meters
and the maximum depth is approximately 700 meters
(Wikipedia, 2007). The eastern parts are stratified due to the
influence of brackish water of Baltic origin, whereas in the
western part oceanic water extends to the surface. The depth of
the halocline varies in the eastern part between a few meters to
about 25 meters depending on wind and water mass supply.
Figure 1. The area of study,
Skagerrak bottom water is normally well oxygenated and
Skagerrak and Kattegat.
exhibits only small seasonal variations (Aure & Dahl, 1994).
The Kattegat is to the south bordered by Öresund (FIFS, 2004). The Kattegat is a more
shallow sea than Skagerrak, with a mean depth of 23 meters and a maximum depth of
approximately 130 meters. The salinity is lower in Kattegat due to the outflow of brackish
surface water from the Baltic Sea. This produces a strong halocline, present at about 15
meters between the brackish surface water of Baltic origin and the deeper more saline water
(~34‰) from the Skagerrak. As a consequence of Kattegat being connected with the Baltic
Sea through narrow straights and shallow bottoms, together with the strong halocline, the
bottom water circulation in the southern Kattegat is reduced, which makes this area
susceptible to seasonal hypoxia in late summer and autumn (Rydberg et al, 1990).
2.2
Materials
2.2.1
Vessel and trawling equipment
Since otter trawls is frequently used as demersal fishing equipment in Sweden, and all around
the world (Collie et al, 2000), this trawl type have been used in this study.
An otter trawl is constructed by a pair of large rectangular boards or doors, which are rigged
to pull the mouth of the trawl net open (Figure15, Appendix). A ground rope runs between
them, often fitted with rubber or heavy metal bobbins. Otter trawls are most frequently towed
over relatively flat soft-bottoms, but can also be towed over relatively rough bottoms when
fitted with “rock-hopper” gear (Hopkins, 2003).
The trawling was executed from three different vessels, two research vessels and one
commercial shrimp trawler.
7
U/F Argos, the largest research vessel belonging to the Swedish Board of Fisheries (Table 3,
Appendix), was during this study participating in the of ICES (International Council of the
Exploration of the Sea) yearly coordinated IBTS (the International Bottom Trawling Survey),
with standardized methods of trawling. The trawling events took place during the first week
in September. During the expedition a French otter trawl, GOV (Grande Ouverture Verticale)
36/47m, with a mesh size of 20 millimetres was used (Figure 16, Appendix). This is a trawl
that targets mainly demersal fish and herring, Clupea harengus. The otter boards on each side
had a weight of 1050 kilos each, and the mean distance between them was nearly 90 meters
and the net opening had a height of about 4 meters. The rubber discs had a diameter of 10
centimetres and 20 centimetres further in and the spherical bobbins had a diameter of 40
centimetres (Figure 18, Appendix). The lining consisted of 400 stretched meshes of 20
millimetres each, giving a total length of 8 meters and the total circumference of the lining
was 600 meshes. A sweep length of 60 meters (including back strops) and minimum warp
length of 150 meters was used throughout the survey. Standard fishing speed was 3,7 knots,
measured as trawl speed over the ground. The duration of each haul was 30 minutes and a
distance of approximately 1,85 nautical miles was being trawled. Trawling was conducted
only during daylight conditions in Skagerrak, from an area close to the lighthouse of Vinga,
outside the archipelago of Gothenburg, extending towards the boarder of the North Sea. The
trawled area had a depth ranging from 20 meters to 231 meters. The area being trawled at
each fishing effort ranged from approximately 23 to 38 ha. The fish trawling efforts had an
average trawling area of approximately 30 ha (Figure 12, Appendix).
U/F Ancylus is a smaller research vessel of the Swedish Board of Fisheries (Table 3,
Appendix) and is used for trawling the inner and more sheltered parts of Skagerrak and
Kattegat. The trawling events were part of a yearly conducted survey of coastal fish densities.
A Nephrops trawl was used with a 140 feet (approximately 43 meter) ground gear with rock
hoppers with a diameter of 10 centimetres and a 70 millimetre diagonal mesh (Figure 19,
Appendix). (Diagonal meshes are prohibited and are therefore not being used by commercial
Nephrops trawlers in Sweden). This diagonal mesh increases the catch of small sized fishes
and undersized Norwegian lobster, but the resulting catch differences should not be of too
much importance, according to Anders Svenson, at the Swedish Board of Fisheries (personal
comment). The lining consisted of 400 stretched meshes. The otter boards weighted 185 kilos
each and the distance between them was approximately 27 meters when trawling. This trawl
targets mainly demersal fish and N. norvegicus. The trawling events in this study were
conducted during October and November in daylight conditions and with the duration of 30
minutes per trawling effort at a trawling speed of 2,5 knots measured as trawl speed over the
ground. The area being trawled in each fishing effort ranged from approximately 6 to 9 ha.
The Nephrops trawling efforts had an average trawling area of approximately 8 ha (Figure 12,
Appendix).
GG 707 Arkö av Dyrön a commercial shrimp trawler (Table 3, Appendix) conducted shrimp
trawling in the Skagerrak, northwest of Skagen, by fishermen called “the Danish corner”,
during this study. The trawls were of Norwegian origin, 2 stropped shrimp trawl (OTB)
Fläckeröj type (Skagerrak) (Figure 20, Appendix) with a mesh size of 35 millimetres, 8 inches
(approximately 20,3 cm) bobbins in the front, 12 inches (approximately 30,5 cm) further in
and a ground gear of 70 meters. The width of each trawl was 30 to 35 meters and the hanging
ratio was 15 to 16 meters. In front of the cod end the net included square meshes on the
superior side, called escape panel. The lining of the cod end consisted of approximately 300
to 400 meshes of 35 millimetres each and the total circumference of the lining was 2200
meshes. The distance between each pair of otter boards, weighting 350 kilos each, varied
between 130 to 140 meters. Two trawls of the same design were used during each haul and
8
between the two trawls a chain matrix of 1500 to 1600 kilos was hanging to down weight
both of the trawls. The trawling took place during a week in the middle of September and the
hauls were done throughout the whole day and night. The trawling depth ranged from 275
meters to 410 meters, the duration of trawling varied between 5 to 7 hours and the speed of
trawling was approximately 1,5 knots. With a trawling speed of 1,5 knots and a trawling
duration of around 7 hours a distance of approximately 10 nautical miles were being trawled.
The area being trawled at each fishing effort ranged from approximately 375 to 600 ha (Figure
12, Appendix). The shrimp trawling efforts had an average trawling area of approximately
510 ha this average trawled area was 17 times larger than the average swept area of the fish
trawl and 67 times larger than the average swept area of the Nephrops trawl.
When the quota of shrimps were filled, a change was done to two fish trawls with a mesh size
of 90 millimetres and a disc size of approximately 10 centimetres. When exchanging to the
fish trawls more extended sweeps/ bridles were used and the distance between the two otter
boards broadened to approximately 160 meters. The area being trawled at each fishing effort
with the fish trawls was approximately 830 ha (Figure 12, Appendix).
2.3
Methods
2.3.1
Field sampling
Table 1. The positions of each trawling station, trawling depths, dates of trawling and the
names of each trawling station, where F stands for fish trawling, S for shrimp trawling,
CF for commercial fish trawling and N for Nephrops trawling.
Trawling
station
Date ShootLat ShootLong HaulLat HaulLong
Depth (m)
F1
F2
F3
F4
F5
F6
F7
F8
F9
F10
F11
F12
F13
F14
F15
F16
S1
S2
S3
S4
S5
CF1
CF2
N1
N2
20070903
20070904
20070904
20070904
20070904
20070905
20070905
20070905
20070905
20070905
20070905
20070906
20070906
20070906
20070906
20070906
20070917
20070917
20070917
20070918
20070918
20070919
20070919
20071015
20071015
57°40´18
57°28´30
57°29´21
57°27´07
57°30´01
57°47´22
57°41´25
57°35´02
57°29´14
57°22´21
57°35´10
57°42´47
57°44´34
58°02´25
58°07´37
57°52´43
58°15´00
58°10´00
58°16´00
58°13´00
58°14´00
58°11´00
58°07´00
57°32'15
57°32'55
11°12´08
08°00´25
08°18´55
08°31´07
08°50´56
08°47´42
08°45´11
08°57´51
09°07´55
09°08´08
09°06´03
09°24´45
09°44´51
09°55´15
10°43´54
10°59´54
10°21´00
09°56´00
10°23´00
10°04´00
10°21´00
10°28´00
10°01´00
11°20'02
11°23'40
57°39´08
57°29´12
57°30´15
57°27´20
57°30´51
57°46´13
57°40´19
57°34´14
57°29´12
57°22´30
57°36´12
57°41´17
57°45´25
58°03´20
58°05´48
57°51´27
58°13´00
58°15´00
58°13´00
58°18´00
58°11´00
58°07´00
58°09´00
57°32'55
57°32'53
11°09´28
08°03´07
08°21´43
08°34´22
08°53´49
08°45´11
08°42´26
08°54´49
09°04´32
09°04´45
09°08´51
09°26´44
09°47´49
09°58´19
10°44´14
10°57´25
10°04´00
10°18´00
10°04´00
10°24´00
10°35´00
10°01´00
10°31´00
11°23'04
11°22'21
38
144
94
54
37
223
136
45
23
20
33
32
37
149
231
74
350-390
350-410
385-390
395-365
300-275
220-230
230-210
31-31
19-30
9
N3
N4
N5
N6
N7
N8
N9
N10
N11
N12
N13
N14
N15
N16
20071017
20071018
20071018
20071023
20071023
20071023
20071023
20071106
20071106
20071106
20071107
20071107
20071108
20071108
58°10'17
58°08'36
58°09'40
57°05'02
56°31'36
56°23'27
56°21'14
58°11'35
58°15'30
58°14'45
58°22'44
58°33'01
58°29'57
58°25'43
11°18'19
11°15'08
11°10'30
11°29'48
11°33'26
12°07'05
12°12'07
11°19'30
11°24'20
11°21'26
11°09'18
11°01'26
10°29'02
10°33'59
58°09'48
58°09'15
58°09'05
57°04'20
56°31'21
56°22'54
56°20'34
58°12'13
58°15'01
58°14'09
58°22'02
58°32'26
58°29'19
58°26'11
11°17'17
11°15'40
11°10'50
11°29'40
11°32'13
12°07'50
12°12'42
11°20'16
11°23'18
11°20'45
11°08'50
11°02'17
10°28'16
10°35'00
72-56
25-41
40-40
51-52
40-41
42-41
37-35
115-150
26-42
34-32
16-20
110-169
98-80
67-69
2.3.2
Classification
The non-commercial invertebrates, hereby called (NCI), in each catch were sorted according
to species. The organisms were determined by species either on board when possible,
otherwise preserved in 96% ethanol and brought in to laboratory for further inspection, and in
some cases external professional support was needed. To make sure that each taxa received
the correct and accepted scientific name the web based platform of marine biodiversity
MarBEF, Marine Biodiversity and Ecosystem Functioning an EU Network of Excellence, was
used as a reference. Some organisms could not be determined to species so they were
determined down to the lowest taxa possible. The individuals were counted and the biomass
weighted (wet weight), with an accuracy of 1 gram. Each individual were classified according
to their functional groups describing their dominant trophic mode and predominant foraging
habit.
The classifications of the species were the following:
1 = predator-scavenger
a = burrowing dwelling
2 = suspension feeder
b = sedentary
3= deposit feeder
c = motile
This grouping of individuals were done according to Coggan et al, 2001.
Since the by-catches of NCI were small when based on the trawled area (m2), these numbers
are not presented in the result.
2.3.3
Standardization
To be able to compare the different trawl types with different trawling durations in a suitable
way, standardization has been performed. The swept area of each trawling event (Figure 12,
Appendix) has been calculated as:
Swept area (m2) = Trawling duration (s) x Towing speed over ground (m/s) x Door spread (m)
The door spread is the distance between the two otter boards. After calculating the swept area
for each trawling event the by-catch of invertebrates was divided by the swept area to receive
a comparable measure of the catch per trawled area. The swept area could have included only
the width of the trawl not the distance between the otter boards, but due to lack of information
about the trawl width during trawling the distance between the trawl doors was used instead,
as often done in fisheries research.
10
2.3.4
Statistical methods/ Data analysis
Correlation analyses were performed, using SPSS 13.0 for Windows® software release 13.0
(2004, Chicago: SPSS Inc.), in order to verify if there were any correlations between the total
catch of target species, trawling depth and the by-catch of non-commercial invertebrates.
These analyses of correlation were of non-parametric origin, Spearman’s rank correlation,
since the distributions were skewed and the data are non-linear, which converts the data to
ranks before calculating the correlation coefficient. The correlation coefficient (Spearman’s
rho) can vary between -1 to +1, a value close to either -1 or +1 indicates a high correlation
between the two variables and a value close to 0 a low correlation. A given p-value of less
than 0,05 (5%) is a correlation of significance.
To evaluate if the different trawl types generated different amount of invertebrate by-catch,
species composition and individual abundance, multivariate statistics in the software
PRIMER (Plymouth Routines In Multivariate Ecological Research) version 5 was used.
Cluster analysis exposing the similarities based on the Bray Curtis distance method and
square root transformation of data was done in order to down-weight the importance of the
very abundant taxa and to allow taxa of intermediate and rare abundance to contribute to the
similarities between different trawl types. A dendrogram of group average similarities
between different trawling events was plotted representing the result of hierarchical
clustering, with the x axis representing the full set of samples and the y axis defining a
similarity level at which two samples or groups are considered to have fused (Clarke &
Warwick, 1994). Non-metric Multi Dimensional Scaling (MDS) generating MDS plots
exposing the results in a two dimensional scale and samples close to each other is to be
interpreted as being similar with respect to all species. The resulting graph is presented with a
stress value that describes how well the result reflects the reality and when below 0, 2 the
graph could be interpreted as corresponding to reality. The stress level increases with
reducing dimensionality but also increasing quantity of data (Clarke & Warwick, 1994). The
number of restarts was set to 30 times and in some cases, stress for the 2-D ordinations is at
the upper level of what is considered an acceptable representation and ordinations should be
interpreted cautiously.
Formal significance tests for the degree of separation in terms of assemblage similarity
between sites were examined using the ANOSIM (Analysis of Similarities) permutation test.
A one-way analysis was chosen where the factors have been set as trawl types: Shrimp trawl,
Nephrops trawl and Fish trawl. Global R, which can vary between -1 and +1, and
corresponding pair wise comparisons were used to measure the degree to which trawling
events differed and the significance of these tests was determined via randomisation tests
(n=999 permutations). If Global R > 0,1 the difference between groups (trawl types) is larger
than within groups (trawl types) at a significance level of 5%. The null hypothesis that there is
no difference between trawl types (Global R = 0) should be rejected, and the trawl types are
statistically proven to be significantly different. The pair wise comparison generates a
statistical R for each comparison between trawl types ranging from 0 to 1 and an R of > 0,75
means that the different groups are well separated, R > 0,5 overlapping, but clearly different,
R < 0,25 barely separated at all (Clarke & Warwick, 1994).
The species primarily accounting for observed differences in species assemblages etc.
between trawl types were determined using SIMPER (similarity percentages) with
unstandardized square root transformed data with a cut off for low contributions at a level of
50%. The average similarity within each trawl type was given and the average dissimilarity
was measured between each group of two and the factors chosen were trawl types. The
diversity and species evenness (in numbers) of each trawling event was determined by
Shannon-Wiener diversity index:
H´= Σ -Pi*(log Pi)
11
Where Pi (e) is the proportion of the total count (or biomass etc) arising from the i species
(Clarke and Warwick, 1994). A trawling event represented by a single or very few
invertebrate species will generate a low diversity index while a trawling event with many
different invertebrate species, included in the by-catch, being even in numbers, will receive a
high index. These calculations were made with the natural logarithm (e) as base.
Evenness, how evenly individuals are distributed among the different species, was calculated
using Pielou´s evenness index:
J´= H´/ Log (S)
Where S is the species richness, which is the total number of species per trawling effort.
2.3.5
GIS application
All trawling coordinates were processed in the GIS software MapInfo version 8.0 and a
map/sea chart exposing each of the trawling positions produced (Figure 1, Appendix).
3
Results
To be able to separate the different trawl types and trawling occasions each trawling received
a unique identity. The first letter of each trawling event refers to the type of trawling, S for
shrimp trawling, F for fish trawling, N for Nephrops trawling and CF for fish trawling from
the commercial vessel. The last digits refer to the specific trawling location. To find out how
and to what degree the different trawling methods influence the benthic invertebrates, the
trawl data is used in different statistical methods and views.
3.1
Functional groups
3.1.1
Based on species diversity
In this study the functional group of motile predators and scavengers are absolutely dominant
of the species found in the by-catch of invertebrates (Figure 2). The second most common
functional group included in all the by-catch together was the sedentary suspension feeders
followed by burrowing, dwelling deposit feeders. All the by-catches of the different trawl
types in this study consisted predominantly of motile predators and scavengers (Figure 2).
The fish and Nephrops trawling also generated second most of by-catches of sedentary
suspension feeders followed by burrowing and dwelling deposit feeders. The by-catches of
the shrimp trawling and the commercial fish trawling had the same composition although with
another order, the burrowing and dwelling deposit feeders made up the second largest
compound while the sedentary suspension feeders the third. The commercial fish trawl had a
composition similar to the shrimp trawl although with less motile predators and scavengers
and the burrowing, dwelling deposit feeders and sedentary suspension feeders were more
common and in the commercial fish trawl there were another functional group of motile
deposit feeders.
Fish
Nephrops
Shrimp
Functional groups included in the Nephrops trawl
Functional groups included in the Fish trawl
68% Motile Predator/Scavenger
14% Sedentary Suspension feeder
12% Burrowing/dwelling Deposit feeder
3% Motile Deposit feeder
3% Burrowing/dwelling Suspension feeder
1% SedentaryPredator/Scavenger
1% Motile Suspension feeder
Functional groups included in the Shrimp traw l
8% Burrow ing/dw elling Deposit feeder
12
Functional groups included in the Fish traw l
1% Burrow ing/dw elling Suspension feeder
Functional groups included in the Nephrops trawl
1% SedentaryPredator/Scavenger
Functional groups included in the Shrim p traw l
Figure 2. The functional groups of invertebrates most frequently caught as by-catch when
trawling with the specified trawl types, with their relative order. From the left Fish trawl,
Nephrops trawl and Shrimp trawl. The % levels were based on the species diversity.
13
3.1.2
Based on individual abundance
When the individuals of invertebrates, per trawled area, in the by-catch were grouped in
functional groups the motile predators and scavengers were absolutely dominant as when
based on the species diversity (Figure 3). Also the second most common functional group
included in the by-catch was the sedentary suspension feeders followed by burrowing,
dwelling deposit feeders. The individual abundance grouped in functional groups gave the
same result as for the same procedure for species diversity, with a few exceptions (Table 2).
The third largest functional group being part of the by-catch in the fish trawls were the motile
deposit feeders, this differed from the rest of the trawling types, where the burrowing,
dwelling deposit feeders were a more common part in the by-catch. The shrimp trawling
generated the same contribution of functional group in their by-catch as the Nephrops trawl.
The motile predators and scavengers consisted of many species and individuals. The
sedentary suspension feeders consisted of quite many species but with fewer individuals,
while the burrowing and dwelling deposit feeders of fewer species with fewer individuals.
Fish
Nephrops
Shrimp
Functional groups in the Fish trawl (m2)
Functional groups in the Nephrops trawl (m/2)
Functional groups in the Shrimp trawl (/m2)
90% Motile Predator/Scavenger 7% Sedentary Suspension feeder 2% Motile Deposit feeder 1% Burrow ing/dw elling Deposit feeder
85% Motile Predator/Scavenger 7% Sedentary Suspension feeder 5% Burrow ing/dw elling Deposit feeder 2% Motile Deposit feeder
95% Motile Predator/Scavenger 3% Sedentary Suspension feeder 2% Burrow ing/dw elling Deposit feeder
Functional groups in the Fish traw l (m 2)
Functional groups in the Nephrops traw l (m /2)
14
Functional groups in the Shrim p traw l (/m 2)
Figure 3. The different functional groups and their contribution to the by-catch in all three
trawling types. From the left Fish trawl, Nephrops trawl and Shrimp trawl. The % levels were
based on the individual abundance per trawled area.
3.2
Abundance
3.2.1 Individual abundance per trawled area
The Nephrops trawling efforts generated the highest average individual abundance of
invertebrates in the by-catch per trawled area with a mean of 536,6/ 10 ha, followed by fish
trawling efforts with a mean of 129,9/ 100 ha while the shrimp trawling efforts generated the
lowest average individual abundance with a mean of 0,7/ 10 ha (Figure 3, Appendix). The
dendrogram (Figure 4) and the MDS plot (Figure 5) indicates that the shrimp trawling was
clearly separated from both the fish- and the Nephrops trawling regarding the individual
abundance of non-commercial invertebrate by-catch per trawling area. The ANOSIM analysis
(Table 2) presented the individual abundance in by-catches of different trawling types as
significantly different. The pairwise testing (Table 2) showed that the largest difference
between the by-catch of different trawl types was between shrimp- and Nephrops trawls,
followed by shrimp and fish trawls, all being well separated. The difference between the fish
and Nephrops trawl were clear but overlapping. All differences were significant. The
SIMPER analysis (Table 3) resulted in fish trawling as having the highest average similarity
in individual abundance of by-catch, followed by Nephrops trawling. Shrimp trawling efforts
had the lowest similarity of individual abundance of by-catch within the group of trawl type.
The fish trawling had additional species contributing to the similarity within the trawl group
while the shrimp trawl had the fewest species contributing to their similarity. There was no
significant correlation between individual abundance in the by-catch and the biomass of target
catch (Table 4). The abundance of NCI individuals in the by-catch of fish trawling correlated
to trawling depth, gave a significant but negative correlation (Table 5). Greater trawling depth
results in lower individual abundance. The Nephrops- and shrimp trawl did not have a
significant correlation between individual abundance and trawling depth. The correlation
between abundance of individuals in the by-catch and the trawled area gave a significant but
negative correlation when the fish trawl was used. Larger area did not automatically generate
larger individual abundance. The correlation of Nephrops and shrimp trawling were not
significant. Species most common in the NCI by-catch varied with trawling equipment,
although two species were among the five most common species in two of the three trawl
types; the swimming crabs Liocarcinus depurator and Liocarcinus holsatus (Figure 13,
Appendix)
15
Figure 4. Dendrogram presenting the similarities between each individual trawling effort with
their respective abundance of individuals in the by-catch per trawling area. The marked
trawling efforts were very different from their respective group of trawl types.
<5%
<5%
Figure 5. MDS plot visualizing the different trawling efforts and their similarities regarding
individual abundance in the by-catch per trawled area. The shrimp trawling efforts were
<5% similar to the other trawling efforts and trawling types.
16
Table 2. Results from the ANOSIM analysis presenting the differences, in individual
abundance in the by-catch per trawled area, between trawl types and their levels of
significance. * = significant result!
Individual abundance / swept area (m2)
Fish-, Nephrops- and
Shrimp trawl
ANOSIM
Global R
Significance
level
0,692
0,1%*
Statistic R
Significance
level
0,1%*
0,1%*
0,1%*
Pairwise tests
Shrimp / Nephrops
Shrimp / Fish
Fish / Nephrops
0,995
0,985
0,520
Table 3. SIMPER analysis on invertebrate species contributing the most to the differences, in
composition of individual abundance per trawled area, between trawl types.
* = significant result!
Dissimilarities between the different trawl types
Trawl types
Average
dissimilarity
Shrimp / Nephrops
98,50%
Shrimp / Fish
96,33%
Fish / Nephrops
79,04%
SIMPER
Species
contributing to the
dissimilarity
Liocarcinus depurator, L.
holsatus, Brissopsis lyrifera,
Asterias rubens
Alloteuthis subulata, Asterias
rubens,
Astropecten irregularis,
Loligo forbesi,
Liocarcinus depurator,
L. holsatus
Brissopsis lyrifera,
Liocarcinus depurator, L.
holsatus
Alloteuthis subulata, Asterias
rubens, Loligo forbesi.
% contribution
54,03%
53,69%
51,47%
Table 4. The individual abundance of invertebrates in the by-catch / m2 correlated to the
biomass of target catch / m2. Fish-, Nephrops- and Shrimp trawling. * = significant result!
Correlation:
Individual abundance / Biomass of Target catch (m2) Spearman's rho
Trawl types:
Fish trawl
Nephrops trawl
Shrimp trawl
Correlation
Coefficient
Sig. (2-tailed)
N
0,164
0,314
-0,029
0,529
16
0,220
16
0,957
5
17
Table 5. The abundance of invertebrate individuals in the by-catch/ m2 correlated to the
trawling depth. Fish-, Nephrops- and Shrimp trawling. * = significant result!
Spearman's rho
Correlation:
Individual abundance (m2) / Trawl depth
Trawl types:
Fish trawl
Nephrops trawl
Shrimp trawl
Correlation
Coefficient
Sig. (2-tailed)
N
-0,615
-0,028
-0,205
0,011*
16
0,918
16
0,741
5
Table 6. The abundance of invertebrate individuals in the by-catch/ m2 correlated to the
trawling area (m2). Fish-, Nephrops- and Shrimp trawling. * = significant result!
Spearman's rho
Correlation:
Individual abundance (m2) / Trawl area (m2)
Trawl types:
Fish trawl
Nephrops trawl
Shrimp trawl
Correlation
Coefficient
Sig. (2-tailed)
N
-0,562
-0,177
-0,477
0,024*
16
0,512
16
0,450
5
3.2.2 Individual abundance, excluding the shrimp trawling, per trawled area (m2)
The dendrogram (Figure 6) and the MDS plot (Figure 7) show a tendency of an existing
difference between Nephrops and fish trawls. The ANOSIM analysis
can be interpreted as the differences between fish and Nephrops
trawling were clearly and significantly different, but overlapping
(Table 7). Trawling stations that did not naturally cluster with its
respective trawl gear was F15, F01, F05, N03 and N10 (Figure 6 and Figure 7).
Figure 6. Dendrogram presenting the similarities between each individual trawling effort,
excluded the shrimp trawling, with their respective abundance in invertebrate by-catch of
individuals per area of trawling.
18
>20%
Figure 7. MDS plot visualizing the different trawling efforts, excluded the shrimp trawling, and
their similarities regarding the individual abundance invertebrates in the by-catch, per trawled
area. The similarity between the Nephrops and fish trawling efforts were >20%.
Table 7. ANOSIM analysis presenting the differences between fish- and Nephrops trawl
regarding the individual abundance of invertebrates in the by-catch, per trawled area and the
level of significance. * = significant result!
Factor
Fish- and Nephrops
trawl
ANOSIM
Global R
Significance level
0,507
0,1%*
Table 8. SIMPER analysis presenting the most different trawling efforts based on the
individual abundance of invertebrates in the by-catch per trawled area, their average
dissimilarities and the species explaining these differences.
Trawling effort
Average
dissimilarity
F15
85,68% to all of the
other trawling effort
F01 & F05
67,05% to the other
fish trawling efforts
81,22% to the other
Nephrops trawling.
N03 & N10
SIMPER
Species
contributing to the
dissimilarity
Crangon allmanii,
Pasiphaea sivado,
Pasiphaea multidentata.
Alloteuthis subulata
% contribution
Brissopsis lyrifera
24, 89%
17,79%
21,66%
19
3.3
Biomass
3.3.1
Biomass per trawled area (m2)
The Nephrops trawling efforts generated the highest biomass per trawled area from the bycatch of non-commercial invertebrates with a mean of 15,033 kg/10 ha, followed by the fish
trawling efforts with a mean of 2,130 kg/10 ha (Figure 7, Appendix). The by-catch of shrimp
trawling efforts had the lowest biomass of non-commercial invertebrates with a mean of 0,005
kg/10 ha (Figure 7, Appendix). The different trawl types were significantly different regarding
the biomass of non-commercial invertebrates in the by-catch (Table 9). The cluster analysis
(Figure 8) and the MDS-plot (Figure 9) indicate a clear difference between shrimp, Nephrops
and fish trawls. According to the one-way ANOSIM test (Table 9) the trawl types that were
most different was shrimp and Nephrops trawl, the two trawl types are very well separated.
The fish and Nephrops trawl were least separated, they are overlapping but clearly different.
The SIMPER analysis (Table 11) determined 8 species contributing to more than 50% of
these differences.
There was no significant correlation between the biomass of the by-catch and the biomass of
target catch (Table 11). This indicates that the amount of target catch did not influence the
amount of non-commercial invertebrate by-catch to a large extent. The correlation analysis of
the fish trawling efforts generated biomasses of by-catch and target catch had the strongest
correlation although not strong, followed by the shrimp trawling (Table 11), which had a
negative correlation. The Nephrops trawling had the lowest correlation, but none of the
trawling types had a significant correlation between biomasses of by-catch and target catch
(Table 11). The biomass of invertebrates in the by-catch had a negative correlation with the
depth of trawling (Table 12). Greater depth of trawling resulted in lower NCI biomass. There
was also a negative correlation, although not significant, between the biomass of invertebrates
in the by-catch and the trawled area (Table 13). The larger area being trawled the less biomass
of invertebrate by-catch.
The Nephrops trawl generated the highest proportion of by-catch biomass compared to target
catch (Table 14), where the target catch was fish and commercial invertebrates, including all
the size classes not only the marketable ones. The fish trawl generated the second largest
proportion of by-catch biomass compared to target catch and the target catch was equal to the
Nephrops. (This means that the Nephrops and fish trawls actually generated an even greater
by-catch of invertebrates, than shown in this study.) The shrimp trawl generated the lowest
proportion of by-catch biomass compared to target catch and their target catch was mainly
shrimps, other commercial Crustaceans and fish, of marketable size and quality (Figure 2,
Appendix). Species dominating the biomass of NCI by-catch varied with trawling equipment,
although two species were among the five species contributing mostly to the biomass in two
of the three trawl types; the swimming crabs Liocarcinus depurator and Liocarcinus holsatus
(Figure 14, Appendix).
20
their respective by-catch biomass per area of trawling.
<5%
<5%
Figure 9. MDS plot visualizing the different trawling efforts and their similarities regarding the
biomass of by-catch per trawled area. The shrimp trawling efforts were only <5% similar to
the other trawling efforts and trawling types.
21
Table 9. Results from the ANOSIM analysis presenting the differences between trawl types
and their levels of significance regarding the biomass of by-catch per trawled area.
Biomass / swept area (m2)
ANOSIM
Factor
Global R
Fish-, Nephropsand Shrimp trawl
0,714
Significance
level
0,1%*
Pairwise tests
Statistic R
Shrimp /
Nephrops
Shrimp / Fish
Fish / Nephrops
0,999
Significance
level
0,1%*
0,993
0,546
0,1%*
0,1%*
Table 10. SIMPER analysis on invertebrate species contributing the most to the difference in
the biomass of by-catch per trawled area between trawl types. * = significant result!
Trawl type
Shrimp /
Nephrops
Average
dissimilarity
99,16%
Shrimp / Fish
97,21%
Fish / Nephrops
81,82%
SIMPER
Species
contributing to the dissimilarity
Brissopsis lyrifera, Liocarcinus
depurator, Asterias rubens,
Pagurus bernhardus.
Loligo forbesi, Asterias rubens,
Liocarcinus depurator, L. holsatus,
Alloteuthis subulata, Alcyonium
digitatum.
Brissopsis lyrifera, Liocarcinus
depurator, L. holsatus Loligo forbesi,
Asterias rubens, Pagurus
bernhardus
% contribution
52, 96%
50, 25%
52, 03%
Table 11. The biomass of invertebrates in the by-catch / trawled area m2 correlated to the
biomass of target catch/ trawled area m2. Fish-, Nephrops- and shrimp trawling.
Correlation:
Biomass of by-catch (m2) / Biomass of target catch (m2)
Spearman's rho
Trawl types:
Fish trawl
Nephrops trawl
Shrimp trawl
Correlation
Coefficient
Sig. (2-tailed)
N
0,412
0,229
-0,400
0,113
16
0,393
16
0,505
5
22
trawling depth. Fish-, Nephrops- and Shrimp trawling. * = significant result!
Spearman's rho
Correlation:
Biomass of by-catch (m2) / Trawl depth
Trawl types:
Fish trawl
Nephrops trawl
Shrimp trawl
Correlation
Coefficient
Sig. (2-tailed)
N
-0,531
-0,074
-0,205
0,034*
16
0,787
16
0,741
5
trawled area. * = significant result!
Correlation:
Biomass of by-catch (m2) / trawled area (m2)
Spearman's rho
Trawl types:
Fish trawl
Nephrops trawl
Shrimp trawl
Correlation
Coefficient
Sig. (2-tailed)
N
-0,476
-0,270
-0,447
0,062
16
0,312
16
0,450
5
Table 14. The different trawl types and the proportion of non-commercial invertebrate by-
catch biomass compared to the biomass of target catch.
By-catch biomass/ target catch biomass
Trawl type
Nephrops trawl
Fish trawl
Shrimp trawl
% by-catch biomass/
target catch biomass
47,69%
2,24%
0,05%
3.3.2
Phyla contributing to the biomass per trawled area (m2)
The biomass of invertebrate by-catch consisted of mainly three most dominating phyla,
Echinodermata, Arthropoda and Mollusca. These were to more than 95% responsible for the
total invertebrate by-catch in all the trawling efforts, regardless which trawl type was being
used.
Depending on which of the trawl types were used during this study, there were obvious
differences in which phyla were the dominating ones in the NCI by-catch.
Echinodermata made up almost 50% of this total by-catch. The order of most dominating
phyla varied according to trawl type. The by-catch of Nephrops trawling had the same order
as for the total biomass of all trawling efforts and these three phyla presented almost 98% of
the by-catch biomass (Figure 10). In the fish and shrimp trawls the Echinodermata were only
the third most common phyla in the by-catch. The by-catch of the fish trawling consisted
predominantly of the phylum Mollusca, followed by the phyla Arthropoda and together with
Echinodermata they were responsible for over 90% of the by-catch biomass (Figure 10). In
the shrimp trawling by-catch Arthropoda were more common than Mollusca, these two phyla
and Echinodermata made up slightly over 96% of the by-catch biomass (Figure 10).
23
Fish
Nephrops
Mol 71%
Art 14%
Ech 8%
Cni 6%
Shrimp
Phyla of by-catch (biomass) / m2 -Shrimp trawl
Phyla of by-catch (biomass) / m2 -Nephrops trawl
Phyla of by-catch (biomass) / m2 -Fish trawl
Ech 56%
Ann 1%
Art 39%
Mol 3%
Por 1%
Cni 1%
Art 73%
Mol 15%
Ech 8%
Cni 4%
Phyla of by-catch (biom ass) / m 2 -Fish traw l
Mol 71%
Art 14%
Ech 8%
Cni 6%
Ann 1%
Phyla of by-catch (biom ass) / m 2 -Nephrops traw l
Ech 56%
Art 39%
Mol 3%
Por 1%
Cni 1%
Phyla of by-catch (biom ass) / m 2 -Shrim p traw l
Art 73%
Mol 15%
Ech 8%
Cni 4%
Figure 10. The phyla responsible for the highest biomass in the invertebrate by-catch. From
the right Fish trawl, Nephrops trawl and Shrimp trawl.
24
3.3.3
Biomass per trawled area (m2), excluded shrimp trawling
There seemed to be a clear difference between Nephrops and fish trawling, with one
exception of a fish trawling event, F01 (Figure 11 & 12). The ANOSIM analysis (Table 15)
tells that these different trawl types were significantly and clearly different but overlapping.
The species diversity was low (H´= 0,925) and the evenness was medium (J´= 0,516). The
trawling station was situated in the inner parts of Skagerrak, at low depth and with a small
total target catch. There was no significant correlation between trawling depth and biomass of
non-commercial invertebrate by-catch, when the shrimp trawling were excluded.
Figure 11. Dendrogram presenting the similarities between each individual trawling effort,
excluded the shrimp trawling, with their respective by-catch biomass per area of trawling.
<20%
the biomass of by-catch per trawled area, excluded shrimp trawling. The Nephrops and
shrimp trawling efforts were <20% similar to each other.
25
Table 15. ANOSIM analysis presenting the difference between fish and Nephrops trawl and
the level of significance regarding biomass of invertebrate by-catch. * = significant result!
Factor
Fish-, Nephrops
trawl
ANOSIM
Global R
Significance level
0, 54
0,1%*
Table 16. SIMPER analysis presenting the most different trawling effort, as biomass of
invertebrate by-catch, its average dissimilarities and the species explaining this difference.
SIMPER
Trawling effort
Average
dissimilarity
F01
79,44% from the rest of
the F
3.4
Species
contributing to the
dissimilarity
Liocarcinus depurator
% contribution
23,74%
Diversity
3.4.1
Species diversity of the by-catch
A species diversity of almost 100 different species (Table 4, Appendix) was found altogether
in the by-catches during this study. The number of different species varied from 4 to 24 per
trawling event. The three trawling efforts that generated the highest diversity (e.g. number of
taxa) of 20 or more different species in the by-catch, were all using the fish trawl. These were
F02, F07 and F16 and these catches were fished at the greatest depths among the fish trawling
events. The fewest species occurred at one shrimp and one Nephrops trawling event; S05 and
N08, and their by-catches consisted of only 4 different species. The most common species in
the by-catches during this study were Liocarcinus holsatus, Liocarcinus depurator, Pagurus
bernhardus, Brissopsis lyrifera and Asterias rubens which all occurred in more than 20 of the
total 39 trawling occasions.
3.4.2
Species diversity per trawling type
The fish trawling events, both the non commercial and the commercial ones, generated the
most diverse by-catches, as mean per trawling, followed by Nephrops trawling (Figure 13).
The shrimp trawl received the lowest diversity of species in the by-catch of non-commercial
invertebrates, as mean per trawling event. The NCI species diversity of each trawling effort
can be viewed in figure 5, Appendix.
26
Species diversity/ trawling type
30%
33%
CF
F
N
S
15%
Species diversity per trawling type
Trawl type
Commercial fish trawl
(CF)
Fish trawl
(F)
Nephrops trawl
(N)
Shrimp trawl
(S)
22%
Mean no. species
17
15
11
8
Figure 13. A chart presenting the comparison of mean number of species-species diversity
per trawl type.
According to the dendrogram (Figure 14) the shrimp trawling seemed most different from the
rest, together with one single fish trawling event (Figure 14 & 15), F15, taking place at the
greatest depth of the fish hauls and having the lowest amount of total catch of the fish
trawling. The ANOSIM analysis (Table 17) shows the different trawl types to be significantly
separated from each other. The species diversity between the different trawl types was
overlapping but clearly different. There are differences between fish and Nephrops trawl, but
they are not clearly separated. The SIMPER analysis (Table 18) presented the commercial
fish trawling events as having the highest average similarity, this group was most similar in
respect to species diversity of all included in this survey, but was also represented by the
fewest replicates, only two. With the shrimp trawl fewer species were responsible for more
than 50% of the similarity within the trawl group. The trawling types with the highest
dissimilarities between trawl types were Nephrops and shrimp trawls. The groups with the
lowest dissimilarity were fish and Nephrops trawling. F15 dissimilarity from the rest of the
fish and Nephrops trawl, was consistent with the results from the individual abundance.
Correlation analyses were made to see if the depth correlated with species diversity and if the
trawling area related to species diversity. But there were no significant correlation in either
case. Although a significant and positive correlation existed, only in the shrimp trawling
efforts, between species diversity and the biomass of target catch/m2, Spearman’s rho
correlation coefficient 0,975 a significance level of 0,005.
27
their respective species diversity.
~20%
species diversity. The shrimp trawling efforts were ~20% similar to the other trawling efforts
and trawling types.
28
Table 17. Results from the ANOSIM analysis presenting the differences between trawl types
and their levels of significance regarding species diversity. * = significant result!
Species diversity / trawl station
ANOSIM
Global R
0,549
Fish-, Nephropsand Shrimp trawl
Significance level
0,1%*
Pairwise tests
Shrimp / Fish
Shrimp / Nephrops
Shrimp / Comm.
Fish
Fish / Comm. Fish
Nephrops / Comm.
Fish
Fish / Nephrops
Statistic R
0,970
0,951
Significance level
0,1%*
0,1%*
0,836
0,789
4, 8%*
0,7%*
0,463
0,254
0,7%*
0,1%*
Table 18. SIMPER analysis on invertebrate species contributing the most to the difference in
species diversity per trawled area between trawl types.
Species diversity / trawl station
Trawl type
SIMPER
Average
dissimilarity
Shrimp / Nephrops 91,75%
Species
contributing to the dissimilarity
Liocarcinus depurator, Pagurus bernhardus,
Pasiphaea tarda, P. sivado, Brissopsis lyrifera
%
contribution
22,63%
Shrimp / Comm.
Fish
91,13%
22,84%
Shrimp / Fish
88,21%
Fish / Comm. Fish
77,94%
Nephrops /
Comm. Fish
Fish / Nephrops
74,08%
Psilaster andromeda, Neptunea antiqua,
Ophiuroidea sp., Pagurus bernhardus,
Parastichopus tremulus
Liocarcinus depurator, Asterias rubens,
Astropecten irregularis, Alloteuthis subulata,
Pasiphaea tarda
Liocarcinus holsatus, Parastichopus tremulus,
Liocarcinus sp., Lithodes maja, Asteronyx
loveni
Ophiuroidea sp., Munida sp., Liocarcinus sp.,
Asteronyx loveni, Spatangus purpureus
Alcyonium digitatum, Loligo forbesi,
Astropecten irregularis, Alloteuthis subulata,
Marthasterias glacialis, Brissopsis lyrifera
F15
65,39%
71,62%
from the other
F & N trawling
Spirontocaris lilljeborgii, Pandalus propinquus,
Pasiphaea multidentata, P. sivado, Euphasia
sp., Crangon allmanni
21,09%
20,97%
24,25%
20,26%
28, 47%
29
3.5
Red Listed species
Threatened non-commercial invertebrate species included in the by-catch of this study was 11
different species; 7 Crustacean species, 3 Echinoderm species and 1 species of Cnidaria
(Table 19). Most of these are considered to be deep living organisms and a significant and
positive correlation, between depth and number of red listed species existed (Table 20).
Greater trawl depth generated more red listed species. Most taxa and number of individual
specimen were in this study found in a depth range of 200-300 meters (Figure 16). Both the
two commercial fish trawling efforts generated by-catches of the most diverse composition of
red listed species, included were also a large number of specimen and three of these red listed
species occurred in both the trawling events (Table 1, Appendix). As for the shrimp and
Nephrops trawling (S05) and (N16) caught three different red listed species, while two of the
Nephrops trawling fewer species but several specimens (Table 1, Appendix). A positive and
significant correlation existed between number of red listed species and depth of trawling
when fishing was done with the fish trawl (Table 20), when the depth increased so did the
number of red listed invertebrates. There was a significant correlation between number of red
listed species included in the by-catch and the trawled area (m2) when fishing with the fish
trawl (Table 21), when the trawling area increased so did the number of red listed species in
the NCI by-catch. No significant correlation existed between number of red listed species in
the by-catch and biomass of target catch (Table 22).
Table 19. The red listed species included in the by-catch, the phylum they belong to and their
red listed category, during this study.
Species
Phylum
Red listed category
VU
Funiculina quadrangularis
Cnidaria
EN
Asteronyx loveni
Echinodermata
NT
Psilaster andromeda
Echinodermata
VU
Spatangus raschi
Echinodermata
DD
Calocarides coronatus
Arthropoda
NT
Hyas coarctatus
Arthropoda
DD
Munida rugosa
Arthropoda
DD
Munida sarsi
Arthropoda
DD
Munida tenuimana
Arthropoda
DD
Geryon trispinosus
Arthropoda
DD
Pontophilus norvegicus
Arthropoda
30
Species/ depth range
no. specimen
75
60
45
30
15
0
0-50m 50-100m
100200m
200300m
depth
300400m
F.quadrangularis
C.coronatus
H.coarctatus
M.rugosa
M.sarsi
M.tenuimana
G.trispinosus
P.norvegicus
A.loveni
P.andromeda
S.raschi
Figure 16. Chart presenting the red listed species distribution in different depth intervals.
Table 20. The number of red listed species in the by-catch / trawled area (m2) correlated to
the trawling depth. Fish-, Nephrops- and Shrimp trawling. * = significant result!
Correlation:
No. red listed / Trawl depth
Spearman's rho
Trawl types:
Fish trawl
Nephrops trawl
Shrimp trawl
Correlation
Coefficient
Sig. (2-tailed)
N
0,790
0,265
-0,574
0,0001*
16
0,322
16
0,312
5
the trawled area (m2). * = significant result!
Correlation:
No. red listed / trawled area (m2)
Spearman's rho
Trawl types:
Fish trawl
Nephrops trawl
Shrimp trawl
Correlation
Coefficient
Sig. (2-tailed)
N
0,766
0,324
-0,500
0,001*
16
0,221
16
0,391
5
the biomass of target catch/ trawled area (m2). Fish-, Nephrops- and shrimp trawling.
Spearman's rho
Correlation:
No. red listed / Biomass of target catch (m2)
Trawl types:
Fish trawl
Nephrops trawl
Shrimp trawl
Correlation
Coefficient
Sig. (2-tailed)
N
-0,140
0,345
-0,447
0,604
16
0,191
16
0,450
5
31
4
4.1
Discussion
Functional groups
The composition of the by-catch could act as an indicator of the status of the area being
trawled, the degree of disturbance and exposure to trawling frequencies.
In this study the functional group of motile predators and scavengers are absolutely dominant,
they represented almost 70% of the species found in the by-catch of NCI. The second most
common functional group included in the by-catch was the sedentary suspension feeders
followed by burrowing, sediment dwelling deposit feeders. Since the by-catches mainly
consisted of mobile predators or scavengers, this could indicate that the trawled areas have
been frequently fished prior to these trawling efforts (Hopkins, 2003). Although where large
erect and sessile suspension feeders were a considerable large part of the by-catch this could
indicate that that specific area has not prior been exposed to frequent heavy fishing pressure,
since they are in trawling aspects the most vulnerable organisms. They can not escape and are
large and erect enough to be caught in the trawl. Therefore they are the ones that ought to be
affected firstly followed by burrowing suspension or deposit feeders. The mobile predators
and scavengers can temporarily benefit from these when being returned as discard.
The different trawl types had NCI by-catches of different composition of functional groups.
The fish trawling efforts generated larger proportion of sedentary suspension feeders than the
Nephrops and shrimp trawling did. This could be due to the trawling areas prior less subjected
to intensive trawling. The Nephrops trawl caught burrowing and dwelling deposit feeders to a
larger extent than the fish trawling did, which was not a surprising result since the Nephrops
trawls has the closest contact with the bottom substrata and are therefore able to catch also
infauna to a large extent. Deposit feeders tends to predominate in muddy sediments, the
habitat of Nephrops norvegicus, as more detritus settle in areas of low turbulence (Castro &
Huber, 2005), which could be an explanation of the deposit feeders being more common in
the by-catches of the Nephrops trawling than the other trawling types.
The NCI by-catch of the shrimp trawls had the highest percentage of motile predators and
scavengers. These trawls received a by-catch of NCI of a low species diversity and individual
abundance with a lower biomass and as these trawls has the least contact with the sea floor,
this was an expected result. The area of fishing is also a well known and frequently used
fishing ground.
4.2
Abundance
In this study the tree different trawl types generated significantly different abundance of
individual invertebrate organisms per trawled m2. The Nephrops trawl had the highest
abundance of individual invertebrates in their by-catch, the fish trawl gained the second
highest abundance while the shrimp trawling the lowest abundance of individual invertebrates
(Figure 3, Appendix). This result is what could have been expected since the shrimp trawl is
the trawl type most distant from the sea floor and the depth of trawling was the greatest on
average of all the three trawling types. This could indicate that the mesh size has less
importance than the distance from the sea bed, but this needs to be further evaluated since the
areas of trawling were geographically separated. The Nephrops trawling occur in similar
biotopes and since these nets have a close contact with the bottom substrata this also
facilitates for more species and individuals, also burrowing infauna to be caught in these
trawls.
The by-catch of shrimp trawling lacked specimen of Liocarcinus depurator, Alloteuthis
subulata, Loligo forbesi, Astropecten irregularis, Asterias rubens and only one Brissopsis
lyrifera at one trawling effort. Most of this absence was probably due to the great depth of
trawling, many of these species has a shallower habitat reference and for some the trawl depth
32
lay in their upper limit of depth distribution. The Nephrops trawl caught a few cephalopods of
the species Alloteuthis subulata and Loligo forbesi, possibly due to these trawling efforts
taking place at lower depths in general and at lower water temperature possibly inducing
seasonal migration.
The trawl station F15 was dissimilar to the other fish and Nephrops trawling (Figure 6). This
was mostly explained by Alloteuthis subulata, a ten-armed Cephalopoda, being missing in the
by-catch at this trawling station while Crustaceans such as Crangon allmanii, Pasiphaea
sivado and Pasiphaea multidentata were an important part of the individual abundance of
invertebrate by-catch (Table 8), but not in the other fish trawling efforts. The individual
abundance was low in the by-catch of this trawling effort F15 which could be expected as the
depth of trawling was the deepest among the fish and also Nephrops trawling events. This
trawling also generated the lowest amount of target catch among the fish trawling efforts and
this station is situated in an area frequently trawled. Two of the fish trawling efforts, F01 and
F05, which occurs within the groupings of Nephrops trawl was dissimilar from the other fish
trawling efforts. This difference was due to a large by-catch of NCI comparable with those of
the Nephrops trawling, these two trawling also caught a large number of Alloteuthis subulata.
The trawling station F01 had the highest individual abundance of the all the fish trawling
efforts during this study (Figure 5, Appendix). This station was situated, in rather shallow
waters where the access to important nutrient is superior to deeper waters, close to the border
of Skagerrak and Kattegat, where two large currents meet, the surface current from the Baltic
Sea and the Jutland deep sea current. The biomass of target catch was low and the species
diversity was the lowest among the fish trawling events, which could indicate that the
community in the area consisted of few species but with high individual abundance. Since the
species diversity is generally lower in less saline water, this is as could be expected. The two
Nephrops trawling N03 and N10 was separated from the rest of the Nephrops trawling efforts.
This difference was mostly explained by the sea urchin Brissopsis lyrifera being present in the
NCI by-catch in very large numbers (Table 8).
The Nephrops trawl in general caught a lot of Brissopsis lyrifera while the fish trawl caught a
lot of Alloteuthis subulata (Figure 13, Appendix). The high individual occurrence of
Alloteuthis subulata in the fish trawling by-catch, while much less frequent in the by-catches
of the other trawl types, could be a consequence of the finer mesh size of the fish trawl and
the coarser mesh size of the other trawl types facilitating escape from the cod-end, since most
of the specimen caught were of a rather small body size. Brissopsis lyrifera was only
presented by one specimen in all of the shrimp trawling efforts, this was probably due to the
shrimp trawls higher placement over the bottom substrata and it could possibly also be
explained by the deeper depth of trawling, since Brissopsis lyrifera is a sub-surface deposit
feeder and lives within the depth interval of 5 to 365 meters, according to Budd, 2004. There
was no significant correlation between individual abundance in the by-catch and the biomass
of the target catch per trawled area. This was not expected since one could expect that a
successful catch with a large biomass of target catch would automatically facilitate for a
higher individual abundance of by-catch and probably smaller organisms. Since these could
be squeezed in between the large amount of target catch without a possibility of escape
through the cod-end. While a less successful catch could be thought of as generating a bycatch of fewer but larger specimen in the by-catch.
There was a negative correlation between individual abundance of invertebrates in the bycatch and the depth of trawling, with an increasing depth the number of individuals decreased
in this study. This could be expected since deeper habitats usually offer more harsh
conditions. Depth of between 200 to 500 meters offers similar conditions and therefore their
might be little or hardly no differences in species composition within these habitats. The food
sources are limiting, not the habitats, so less competition occurs than in more shallow areas.
There were almost no correlation when the fishing was executed from the Nephrops trawl,
33
this was probably caused by the fact that these trawling efforts occurred at lower depths with
a maximum depth of 170 meters. A correlation existed between abundance and the swept
area, with an increasing area being trawled the abundance decreased. The area being swept by
the shrimp trawl was at an average 17 times larger than the swept area by the fish trawl and
67 times larger than the trawled area at the Nephrops trawling efforts. This larger area was
due to 10 to 14 times longer trawling duration and also the fact that two identical trawls were
used simultaneously at each shrimp trawling effort. Shrimp and Fish trawling occurred during
this study in the outer parts of Skagerrak (closer geographically), while the Nephrops trawling
events took place in the inner parts and also in Kattegat and the shrimp and fish trawling
efforts occurred at an earlier date of the autumn. These circumstances might influence the
result.
4.3
Biomass
All the trawl types were significantly different when taking the biomass of NCI by-catch into
account. The Nephrops trawling generated the highest proportion of NCI by-catch biomass
per m2 compared to biomass of target catch per m2, while the fish trawling the second highest
and shrimp trawling the lowest (Figure 2, Appendix). As for the shrimp trawling this is what
would be expected as it occurs at a deeper depth and more distant from the sea floor. The
shrimp trawling had a more limited target catch, since it included solely marketable catch.
These shrimp trawling also had less species and individuals in the NCI by-catch which makes
this lower biomass of by-catch an expected result. The large differences between shrimp and
Nephrops trawl could be explained by Brissopsis lyrifera only caught once in the shrimp
trawling events and that Liocarcinus depurator as well as both Asterias rubens and Pagurus
bernhardus did not appear at all in these shrimp trawling events (probably due to the greater
depth in shrimp trawling). Both Liocarcinus depurator and Brissopsis lyrifera were found in
the by-catch of all but three Nephrops trawling efforts. The proportion of NCI biomass of bycatch would actually be even greater in the trawling efforts performed by the fish and
Nephrops trawl if the target catch would have included only marketable catch, as in the case
with the shrimp trawling efforts.
The Nephrops trawling had clearly the largest biomass of NCI by-catch per m2 compared to
the biomass of target catch per m2, approximately 50%. This part would actually be even
larger, as with the fish trawling induced by-catch, if the target catch would have consisted of
only marketable fish and invertebrates. The high level of invertebrate by-catch in Nephrops
trawling is consistent with Bergmann et al, 2002 and Bergmann et al 2001, where the
invertebrates accounted for up to 90% of the discard in the Clyde Sea Nephrops fishery. The
fish had less proportion of NCI by-catch almost 2,25% while the shrimp trawl had the lowest
proportion of NCI by-catch, 0,05%. This is probably due to the greater depth of trawling and
the larger distance from the trawl to the bottom substrata. The biomass of invertebrates in the
by-catch had a negative correlation with the depth of trawling. The greater depth of trawling
the less biomass of invertebrate by-catch and this is what could be expected since the
individual abundance also decreased with depth.
Since there was a significant and negative correlation between biomass of invertebrates and
the area being trawled this meant that when the trawling area increased, the biomass of
invertebrates in the by-catch decreased. This was consistent with the correlation result for
individual abundance as would be expected.
Depending on which trawl type was being used the by-catch composition of phyla varied.
The phyla responsible for the highest percentage of the non-commercial invertebrate by-catch
biomass were Echinodermata, followed by Arthropoda and Mollusca, when taking all the
different trawling effort into account. The by-catch of the Nephrops trawl had the same
composition (Figure 14, Appendix). These major groups were consistent with the composition
of invertebrate discard in the Nephrops fisheries in the Clyde Sea (Bergmann et al, 2002 and
34
Bergmann et al 2001). In the fish trawl the invertebrate by-catch consisted mainly of
Mollusca, followed by Arthropoda and Echinodermata and these had an uneven spread as
Mollusca were highly dominant (Figure 14, Appendix). In the shrimp trawl by-catch
Arthropoda, Mollusca and Echinodermata were a major part and the Arthropoda were highly
dominant, while the rest contributed less (Figure 14, Appendix).
The ranking order of the phyla most responsible for the by-catch biomass varied between
trawl types which could be due to different mesh sizes, different geographical distribution of
trawling, different depths of trawling.
The trawling station F01 was grouped together with the Nephrops trawling, as it was when
analysing the individual abundance. In this trawl the swimming crab Liocarcinus depurator
dominated the biomass and they appeared in a large number of individuals (Table 16).
Species not part in this by-catch were Brissopsis lyrifera, Asterias rubens, Alcyonium
digitatum and Pagurus bernhardus. The trawling station was situated in the inner parts of
Skagerrak close to Kattegat where the Baltic surface current meets the Jutland deep sea
current, at low depth and with a small total target catch and with a low species diversity of
NCI in the by-catch.
Shrimp and fish trawling occurred during this study in the outer parts of Skagerrak (closer
geographically), while the Nephrops trawling events took place in the inner parts and also in
Kattegat and the shrimp and fish trawling efforts occurred at an earlier date of the autumn.
These circumstances might influence the result.
4.4
Diversity
The total species diversity of invertebrates of all the trawling efforts in total in this study
included almost 100 different taxa (Table 4, Appendix). The species most often included in
the by-catches in this study was four species of predators and scavengers and one deposit
feeding burrowing species. Predator and scavenger species are thought to be benefited by
intensive trawling so this could be interpreted as being consistent with studies performed by
Demestre et al, 2000, Hopkins, 2003, Ocean Studies Board, 2002.
The fish trawling events, both the non commercial and the commercial, generated the most
diverse by-catches while the shrimp trawling occasions gained the least diverse NCI bycatches. This could indicate that the fish trawling reflects the benthic community more
accurately than the other trawl types do. Consistent with the former results the shrimp
trawling generated the least diverse NCI by-catch. The shrimp trawling were conducted at
greater depths which could be of importance to the species composition and the depths of
trawling were more homogenous with a similar species composition. The shrimp trawling
occurred most distant from the sea floor of all the trawl types included in this study which
makes it possible for a large amount of invertebrates to escape the trawl as it passes above
them and invertebrates normal escape behaviour is to flee down towards or stay as close to the
bottom as possible. Another reason for the shrimp trawling receiving less diverse by-catch
could be due to the selection panel in front of the cod end on the dorsal side, which facilitates
an upward escape of organisms, but since invertebrates normally has a downward escape
behaviour this should not influence the amount and diversity of invertebrate by-catch.
Depth did not seem to affect the species diversity in this study, nor did the size of the area
being swept by the trawl, as there was no significant correlation between the variables.
The species most often included in the by-catches in this study were Liocarcinus holsatus,
Liocarcinus depurator, Pagurus bernhardus, Brissopsis lyrifera and Asterias rubens (Figure
10, Appendix) which all occurred in more than 20 of the total 39 trawling occasions. Catches
in numbers and biomass were usually dominated by only two to four species per location as in
the study performed by Lindeboom & de Groot, 1998. The species compositions in the bycatch are consistent or similar with the discarded invertebrate species in the Clyde Sea
Nephrops trawling industry (Bergmann et al, 2001, Bergmann et al 2002) and also with those
35
of studies performed by Bradshaw et al (1999) and Lindeboom & de Groot (1998).
Groenewold et al (2000) found Liocarcinus holsatus, Pagurus bernhardus, Asterias rubens
among the main active scavengers of different kinds of food sources. The scavenging fauna of
the southern North Sea are dominated by a few abundant and opportunistic species, such as
swimming crabs, hermit crabs, starfish, ophiuroids and shrimp and locally also whelks and
edible crabs (Lindeboom & de Groot, 1998), these could be the dominant ones in Skagerrak
too, since those species were often caught in the trawls during this study. Liocarcinus holsatus
and depurator, both swimming crabs, are fast-growing highly mobile species, reaching an
early maternity (Lindeboom & de Groot, 1998) and they are able to reproduce several times a
year (Wear, 1974) which should make recovery from disturbance of fishing effort high (Hill,
2007). The hermit crab, Pagurus bernhardus is a scavenger found on most substratum, from
soft sediment and muddy sand to hard rocky outcrop and also up in the shallow sea grass beds
and in the algal zone. They are most common between 0–50 meters but can be found down to
500 meters depth.
Brissopsis lyrifera has a fragile test that is likely to be damaged by an abrasive force, such as
movement of trawling gear. Although populations of Brissopsis lyrifera are likely to recover
from effects of physical disturbance, such as fishing impacts, rapidly as the species is fecund
and recruits annually with pelagic larvae which enables for a wide distribution and together
with their fast growing and short life history they are suited for a variable environment (Budd,
2004). Asterias rubens is likely to be damaged by physical abrasion, especially removal of
arms or damage to superficial tissue. However, they are quite resilient and probably suffer
low mortality because of their regenerative abilities following autotomy of arms.
Recoverability of Asterias rubens is also likely to be high as it is widespread, fecund with an
annually reproduction of long living pelagic larvae having a high dispersal potential and are
able to settle upon a variety of benthic substrata (Clark & Downey, 1992). Fishing activities
increase the input of carrion to benthic communities (Ramsay et al, 2000). Asterias rubens is
an opportunistic scavenger that has been shown to gain extra food by foraging in fished areas
upon damaged and displaced organisms (Ramsay et al, 1998) and also feeds on fisheries
discards (Lindeboom & de Groot 1998 and Ramsay et al, 1997). Overall taking account of the
importance of discards as a source of food and the resilience of Asterias rubens to physical
impact, fishing activity may favour populations of Asterias rubens. The species is an
important food source for other star fish, demersal fish, crustaceans and sea birds (Budd,
2007). The species most frequently part of the by-catch in this study were opportunistic
species with planktonic larvae which enables a wide distribution, high productivity and they
could be favoured by discard from the fishing industry as an additional food source. Most of
them have a maturity age of only 1 year and all but the Brissopsis lyrifera are predator or
scavengers. Predicted by disturbance paradigm it is a likely favourable situation for more
opportunistic recolonizers being short-lived, highly motile or dispersing species with high
reproduction rates which will recover from disturbance faster than long-lived, sessile, lowdispersing species will (Collie et al, 2000, Hansson et al, 2000, Kaiser & Spencer, 1994,
Pickett & White, 1995). This could be supported by the species most common in the bycatches during this study.
The species that were most often included in the by-catch of invertebrates were all rather large
epifauna, except for Brissopsis lyrifera which is large but an infaunal species. Smaller bodied
organisms were not a common part of the by-catch which could be explained by the fact that
they are more likely to escape through the meshes of the trawl or that they could be displaced
by pressure waves that form in front of fishing gear as they move through the water, as
showed by Gilkingson et al, 1998.
The trawl station F15 was dissimilar to the other fish and Nephrops trawling, as it was when
taking the individual abundance into account. This could be explained by a large number of
Crustaceans included in the invertebrate by-catch of F15 while Pagurus bernhardus,
36
Astropecten irregularis, Alloteuthis subulata, Alcyonium digitatum, Aphrodita aculeata and
Neptunea antique was not, this in contrary with the other fish and Nephrops trawling there
they always were part of the by-catch. This fish trawling effort also generated the lowest
amount of target catch among the fish trawling events. The low diversity of species could be
explained by the greater depth of approximately 230 meter, making this trawling event the
deepest trawling among the fish and Nephrops trawling events. According to the correlation
analysis this is what could be expected. The station is also situated in a frequently trawled
area.
In the Gullmarfjord the by-catches included large amounts of Echinoderms which is
consistent with the research of effects of shrimp trawling in the Gullmarfjord done by
Hansson et al 2000, showing a significant trend of decreasing number of echinoderms after
trawling.
This study was conducted only during one season and the species diversity might shift over
the seasons, making a future study on a temporal as well as spatial scale important. Some of
the trawling efforts could have gained a different abundance, species diversity or biomass per
swept area which could be explained by “unsuccessful” trawling event due to poor contact
with bottom substrata, wrong angle of towing gear, poor fishing ground due to over fishing or
poor habitat or just according to the high depth of trawling. Shrimp and Fish trawling
occurred during this study in the outer parts of Skagerrak (closer geographically), while the
Nephrops trawling events took place in the inner parts and also in Kattegat and the shrimp and
fish trawling efforts occurred at an earlier date of the autumn. These circumstances might
influence the result.
4.5
Red listed species
In the total NCI by-catch there was a total of 11 red listed species (Table 2, Appendix), of
these, 7 belonged to Crustacea, 3 to Echinodermata and 1 to Cnidaria, most of these were
caught at a depth ranging from 200 to 300 meters. Both the two commercial fish trawling
efforts generated by-catches of the most diverse composition of red listed species, included
were also a large number of specimen. The areas being trawled were probably not prior
subject to intensive demersal trawling, especially since the by-catch included Funiculina
quadrangularis which is a sessile erect and large sized species, most likely to be caught in the
trawl net. Asteronyx loveni, an often associated species, was also included. There were more
specimen of Psilaster andromeda being caught in the trawls, but many of them got stuck on
top of or in the net and did not enter the sorting table, which made it impossible to be able to
quantify them correctly as they were wound together with the trawl around the net drum when
hauling.
The species on the red list were Funiculina quadrangularis, Calocarides coronatus, Hyas
coarctatus, Munida rugosa, Munida sarsi, Munida tenuimana, Geryon trispinosus,
Pontophilus norvegicus, Asteronyx loveni, Psilaster andromeda and Spatangus raschi.
Funiculina quadrangularis belongs to Cnidaria and is classified as vulnerable (VU) and since
it settles on soft sediment, has a large erect size and a slow growth rate it is considered to be
seriously threatened by intensive demersal trawling. Asteronyx loveni an Echinodermata is
categorised as endangered (EN) since it is closely associated with the former species F.
quadrangularis. Psilaster andromeda is classified as nearby threatened (NT), but close to be
regarded as vulnerable (VU), as they have diminished probably due to demersal trawling.
Spatangus raschi another Echinodermata is only found at few locations and is regarded as
vulnerable (VU), probably negatively effected by demersal trawling. A large quantity of
specimen were part of the by-catch at Torgestad in the Gullmarfjord and several in the
trawling in the Kosterfjord. The data is considered deficient (DD) regarding the burrowing
crustacean Calocarides coronatus which means it could be negatively effected by demersal
trawling. The Crustacean Hyas coarctatus is according to the red list nearby threatened (NT)
37
since it has been heavily reduced, especially in shallow areas near the coastline, instead of
demersal trawling, eutrophication is thought of as being the reason for their decline. Munida
rugosa, Munida sarsi, Munida tenuimana and Geryon trispinosus could probably be
negatively effected by demersal trawling and the data is said to be deficient (DD). Another
crustacean were the data is deficient (DD) is Pontophilus norvegicus, which also probably is
negatively affected by demersal trawling (ArtDatabanken, 2007).
4.6
Trawling damages on invertebrates
Demersal trawls will affect the fauna of a given location to some degree, but the magnitude
and duration of the effect depends on several factors, including gear configuration, towing
speed, water depth, and the substrate over which the towing occurs (Auster & Langton, 1999).
Physical disturbances from otter trawls on muddy sediment could lead to community changes
in the benthos. These changes include reduction in diversity, biomass and of individual
organism size. The effects of otter trawling on the infauna include a reduction in the
abundance of large-bodied fragile organisms and an increase in abundance of opportunists,
and may ultimately lead to an altered but stable community comprising a reduced number of
species and faunal diversity, and with the fauna comprising primarily of small organisms
(Collie et al, 2000, Hansson et al, 2000). The observed damages to the NCI in the by-catch
were depending on species. Some species seemed to be relatively resistant while others
seemed to suffer severely. Many of the Echinoderms such as sea urchins were being crushed
or losing several spines and these damages could have been caused by different parts of the
trawling gear or in interaction with other species in the trawl. Asterias rubens, Psilaster
andromeda and Astropecten irregularis seemed highly resistent to the effects of trawling.
Some Crustaceans such as swimming crabs, Liocarcinus, had broken or crushed carapaces
and missing or having their appendages partly or totally removed, which reduce there chances
of survival. These damages could have been caused by different parts of the trawling gear or
in interaction with other species in the trawl. Mortality of hermit crabs, Pagurus, seemed
relatively low, but some had abandoned their protective gastropod shell probably due to
heavily stress. Molluscs as for example Buccinum undatum, Neptunea antiqua and Pecten
maximus seem to have survived probably due to their well protecting thick shell. The
polychaete Aphrodita aculeata, seemed to survive mostly when being caught in the trawl. The
species that seemed resistant to or vulnerable to demersal trawling were consistent with
Kaiser et al, 1995 and Kaiser & Spencer’s studies from 2005, where they concluded that an
individuals and species ability to survive the trawling is directly related to its physiology,
morphology and behaviour in response to the trawling gear being used (Kaiser & Spencer,
1995). Bergmann et al suggested, in their study 1999, that direct mortality within a species
may vary with sex and density. For example species where the females burrow in the
sediment appear to be more protected than the males living mostly on top of the seabed.
Trawling might induce changes in the sex structure of such populations. Since this study did
not included determination of sex this is not comparable. The effects on non-commercial
invertebrates by otter trawling is world wide issue since the invertebrates play an important
role in the whole marine ecosystem, contributing essentially to the food source of several
commercially important fish species. These results are important to highlight also in an
international aspect since demersal otter trawling is a commonly world wide used trawling
equipment and some of the trawling grounds included in this study belong not only to Sweden
but to Denmark and Norway and other fishing nations are also allowed to trawl in these areas,
and the guidelines for fishing and quotas are decided within the European community.
38
5
Conclusions
This study is a prestudy representing the differences between marine benthic macrofauna
exposed to different trawling methods. The relatively low frequency of trawling efforts and
the short temporal scale makes the conclusions more of assumptions rather than reflecting the
reality, but are nevertheless of importance. An overall trend existed in this study; the bycatches of the shrimp trawling efforts were most and significantly different of all the trawl
types included in this study regardless of analysing the individual abundance, species
diversity or biomass in the by-catches of invertebrates. This trawl was affecting the species
diversity, the individual abundance and the biomass the least of the three compared trawling
types during this study. The Nephrops trawl caught the highest individual abundance and
biomass of NCI by-catch. The fish trawling efforts resulted in the highest species diversity of
the NCI by-catch, followed by the Nephrops trawling efforts.
The Nephrops trawling efforts generated the highest proportion of by-catch biomass
compared to target catch (almost 50%) while the shrimp trawling the lowest, even though the
target catch of the shrimp trawling were only marketable catch and therefore should be less.
The fish trawling efforts generated by-catches most homogenous, where more different
species contributed to the similarity within the trawl group than the other trawl types did,
regardless of analysing the individual abundance, species diversity or biomass in the bycatches of invertebrates. Motile predators and scavengers were the functional group that most
frequently were part of the by-catch. This could reflect the fact that the trawled areas were
former subject to intensive trawling pressure. The three different trawl types generated
different compositions of Phyla in their by-catches, but the three most common Phyla in all
by-catches were Echinodermata, Arthropoda and Mollusca.
This study indicates that the shrimp and fish trawling industry could have a more direct and
aimed fishing, than the Nephrops trawling has. The largest differences between the three trawl
types and their generated by-catch of invertebrates were the comparison between the different
biomass of by-catch, secondly the individual abundance and the least differences occurred
when comparing the species diversity.
A lot of factors could cause differences in species diversity, individual abundance and
biomass of NCI by-catches. These could for example be shifting environmental conditions
such as currents, temperature, variation, natural migration, storm activity (Ocean Studies
Board, 2002), differences in bottom substrata, depth, salinity and level of exposure. While
other differences may not have been detected because the species were relatively rare, their
distribution was patchy, or the sample sizes were small (Castro & Huber, 2005 and Freese et
al, 1999). The data from the commercial shrimp trawler is not as accurate as the data from
both the research vessels, since the research trawling are always executed in an standardized
way, with an objective of receiving comparable data while the commercial fishing has their
aim set on maximizing the target catch. Demersal otter trawling affects both the benthic
organisms and their habitat negatively.
In an ideal study multiple replicates with control sites of untrawled areas would be compared
to frequently trawled areas with as similar habitat construction and physical conditions as
possible, although untrawled areas might have been exposed to different fishing pressure in
the past. The different trawling types ought to be standardized regarding trawling duration,
towing speed, towing distance etc. and this research should include not only the spatial scale
but also the temporal scale, preferably stretched over a long period of time to receive an
accurate result. Future studies on the catch efficiency of demersal otter trawls for epibenthic
invertebrates and effects of injury at a population level are needed to elucidate community
effects, although unfortunately, the lack of quantitative historical data sets and of unfished
control areas makes it difficult to evaluate the impact of trawling.
39
6
Acknowledgements
I would like to express my gratitude to all those who gave me the possibility to complete this
thesis. First of all to my supervisor Matz Berggren whose help, stimulating suggestions and
encouragement helped me throughout the research for and writing of this thesis. Thank you
for spending time reading and commenting on my thesis also late evenings and weekends! I
am also grateful for the support from Anders Svenson, Ann-Christine Rudolphi and Barbara
Bland-Johnson at the Swedish National Board of Fisheries in Lysekil and all the personnel
and participants of the U/F Argos and U/F Ancylus expeditions.
Roger Aronsson, Mathias Ivarsson, Leif Pettersson and Olle Karlsson at GG 707 Arkö av
Dyrön, I am really thankful for you having me on board and your patience in answering all
my questions and not to forget all the nice meals you supplied. I would also like to thank
Stefan Agrenius at Kristineberg Marine Research Station, Anna Dimming, Andrea Belgrano
at the Swedish National Board of Fisheries in Lysekil, Larry Hansson at the Swedish
Coastguards in Gothenburg, Hans G Hansson at Tjärnö Marine Biological Laboratory and the
personnel at Kristineberg Marine Research Station. Tomas Nilsson at AB DFS, Donsö
Fiskeredskap & Skeppsfurnering for sharing valuable knowledge about fishing equipment.
Last but not least I would like to thank the other master thesis students, especially Anders
Olsson and Maria Caules-Bosch, and my colleagues at the Fishery Competence Centre,
Gothenburg, especially Anna Söderlind, you have given me very valuable comments, and my
family and friends who have supported me greatly. I am deeply grateful!
7
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42
Appendix
Table 1. Presenting each trawling station with the respective physical data.
Trawling
station
F01
F02
F03
F04
F05
F06
F07
F08
F09
F10
F11
F12
F13
F14
F15
F16
S01
S02
S03
S04
S05
CF1
CF2
N01
N02
N03
N04
N05
N06
N07
N08
N09
N10
N11
N12
N13
N14
N15
N16
Date
20070903
20070904
20070904
20070904
20070904
20070905
20070905
20070905
20070905
20070905
20070905
20070906
20070906
20070906
20070906
20070906
20070917
20070917
20070917
20070918
20070918
20070919
20070919
20071015
20071015
20071017
20071018
20071018
20071023
20071023
20071023
20071023
20071106
20071106
20071106
20071107
20071107
20071108
20071108
Depth (m)
Duration
(h)
Speed
SOG
(knots)
38
144
94
54
37
223
136
45
23
20
33
32
37
149
231
74
350-390
350-410
385-390
395-365
300-275
220-230
230-210
31-31
19-30
72-56
25-41
40-40
51-52
40-41
42-41
37-35
115-150
26-42
34-32
16-20
110-169
98-80
67-69
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
7
7
8
7
5
7
7
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
0,5
3,7
3,6
3,6
3,6
3,5
3,6
3,6
3,7
3,7
3,7
3,7
3,7
3,6
3,8
3,7
3,7
1,5
1,5
1,5
1,5
1,5
2
2
2,5
2,5
2,5
2,5
2,5
2,5
2,5
2,5
2,5
2,5
2,5
2,5
2,5
2,5
2,5
2,5
Door
spread
(m)
82
109
99
90
84
101
102
86
68
70
78
75
82
108
96
97
270
270
270
270
270
320
320
30
32
35
28
31
35
33
34
31
38
29
29
27
34
40
37
Swept area
(m2)
280948,3976
363362,3969
330026,3971
300023,9974
272243,9976
336693,5971
340027,1971
294653,1975
232981,598
239833,9979
267243,5977
256964,9978
273355,1976
380030,3967
328915,1972
332341,3971
5250419,955
5250419,955
6000479,948
5250419,955
3750299,968
8296959,928
8296959,928
69449,9994
74079,99936
81024,9993
64819,99944
71764,99938
81024,9993
77320,99933
78709,99932
72459,49937
87969,99924
67134,99942
67134,99942
62504,99946
78709,99932
92599,9992
85654,99926
Wind
m/s
Wind
dir.
10
12
14
11
9
12
11
10
10
12
11
6
8
7
6
6
14
10
10
10
5
7
10
5
5
10
5
10
4
4
3
2
4
5
5
5
7
10
10
NNW
NNW
NNW
N
NNE
NNE
NNE
NNE
NNE
NNE
NNE
NNE
NNE
NNE
NNE
NNE
WSW
WSW
NNW
NNW
N
N
SSW
SW
SW
SW
N
N
S
S
ESE
ESE
NNW
NNW
NNW
NW
NW
SW
SW
43
Figure 1. Map over the study area and each trawling station, with red markings at each shoot
position.
F=
CF =
N=
S=
Fish trawling stations
Commercial fish trawling stations
Nephrops trawling
Shrimp trawling
44
Table 2. The red listed species and numbers included in the by-catch at their respective
trawling stations.
Red listed species / Trawling station
F2 F6 F7 F8 F14 F15 F16 S1 S3 S4 S5 CF1 CF2
A. loveni
2
5
2
20
N1
N2 N3 N4 N5
N10
N11 N14 N15 N16
1
(EN)
1
F.quadrangularis
2
1
2
(VU)
2250x
S. raschi
38
(VU)
H. coarctatus
1
1
1
a
4
6
b
b
1
1
a
b
1
1
10
2
1
3
1
(NT)
2
P. andromeda
6
1
3
(NT)
1
C. coronatus
(DD)
G. trispinosus
1
1
1
(DD)
1
M. rugosa
(DD)
11
M. sarsi
4
(DD)
2
M. tenuimana
(DD)
a
P. norvegicus
(DD)
a = a few
b = many
x = approximated number
By-catch biomass / Swept area (1ha=10000m2)
N
16
N
14
N
12
N
10
0
N
08
0
N
06
4
N
04
1
N
02
8
S
05
2
S
03
12
S
01
3
F1
5
16
F1
3
4
F1
1
20
F0
9
5
F0
7
24
F0
5
6
F0
3
28
F0
1
7
Target catch Biomass / Swept area (1ha=10000m2)
Figure 2. Presenting the total target catch (green, right axis) and the total by-catch (red, left
axis) of non-commercial invertebrate biomass (kg) per trawling effort.
45
Individuals/ Swept area (1ha=10000m2)
400
350
300
250
200
150
100
50
N1
6
N1
4
N1
2
N1
0
N0
8
N0
6
N0
4
N0
2
S0
5
S0
3
S0
1
F1
5
F1
3
F1
1
F0
9
F0
7
F0
5
F0
3
F0
1
0
Figure 3. The individual abundance/ ha of the NCI by-catch at each trawling station.
Individuals / Swept area (1ha=10000m2)
400
350
300
250
200
150
100
50
0
N01
N02
N03
N04
N05
N06
N07
N08
N09
N10
N11
N12
N13
N14
N15
N16
Figure 4. The individual abundance/ ha of the NCI by-catch at the Nephrops trawling stations.
Individuals / Swept area (1ha=10000m2)
40
35
30
25
20
15
10
5
0
F01
F02
F03
F04
F05
F06
F07
F08
F09
F10
F11
F12
F13
F14
F15
F16
Figure 5. The individual abundance/ ha of the NCI by-catch at the fish trawling stations.
46
Individuals/ Swept area (1ha= 10000m2)
0,16
0,14
0,12
0,1
0,08
0,06
0,04
0,02
0
S01
S02
S03
S04
S05
Figure 6. The individual abundance/ ha of the NCI by-catch at the shrimp trawling stations.
Biomass / Swept area (1ha=10000m2)
7
6
5
kg
4
3
2
1
N1
6
N1
4
N1
2
N1
0
N0
8
N0
6
N0
4
N0
2
S0
5
S0
3
S0
1
F1
5
F1
3
F1
1
F0
9
F0
7
F0
5
F0
3
F0
1
0
Figure 7. The biomass (kg) / ha of the NCI by-catch at each trawling station.
7
6
5
kg
4
3
2
1
0
N01
N02
N03
N04
N05
N06
N07
N08
N09
N10
N11
N12
N13
N14
N15
N16
Figure 8. The biomass (kg) / ha of the NCI by-catch at the Nephrops trawling stations.
47
1,4
1,2
1
kg
0,8
0,6
0,4
0,2
0
F01
F02
F03
F04
F05
F06
F07
F08
F09
F10
F11
F12
F13
F14
F15
F16
Figure 9. The biomass (kg) / ha of the NCI by-catch at the fish trawling stations.
0,0014
0,0012
0,001
kg
0,0008
0,0006
0,0004
0,0002
0
S01
S02
S03
S04
S05
Figure 10. The biomass (kg) / ha of the NCI by-catch at the shrimp trawling stations.
Species diversity
30
25
No. of taxa
20
15
10
5
N1
6
N1
4
N1
2
N1
0
N0
8
N0
6
N0
4
N0
2
D2
FA
S0
5
S0
3
S0
1
F1
5
F1
3
F1
1
F0
9
F0
7
F0
5
F0
3
F0
1
0
Figure 11. The species diversity (number of taxa) of the NCI by-catch at each trawling
station.
48
Swept area (ha)
900
800
700
600
ha
500
400
300
200
100
N1
6
N1
4
N1
2
N1
0
N0
8
N0
6
N0
4
N0
2
D2
FA
S0
5
S0
3
S0
1
F1
5
F1
3
F1
1
F0
9
F0
7
F0
5
F0
3
F0
1
0
Figure 12. The swept area (ha) of each trawling effort.
Table 3.
Vessel
Facts about the different trawling vessels used in this study.
Engine Length Width Depth
Type
Built
Gross
(m)
(m)
penetration
tonnage effect
(m)
(kW)
(tonne)
U/F
Argos
Steel
stern
trawler
1974
U/F
Ancylus
Steel
stern
trawler
GG 707
Arkö av
Dyrön
Steel
stern
trawler
1261
1325
61
12
5
1971
108
500
24
6
3
1979
205
496
26
8
4
(modified
1993)
49
Table 4.
Trawl
station
F1
F1
F1
F1
F1
F1
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F2
F3
F3
F3
F3
F3
F3
F3
F3
F3
F3
F3
F3
F3
F3
F4
F4
F4
F4
F4
F4
F4
F4
Each trawling station and the taxa, phyla, class and family included in the bycatch. X represents lack of information.
Taxon
Phylum
Class
Family
Brissopsis lyrifera
Liocarcinus holsatus
Loliginidae fam.
Loligo forbesi
Actiniidae sp.
Alcyonium digitatum
Aphrodita aculeata
Ascidia virginea
Asteria rubens
Astropecten irregularis
Brissopsis lyrifera
Buccinum undatum
Colus islandicus
Echinus elegans
Geryon trispinosus
Hyas coarctatus
Luidia sarsi
Neptunea antiqua
Ophiura albida
Pagurus pubescens
Pseudamussium peslutrae
Strongylocentrotus droebachiensis
Alcyonium digitatum
Aphrodita aculeata
Asterias rubens
Buccinum undatum
Neptunea antiqua
Pagurus bernhardus
Polycarpa pomaria
Spatangus purpureus
Todaropsis eblanae
Trachythyone elongata
Alcyonium digitatum
Aphrodita aculeata
Pagurus bernhardus
Mollusca
Echinodermata
Arthropoda
Arthropoda
Mollusca
Mollusca
Cnidaria
Cnidaria
Mollusca
Annelida
Chordata
Echinodermata
Echinodermata
Echinodermata
Mollusca
Mollusca
Echinodermata
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Echinodermata
Mollusca
Echinodermata
Arthropoda
Mollusca
Echinodermata
Cnidaria
Annelida
Echinodermata
Echinodermata
Mollusca
Arthropoda
Arthropoda
Mollusca
Arthropoda
Chordata
Echinodermata
Echinodermata
Mollusca
Echinodermata
Cnidaria
Mollusca
Annelida
Echinodermata
Arthropoda
Arthropoda
Arthropoda
Echinodermata
Cephalopoda
Echinoidea
Malacostraca
Malacostraca
Cephalopoda
Cephalopoda
Hexacorallia
Octocorallia
Cephalopoda
Polychaeta
Ascidiacea
Stelleroidea
Stelleroidea
Echinoidea
Gastropoda
Gastropoda
Echinoidea
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Stelleroidea
Gastropoda
Stelleroidea
Malacostraca
Bivalvia
Echinoidea
Octocorallia
Polychaeta
Stelleroidea
Stelleroidea
Gastropoda
Malacostraca
Malacostraca
Gastropoda
Malacostraca
Ascidiacea
Echinoidea
Echinoidea
Cephalopoda
Holothuroidea
Octocorallia
Cephalopoda
Polychaeta
Stelleroidea
Malacostraca
Malacostraca
Malacostraca
Echinoidea
Loliginidae
Brissidae
Portunidae
Portunidae
Loliginidae
Loliginidae
Actiniidae
Alcyoniidae
Loliginidae
Aphroditidae
Ascidiidae
Asteriidae
Astropectinidae
Brissidae
Buccinidae
Buccinidae
Echinidae
Geryonidae
Majidae
Portunidae
Portunidae
Luidiidae
Buccinidae
Ophiuridae
Paguridae
Pectinidae
Strongylocentrotidae
Alcyoniidae
Aphroditidae
Asteriidae
Astropectinidae
Buccinidae
Portunidae
Portunidae
Buccinidae
Paguridae
Styelidae
Spatangidae
Ommastrephidae
Cucumariidae
Alcyoniidae
Loliginidae
Aphroditidae
Astropectinidae
Portunidae
Portunidae
Paguridae
50
Trawl
station
F5
F5
F5
F5
F5
F5
F5
F5
F5
F5
F5
F5
F5
F5
F5
F6
F6
F6
F6
F6
F6
F6
F6
F6
F6
F6
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
F7
Taxon
Phylum
Class
Family
Actiniidae sp.
Alcyonium digitatum
Ascidia virginea
Asterias rubens
Balanus crenatus
Chamelea striatula
Loligo forbesi
Macropodia rostrata
Neanthes fucata
Ophiura ophiura
Ophiuroidea sp.
Pagurus bernhardus
Actiniidae sp.
Aphrodita aculeata
Asterias rubens
Brissopsis lyrifera
Munida sp.
Ophiura albida
Pagurus bernhardus
Psilaster andromeda
Trischizostoma raschi
Actiniidae sp.
Alcyonium digitatum
Aphrodita aculeata
Asterias rubens
Brissopsis lyrifera
Buccinum undatum
Colus islandicus
Echinus sp.
Gattyana cirrhosa
Hyas coarctatus
Loligo forbesi
Neptunea antiqua
Ophiothrix fragilis
Ophiura albida
Ophiura ophiura
Pagurus bernhardus
Pecten maximus
Phascolion strombi
Psilaster andromeda
Suberites ficus
Todaropsis eblanae
Cnidaria
Cnidaria
Mollusca
Chordata
Arthropoda
Echinodermata
Arthropoda
Mollusca
Arthropoda
Mollusca
Arthropoda
Annelida
Echinodermata
Echinodermata
Arthropoda
Cnidaria
Annelida
Arthropoda
Echinodermata
Arthropoda
Arthropoda
Echinodermata
Arthropoda
Mollusca
Echinodermata
Arthropoda
Echinodermata
Cnidaria
Annelida
Echinodermata
Echinodermata
Echinodermata
Mollusca
Mollusca
Echinodermata
Annelida
Arthropoda
Arthropoda
Arthropoda
Mollusca
Mollusca
Echinodermata
Echinodermata
Echinodermata
Arthropoda
Mollusca
Sipuncula
Echinodermata
Porifera
Mollusca
Hexacorallia
Octocorallia
Cephalopoda
Ascidiacea
Stelleroidea
Stelleroidea
Maxillopoda
Bivalvia
Malacostraca
Cephalopoda
Malacostraca
Polychaeta
Stelleroidea
Stelleroidea
Malacostraca
Hexacorallia
Polychaeta
Stelleroidea
Echinoidea
Malacostraca
Malacostraca
Stelleroidea
Malacostraca
Bivalvia
Stelleroidea
Malacostraca
Hexacorallia
Octocorallia
Polychaeta
Stelleroidea
Stelleroidea
Echinoidea
Gastropoda
Gastropoda
Echinoidea
Polychaeta
Malacostraca
Malacostraca
Malacostraca
Cephalopoda
Gastropoda
Stelleroidea
Stelleroidea
Stelleroidea
Malacostraca
Bivalvia
Sipunculidea
Stelleroidea
Demospongiae
Cephalopoda
Actiniidae
Alcyoniidae
Loliginidae
Ascidiidae
Asteriidae
Astropectinidae
Balanidae
Veneridae
Portunidae
Loliginidae
Inachidae
Nereididae
Ophiuridae
x
Paguridae
Actiniidae
Aphroditidae
Asteriidae
Brissidae
Portunidae
Galatheidae
Ophiuridae
Paguridae
Pectinidae
Astropectinidae
Lysianassidae
Actiniidae
Alcyoniidae
Aphroditidae
Asteriidae
Astropectinidae
Brissidae
Buccinidae
Buccinidae
Echinidae
Polynoidae
Majidae
Portunidae
Portunidae
Loliginidae
Buccinidae
Ophiothrichidae
Ophiuridae
Ophiuridae
Paguridae
Pectinidae
Phascolionidae
Astropectinidae
Suberitidae
Ommastrephidae
51
Trawl
station
F8
F8
F8
F8
F8
F8
F8
F8
F8
F8
F8
F8
F8
F8
F8
F8
F9
F9
F9
F9
F9
F9
F9
F9
F9
F9
F10
F10
F10
F10
F10
F10
F10
F10
F10
F10
F10
F10
F10
F10
F10
F11
F11
F11
F11
F11
F11
F11
F11
F11
F11
Taxon
Phylum
Class
Family
Actiniidae sp.
Aequipecten opercularis
Ascidiella scabra
Asterias rubens
Echinidae sp.
Loligo forbesi
Ophiothrix fragilis
Ophiura albida
Pagurus bernhardus
Pecten maximus
Psilaster andromeda
Suberites ficus
Alcyonium digitatum
Ascidiella scabra
Asterias rubens
Loligo forbesi
Pagurus bernhardus
Pecten maximus
Abietinaria abietina
Alcyonium digitatum
Ascidia virginea
Ascidiella scabra
Asterias rubens
Brissopsis lyrifera
Cephalopoda sp.
Marthasterias glacialis
Ophiura albida
Pagurus bernhardus
Securiflustra securifrons
Abietinaria abietina
Alcyonidium diaphanum
Alcyonium digitatum
Ascidiella scabra
Asterias rubens
Loligo forbesi
Echinodermata
Mollusca
Mollusca
Chordata
Echinodermata
Echinodermata
Echinodermata
Arthropoda
Arthropoda
Mollusca
Echinodermata
Echinodermata
Arthropoda
Mollusca
Echinodermata
Porifera
Cnidaria
Mollusca
Chordata
Echinodermata
Echinodermata
Arthropoda
Arthropoda
Mollusca
Arthropoda
Mollusca
Cnidaria
Cnidaria
Mollusca
Chordata
Chordata
Mollusca
Mollusca
Echinodermata
Mollusca
Arthropoda
Mollusca
Mollusca
Echinodermata
Arthropoda
Bryozoa
Cnidaria
Bryozoa
Cnidaria
Mollusca
Chordata
Mollusca
Mollusca
Arthropoda
Arthropoda
Mollusca
Hexacorallia
Bivalvia
Cephalopoda
Ascidiacea
Stelleroidea
Stelleroidea
Echinoidea
Malacostraca
Malacostraca
Cephalopoda
Stelleroidea
Stelleroidea
Malacostraca
Bivalvia
Stelleroidea
Demospongiae
Octocorallia
Cephalopoda
Ascidiacea
Stelleroidea
Stelleroidea
Malacostraca
Malacostraca
Cephalopoda
Malacostraca
Bivalvia
Hydroidomedusa
Octocorallia
Cephalopoda
Ascidiacea
Ascidiacea
Stelleroidea
Stelleroidea
Echinoidea
Cephalopoda
Malacostraca
Malacostraca
Stelleroidea
Stelleroidea
Malacostraca
Gymnolaemata
Hydroidomedusa
Gymnolaemata
Octocorallia
Cephalopoda
Ascidiacea
Stelleroidea
Stelleroidea
Malacostraca
Malacostraca
Cephalopoda
Actiniidae
Pectinidae
Loliginidae
Ascidiidae
Asteriidae
Astropectinidae
Echinidae
Portunidae
Portunidae
Loliginidae
Ophiothrichidae
Ophiuridae
Paguridae
Pectinidae
Astropectinidae
Suberitidae
Alcyoniidae
Loliginidae
Ascidiidae
Asteriidae
Astropectinidae
Portunidae
Portunidae
Loliginidae
Paguridae
Pectinidae
Sertulariidae
Alcyoniidae
Loliginidae
Ascidiidae
Ascidiidae
Asteriidae
Astropectinidae
Brissidae
x
Portunidae
Portunidae
Asteriidae
Ophiuridae
Paguridae
Flustridae
Sertulariidae
Alcyonidiidae
Alcyoniidae
Loliginidae
Ascidiidae
Asteriidae
Astropectinidae
Portunidae
Portunidae
Loliginidae
52
Trawl
station
F11
F11
F11
F11
F12
F12
F12
F12
F12
F12
F12
F12
F12
F12
F12
F12
F12
F12
F12
F12
F12
F13
F13
F13
F13
F13
F13
F13
F13
F13
F13
F13
F13
F13
F13
F13
F13
F14
F14
F14
F14
F14
F14
F14
F14
F14
F14
F14
F14
F14
Taxon
Phylum
Class
Family
Pagurus bernhardus
Suberites ficus
Alcyonium digitatum
Aphrodita aculeata
Arctica islandica
Ascidia virginea
Ascidiella scabra
Asterias rubens
Buccinum undatum
Loligo forbesi
Modiolus modiolus
Nemertea
Pagurus bernhardus
Actiniidae sp.
Alcyonium digitatum
Aphrodita aculeata
Asterias rubens
Balanus balanus
Echinidae sp.
Loligo forbesi
Macropodia rostrata
Pagurus bernhardus
Pecten maximus
Suberites ficus
Asterias rubens
Brissopsis lyrifera
Buccinum undatum
Geryon trispinosus
Luidia sarsi
Neptunea antiqua
Pasiphaea sp.
Mollusca
Arthropoda
Bryozoa
Porifera
Cnidaria
Mollusca
Annelida
Mollusca
Chordata
Chordata
Mollusca
Mollusca
Mollusca
Arthropoda
Arthropoda
Mollusca
Mollusca
Nemertea
Arthropoda
Bryozoa
Echinodermata
Cnidaria
Cnidaria
Mollusca
Annelida
Mollusca
Mollusca
Arthropoda
Echinodermata
Arthropoda
Arthropoda
Mollusca
Arthropoda
Arthropoda
Mollusca
Bryozoa
Porifera
Mollusca
Echinodermata
Echinodermata
Echinodermata
Mollusca
Arthropoda
Arthropoda
Arthropoda
Echinodermata
Mollusca
Arthropoda
Mollusca
Bryozoa
Stelleroidea
Malacostraca
Gymnolaemata
Demospongiae
Octocorallia
Cephalopoda
Polychaeta
Bivalvia
Ascidiacea
Ascidiacea
Stelleroidea
Stelleroidea
Gastropoda
Malacostraca
Malacostraca
Cephalopoda
Bivalvia
x
Malacostraca
Gymnolaemata
Echinoidea
Hexacorallia
Octocorallia
Cephalopoda
Polychaeta
Stelleroidea
Stelleroidea
Maxillopoda
Echinoidea
Malacostraca
Malacostraca
Cephalopoda
Malacostraca
Malacostraca
Bivalvia
Gymnolaemata
Demospongiae
Cephalopoda
Stelleroidea
Stelleroidea
Echinoidea
Gastropoda
Malacostraca
Malacostraca
Malacostraca
Stelleroidea
Gastropoda
Malacostraca
Bivalvia
Gymnolaemata
Asteriidae
Paguridae
Flustridae
Suberitidae
Alcyoniidae
Loliginidae
Aphroditidae
Arcticidae
Ascidiidae
Ascidiidae
Asteriidae
Astropectinidae
Buccinidae
Portunidae
Portunidae
Loliginidae
Mytilidae
x
Paguridae
Flustridae
Actiniidae
Alcyoniidae
Loliginidae
Aphroditidae
Asteriidae
Astropectinidae
Balanidae
Echinidae
Portunidae
Portunidae
Loliginidae
Inachidae
Paguridae
Pectinidae
Flustridae
Suberitidae
Loliginidae
Asteriidae
Astropectinidae
Brissidae
Buccinidae
Geryonidae
Portunidae
Portunidae
Luidiidae
Buccinidae
Pasiphaeidae
Pectinidae
Flustridae
53
Trawl
station
F15
F15
F15
F15
F15
F15
F15
F15
F15
F15
F15
F15
F15
F15
F15
F15
F15
F16
F16
F16
F16
F16
F16
F16
F16
F16
F16
F16
F16
F16
F16
F16
F16
F16
F16
F16
F16
S1
S1
S1
S1
S1
S1
S1
S1
S2
S2
S2
S2
S2
S2
Taxon
Phylum
Class
Family
Abra nitida
Actiniidae sp.
Asterias rubens
Brissopsis lyrifera
Buccinum undatum
Crangon allmannii
Euphausia sp.
Hyas coarctatus
Neoleanira tetragona
Pandalus propinquus
Pasiphaea multidentata
Pasiphaea sivado
Spirontocaris lilljeborgii
Actiniidae sp.
Alcyonium digitatum
Aphrodita aculeata
Asterias rubens
Brissopsis lyrifera
Bryozoa
Buccinum undatum
Colus islandicus
Geryon trispinosus
Neptunea antiqua
Ophiura albida
Ophiura ophiura
Pagurus bernhardus
Psilaster andromeda
Aega crenulata
Munida sarsi
Pandalus propinquus
Pasiphaea sivado
Pasiphaea tarda
Pontophilus norvegicus
Crangon sp.
Euphausia sp.
Pandalus propinquus
Pasiphaea sivado
Mollusca
Cnidaria
Echinodermata
Echinodermata
Mollusca
Arthropoda
Arthropoda
Cnidaria
Arthropoda
Arthropoda
Arthropoda
Annelida
Arthropoda
Arthropoda
Arthropoda
Bryozoa
Arthropoda
Cnidaria
Cnidaria
Mollusca
Annelida
Echinodermata
Echinodermata
Echinodermata
Bryozoa
Mollusca
Mollusca
Arthropoda
Arthropoda
Arthropoda
Annelida
Mollusca
Echinodermata
Echinodermata
Arthropoda
Echinodermata
Bryozoa
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Bivalvia
Hexacorallia
Stelleroidea
Echinoidea
Gastropoda
Malacostraca
Malacostraca
Octacorallia
Malacostraca
Malacostraca
Malacostraca
Polychaeta
Malacostraca
Malacostraca
Malacostraca
Gymnolaemata
Malacostraca
Hexacorallia
Octocorallia
Cephalopoda
Polychaeta
Stelleroidea
Stelleroidea
Echinoidea
x
Gastropoda
Gastropoda
Malacostraca
Malacostraca
Malacostraca
Polychaeta
Gastropoda
Stelleroidea
Stelleroidea
Malacostraca
Stelleroidea
Gymnolaemata
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Semelidae
Actiniidae
Asteriidae
Brissidae
Buccinidae
Crangonidae
Euphausiidae
Funiculinidae
Majidae
Portunidae
Portunidae
Sigalionidae
Pandalidae
Pasiphaeidae
Pasiphaeidae
Flustridae
Hippolytidae
Actiniidae
Alcyoniidae
Loliginidae
Aphroditidae
Asteriidae
Astropectinidae
Brissidae
x
Buccinidae
Buccinidae
Geryonidae
Portunidae
Portunidae
Sigalionidae
Buccinidae
Ophiuridae
Ophiuridae
Paguridae
Astropectinidae
Flustridae
Aegidae
Portunidae
Galatheidae
Pandalidae
Pasiphaeidae
Pasiphaeidae
Pasiphaeidae
Crangonidae
Crangonidae
Euphausiidae
Portunidae
Pandalidae
Pasiphaeidae
Pasiphaeidae
54
Trawl
station
S2
S2
S3
S3
S3
S3
S3
S3
S3
S4
S4
S4
S4
S4
S4
S4
S4
S4
S4
S4
S4
S5
S5
S5
S5
CF1
CF1
CF1
CF1
CF2
CF1
CF1
CF1
CF1
CF1
CF1
CF1
CF1
CF1
CF1
CF1
CF2
CF2
CF2
CF2
CF2
CF2
CF2
CF2
CF2
CF2
Taxon
Phylum
Class
Family
Pasiphaea tarda
Syscenus infelix
Munida sp.
Munida tenuimana
Pandalus propinquus
Pasiphaea sivado
Pasiphaea tarda
Syscenus infelix
Brissopsis lyrifera
Calocarides coronatus
Crangon sp.
Loligo vulgaris
Munida sp.
Pasiphaea sivado
Pasiphaea tarda
Spatangus purpureus
Syscenus infelix
Todaropsis eblanae
Asteronyx loveni
Munida sarsi
Actiniidae sp.
Aphrodita aculeata
Asteronyx loveni
Brissopsis lyrifera
Geryon trispinosus
Liocarcinus sp.
Lithodes maja
Munida sp.
Neptunea antiqua
Ophiuroidea sp.
Pagurus bernhardus
Polychaeta spp.
Psilaster andromeda
Spatangus purpureus
Actiniidae sp.
Aphrodita aculeata
Asteronyx loveni
Brissopsis lyrifera
Buccinum undatum
Geryon trispinosus
Hyas coarctatus
Liocarcinus sp.
Lithodes maja
Munida sp.
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Echinodermata
Arthropoda
Arthropoda
Arthropoda
Mollusca
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Echinodermata
Arthropoda
Mollusca
Echinodermata
Cnidaria
Arthropoda
Arthropoda
Cnidaria
Annelida
Echinodermata
Echinodermata
Cnidaria
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Mollusca
Echinodermata
Arthropoda
Echinodermata
Annelida
Echinodermata
Echinodermata
Cnidaria
Annelida
Echinodermata
Echinodermata
Mollusca
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Echinoidea
Malacostraca
Malacostraca
Malacostraca
Cephalopoda
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Echinoidea
Malacostraca
Cephalopoda
Stelleroidea
Octacorallia
Malacostraca
Malacostraca
Hexacorallia
Polychaeta
Stelleroidea
Echinoidea
Octacorallia
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Gastropoda
Stelleroidea
Malacostraca
Holothuroidea
Polychaeta
Stelleroidea
Echinoidea
Hexacorallia
Polychaeta
Stelleroidea
Echinoidea
Gastropoda
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Pasiphaeidae
Aegidae
Portunidae
Galatheidae
Galatheidae
Pandalidae
Pasiphaeidae
Pasiphaeidae
Aegidae
Brissidae
Axiidae
Crangonidae
Portunidae
Loliginidae
Galatheidae
Pasiphaeidae
Pasiphaeidae
Pasiphaeidae
Spatangidae
Aegidae
Ommastrephidae
Asteronychidae
Funiculinidae
Portunidae
Galatheidae
Actiniidae
Aphroditidae
Asteronychidae
Brissidae
Funiculinidae
Geryonidae
Portunidae
Lithodidae
Galatheidae
Buccinidae
x
Paguridae
Stichopodidae
x
Astropectinidae
Spatangidae
Actiniidae
Aphroditidae
Asteronychidae
Brissidae
Buccinidae
Geryonidae
Majidae
Portunidae
Lithodidae
Galatheidae
55
Trawl
station
CF2
CF2
CF2
CF2
CF2
CF2
CF2
N1
N1
N1
N1
N1
N1
N1
N1
N1
N1
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N3
N3
N3
N3
N3
N3
N3
N3
N3
N3
N3
N3
N3
N4
N4
N4
N4
N4
N4
N4
N4
N4
Taxon
Phylum
Class
Family
Neptunea antiqua
Ophiuroidea sp.
Pagurus bernhardus
Psilaster andromeda
Spatangus purpureus
Brissopsis lyrifera
Hyas coarctatus
Ophiura ophiura
Pagurus bernhardus
Pennatula phosphorea
Platynereis dumerilii
Psilaster andromeda
Alcyonium digitatum
Ascidiella scabra
Asterias rubens
Brissopsis lyrifera
Carcinus maenas
Hyas coarctatus
Pagurus bernhardus
Psilaster andromeda
Actiniidae sp.
Aphrodita aculeata
Ascidiacea sp.
Asterias rubens
Brissopsis lyrifera
Buccinum undatum
Hyas coarctatus
Mesothuria intestinalis
Neptunea antiqua
Ophiura albida
Ophiura ophiura
Alcyonium digitatum
Aphrodita aculeata
Ascidiacea sp.
Brissopsis lyrifera
Carcinus maenas
Echinocardium cordatum
Hyas sp.
Annelida
Mollusca
Echinodermata
Arthropoda
Echinodermata
Echinodermata
Echinodermata
Mollusca
Echinodermata
Arthropoda
Arthropoda
Arthropoda
Echinodermata
Arthropoda
Cnidaria
Annelida
Echinodermata
Cnidaria
Chordata
Echinodermata
Echinodermata
Echinodermata
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Echinodermata
Arthropoda
Echinodermata
Cnidaria
Annelida
Chordata
Echinodermata
Echinodermata
Mollusca
Cnidaria
Arthropoda
Arthropoda
Echinodermata
Mollusca
Echinodermata
Echinodermata
Cnidaria
Annelida
Chordata
Echinodermata
Arthropoda
Echinodermata
Cnidaria
Arthropoda
Arthropoda
Polychaeta
Gastropoda
Stelleroidea
Malacostraca
Holothuroidea
Stelleroidea
Echinoidea
Cephalopoda
Echinoidea
Malacostraca
Malacostraca
Malacostraca
Stelleroidea
Malacostraca
Octocorallia
Polychaeta
Stelleroidea
Octocorallia
Ascidiacea
Stelleroidea
Stelleroidea
Echinoidea
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Stelleroidea
Malacostraca
Stelleroidea
Hexacorallia
Polychaeta
Ascidiacea
Stelleroidea
Echinoidea
Gastropoda
Octacorallia
Malacostraca
Malacostraca
Holothuroidea
Gastropoda
Stelleroidea
Stelleroidea
Octocorallia
Polychaeta
Ascidiacea
Echinoidea
Malacostraca
Echinodermata
Octacorallia
Malacostraca
Malacostraca
Sigalionidae
Buccinidae
x
Paguridae
Stichopodidae
Astropectinidae
Spatangidae
Loliginidae
Brissidae
Majidae
Portunidae
Portunidae
Ophiuridae
Paguridae
Pennatulidae
Nereididae
Astropectinidae
Alcyoniidae
Ascidiidae
Asteriidae
Astropectinidae
Brissidae
Portunidae
Majidae
Portunidae
Portunidae
Asteriidae
Paguridae
Astropectinidae
Actiniidae
Aphroditidae
x
Asteriidae
Brissidae
Buccinidae
Funiculinidae
Majidae
Portunidae
Synallactidae
Buccinidae
Ophiuridae
Ophiuridae
Alcyoniidae
Aphroditidae
x
Brissidae
Portunidae
Loveniidae
Funiculinidae
Majidae
Portunidae
56
Trawl
station
N4
N4
N4
N4
N4
N4
N4
N5
N5
N5
N5
N5
N5
N5
N5
N5
N5
N5
N5
N5
N6
N6
N6
N6
N6
N6
N6
N6
N7
N7
N7
N7
N7
N7
N7
N7
N7
N7
N7
N8
N8
N8
N8
N9
N9
N9
N9
N9
N9
N9
N9
Taxon
Phylum
Class
Family
Neptunea antiqua
Pagurus bernhardus
Pecten maximus
Polychaeta sp.
Polyphysia crassa
Ascidiacea sp.
Asterias rubens
Carcinus maenas
Hyas coarctatus
Ophiura albida
Pagurus bernhardus
Pisidia longicornis
Asterias rubens
Brissopsis lyrifera
Pagurus bernhardus
Aphrodita aculeata
Asterias rubens
Buccinum undatum
Neptunea antiqua
Pagurus bernhardus
Brissopsis lyrifera
Neptunea antiqua
Pagurus bernhardus
Actiniidae sp.
Aphrodita aculeata
Arctica islandica
Asterias rubens
Brissopsis lyrifera
Echinodermata
Mollusca
Arthropoda
Mollusca
Cnidaria
Annelida
Annelida
Mollusca
Chordata
Echinodermata
Echinodermata
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Echinodermata
Echinodermata
Arthropoda
Cnidaria
Arthropoda
Mollusca
Echinodermata
Echinodermata
Echinodermata
Arthropoda
Arthropoda
Echinodermata
Arthropoda
Mollusca
Annelida
Echinodermata
Echinodermata
Mollusca
Echinodermata
Arthropoda
Arthropoda
Echinodermata
Mollusca
Arthropoda
Echinodermata
Echinodermata
Mollusca
Arthropoda
Cnidaria
Annelida
Mollusca
Echinodermata
Echinodermata
Echinodermata
Arthropoda
Echinodermata
Stelleroidea
Gastropoda
Malacostraca
Bivalvia
Octocorallia
Polychaeta
Polychaeta
Cephalopoda
Ascidiacea
Stelleroidea
Stelleroidea
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Stelleroidea
Stelleroidea
Malacostraca
Octocorallia
Malacostraca
Cephalopoda
Stelleroidea
Stelleroidea
Echinoidea
Malacostraca
Malacostraca
Stelleroidea
Malacostraca
Cephalopoda
Polychaeta
Stelleroidea
Stelleroidea
Gastropoda
Echinoidea
Malacostraca
Malacostraca
Stelleroidea
Gastropoda
Malacostraca
Echinoidea
Stelleroidea
Gastropoda
Malacostraca
Hexacorallia
Polychaeta
Bivalvia
Stelleroidea
Stelleroidea
Echinoidea
Malacostraca
Stelleroidea
Asteriidae
Buccinidae
Paguridae
Pectinidae
Pennatulidae
x
Scalibregmidae
Loliginidae
x
Asteriidae
Astropectinidae
Portunidae
Majidae
Portunidae
Portunidae
Asteriidae
Ophiuridae
Paguridae
Pennatulidae
Porcellanidae
Loliginidae
Asteriidae
Astropectinidae
Brissidae
Portunidae
Portunidae
Asteriidae
Paguridae
Loliginidae
Aphroditidae
Asteriidae
Astropectinidae
Buccinidae
Loveniidae
Portunidae
Portunidae
Asteriidae
Buccinidae
Paguridae
Brissidae
Asteriidae
Buccinidae
Paguridae
Actiniidae
Aphroditidae
Arcticidae
Asteriidae
Astropectinidae
Brissidae
Portunidae
Asteriidae
57
Trawl
station
N9
N9
N9
N10
N10
N10
N10
N10
N10
N10
N10
N10
N10
N10
N10
N10
N10
N10
N10
N10
N11
N11
N11
N11
N11
N11
N11
N11
N11
N11
N11
N11
N11
N12
N12
N12
N12
N12
N12
N12
N12
N12
N12
N12
N13
N13
N13
N13
N13
N13
N13
Taxon
Phylum
Class
Family
Neptunea antiqua
Pagurus bernhardus
Pecten maximus
Actiniidae sp.
Aphrodita aculeata
Arctica islandica
Asterias rubens
Brissopsis lyrifera
Buccinum undatum
Crangon sp.
Echinidae sp.
Mycale (Mycale) lingua
Neptunea antiqua
Ophiura ophiura
Polyphysia crassa
Spatangus raschi
Asterias rubens
Brissopsis lyrifera
Buccinum undatum
Carcinus maenas
Ciona intestinalis
Gattyana cirrhosa
Luidia sarsi
Ophiura ophiura
Pagurus bernhardus
Pecten maximus
Spatangus purpureus
Arctica islandica
Asterias rubens
Brissopsis lyrifera
Carcinus maenas
Neptunea antiqua
Ophiura ophiura
Pagurus bernhardus
Pecten maximus
Alcyonium digitatum
Arctica islandica
Asterias rubens
Buccinum undatum
Carcinus maenas
Crepidula fornicata
Mollusca
Arthropoda
Mollusca
Cnidaria
Annelida
Mollusca
Echinodermata
Echinodermata
Mollusca
Arthropoda
Echinodermata
Echinodermata
Arthropoda
Porifera
Mollusca
Echinodermata
Echinodermata
Annelida
Mollusca
Echinodermata
Echinodermata
Echinodermata
Mollusca
Arthropoda
Chordata
Cnidaria
Annelida
Arthropoda
Echinodermata
Echinodermata
Arthropoda
Mollusca
Echinodermata
Mollusca
Mollusca
Echinodermata
Echinodermata
Echinodermata
Arthropoda
Arthropoda
Mollusca
Echinodermata
Arthropoda
Mollusca
Cnidaria
Mollusca
Echinodermata
Mollusca
Arthropoda
Mollusca
Echinodermata
Gastropoda
Malacostraca
Bivalvia
Hexacorallia
Polychaeta
Bivalvia
Stelleroidea
Echinoidea
Gastropoda
Malacostraca
Echinoidea
Echinoidea
Malacostraca
Demospongiae
Gastropoda
Stelleroidea
Holothuroidea
Polychaeta
Bivalvia
Echinoidea
Stelleroidea
Echinoidea
Gastropoda
Malacostraca
Ascidiacea
Octacorallia
Polychaeta
Malacostraca
Stelleroidea
Stelleroidea
Malacostraca
Bivalvia
Echinoidea
Cephalopoda
Bivalvia
Stelleroidea
Stelleroidea
Echinoidea
Malacostraca
Malacostraca
Gastropoda
Stelleroidea
Malacostraca
Bivalvia
Octocorallia
Bivalvia
Stelleroidea
Gastropoda
Malacostraca
Gastropoda
Echinoidea
Buccinidae
Paguridae
Pectinidae
Actiniidae
Aphroditidae
Arcticidae
Asteriidae
Brissidae
Buccinidae
Crangonidae
Echinidae
Loveniidae
Portunidae
Mycalidae
Buccinidae
Ophiuridae
Stichopodidae
Scalibregmidae
Pectinidae
Spatangidae
Asteriidae
Brissidae
Buccinidae
Portunidae
Cionidae
Funiculinidae
Polynoidae
Portunidae
Luidiidae
Ophiuridae
Paguridae
Pectinidae
Spatangidae
Loliginidae
Arcticidae
Asteriidae
Astropectinidae
Brissidae
Portunidae
Portunidae
Buccinidae
Ophiuridae
Paguridae
Pectinidae
Alcyoniidae
Arcticidae
Asteriidae
Buccinidae
Portunidae
Calyptraeidae
Loveniidae
58
Trawl
station
N13
N13
N13
N13
N13
N13
N13
N14
N14
N14
N14
N14
N14
N14
N14
N14
N14
N14
N15
N15
N15
N15
N15
N15
N15
N15
N15
N16
N16
N16
N16
N16
N16
N16
N16
Taxon
Phylum
Class
Family
Hyas araneus
Modiolus modiolus
Pagurus bernhardus
Pecten maximus
Pisidia longicornis
Actiniidae sp.
Brissopsis lyrifera
Hyas coarctatus
Lithodes maja
Mesothuria intestinalis
Pagurus bernhardus
Sepietta oweniana
Spatangus raschi
Brissopsis lyrifera
Geryon trispinosus
Hyas coarctatus
Lithodes maja
Loligo forbesi
Pasiphaea sp.
Sepietta oweniana
Brissopsis lyrifera
Geryon trispinosus
Hyas coarctatus
Munida rugosa
Sepietta oweniana
Arthropoda
Arthropoda
Echinodermata
Mollusca
Arthropoda
Mollusca
Arthropoda
Cnidaria
Echinodermata
Echinodermata
Arthropoda
Arthropoda
Arthropoda
Echinodermata
Arthropoda
Echinodermata
Mollusca
Echinodermata
Echinodermata
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Mollusca
Arthropoda
Mollusca
Mollusca
Echinodermata
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Arthropoda
Mollusca
Malacostraca
Malacostraca
Stelleroidea
Bivalvia
Malacostraca
Bivalvia
Malacostraca
Hexacorallia
Echinoidea
Echinoidea
Malacostraca
Malacostraca
Malacostraca
Holothuroidea
Malacostraca
Holothuroidea
Cephalopoda
Echinoidea
Echinoidea
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Cephalopoda
Malacostraca
Cephalopoda
Cephalopoda
Echinoidea
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Malacostraca
Cephalopoda
Majidae
Portunidae
Asteriidae
Mytilidae
Paguridae
Pectinidae
Porcellanidae
Actiniidae
Brissidae
Loveniidae
Majidae
Portunidae
Lithodidae
Synallactidae
Paguridae
Stichopodidae
Sepiolidae
Spatangidae
Brissidae
Geryonidae
Majidae
Portunidae
Portunidae
Lithodidae
Loliginidae
Pasiphaeidae
Sepiolidae
Loliginidae
Brissidae
Geryonidae
Majidae
Portunidae
Portunidae
Galatheidae
Sepiolidae
59
10 m ost com m on species in the by-catch (m 2) -Fish traw l
Alloteuthis subulata 40%
Loligo forbesi 14%
Asterias rubens 6%
Liocarcinus depurator 6%
Astropecten irregularis 6%
Liocarcinus holsatus 5%
Loliginidae fam 5%
Alcyonium digitatum 4%
Echinidae sp. 2%
Strongylocentrotus droebachiensis 2%
10 m ost com m on species in the by-catch (m 2) -Nephrops traw l
Spatangus raschi 30%
Brissopsis lyrifera 29%
Hyas sp. 4%
Asterias rubens 3%
Echinidae sp. 2%
Pagurus bernhardus 1%
Pseudamussium peslutrae 1%
Carcinus maenas 1%
10 m ost com m on species in the by-catch (m 2) -Shrim p traw l
Crangon sp. 21%
Munida sp. 12%
Munida sarsi 9%
Pandalus propinquus 5%
Syscenus infelix 4%
Asteronyx loveni 2%
Funiculina quadrangularis 2%
Euphasia spp. 1%
Munida tenuimana 1%
Figure 13. The 10 most common species of the NCI by-catch, based on individual
abundance/m2. Fish trawling efforts, Nephrops trawling efforts, Shrimp trawling efforts
60
10 species w ith highest biom ass (m 2) in the by-catch -Fish traw l
Loligo forbesi 60%
Asterias rubens 7%
Alcyonium digitatum 4%
Alloteuthis subulata 3%
Echinidae sp. 2%
Actiniidae sp. 2%
Buccinum undatum 2%
10 species w ith highest biom ass (m 2) in the by-catch- Nephrops traw l
Hyas sp. 18%
Spatangus raschi 8%
Parastichopus tremulus 6%
Asterias rubens 5%
Echinidae sp. 2%
Pseudamussium peslutrae 1%
10 species w ith highest biom ass (m 2) in the by-catch -Shrim p traw l
Munida sp. 16%
Munida sarsi 14%
Todaropsis eblanae 9%
Crangon sp. 6%
Funiculina quadrangularis 4%
Spatangus purpureus 4%
Asteronyx loveni 3%
Munida tenuimana 1%
Figure 14. The 10 species accounting for the highest biomass in the NCI by-catch.
Fish trawling efforts, Nephrops trawling efforts, Shrimp trawling efforts.
61
Figure 15. Presentation of the main components and the function of an otter trawl.
62
Figure 16. Construction of the 36/47 GOV Trawl used at U/F Argos
Source:
MANUAL FOR THE INTERNATIONAL BOTTOM TRAWL SURVEYS
REVISION VII
The International Bottom Trawl Survey Working Group
63
Figure 17. “Exocet kit” for the 36/47 GOV Trawl used at U/F Argos
Source:
REVISION VII
64
Figure 18. Rigging of the 36/47 GOV Trawl used at U/F Argos
Source:
REVISION VII
65
Figure 19. Construction of the Nephrops trawl used at U/F Ancylus
66
Figure 20. This construction represents the type of trawl used by the commercial shrimp
trawling vessel.
Source: Kristiansands Fiskegarnsfabrik A/S, Norway
67

By-catches of non-commercial invertebrate taxa in Skagerrak

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