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CSG 15
DEPARTMENT for ENVIRONMENT, FOOD and RURAL AFFAIRS
Research and Development
Final Project Report
(Not to be used for LINK projects)
Two hard copies of this form should be returned to:
Research Policy and International Division, Final Reports Unit
DEFRA, Area 301
Cromwell House, Dean Stanley Street, London, SW1P 3JH.
An electronic version should be e-mailed to [email protected]
Project title
Can demersal fishing cause long-term changes in benthic community
structure
DEFRA project code
MF0716
Contractor organisation
and location
CEFAS Conwy Laboratory (04/ 1997- 01/ 1999:
contract leader MJ Kaiser)
CEFAS Lowestoft Laboratory (01/ 1999- 03/ 2002:
contract leader S Jennings)
Total DEFRA project costs
Project start date
£
01/04/97
Project end date
31/03/02
Executive summary (maximum 2 sides A4)
Can demersal fishing cause long-term changes in benthic community structure?
Demersal fishing is a source of chronic and widespread disturbance in the marine environment. Concerns
about the effects of fishing on the seabed have been raised at national and European levels, and DEFRA
requires supporting science in order to assess whether these effects are sustainable. The project ‘Can
demersal fishing cause long-term changes in benthic community structure’ was commissioned to provide the
scientific basis for assessing the impacts of demersal fishing disturbance on the marine environment.
In this project, the impacts of demersal fishing were assessed at large spatial scales on real fishing grounds in
the North Sea, Irish Sea and English Channel. This approach provided many advantages over the small-scale
experimental approach, since the results of small-scale experiments could not be reliably extrapolated to the
larger scales on which demersal fishing impacts occurred in real fisheries.
Large-scale comparisons between areas subject to different levels of demersal fishing disturbance showed
that trawling and scallop-dredging disturbance has altered benthic community structure in the North and Celtic
Seas. In areas subject to low levels of natural disturbance, benthic communities in infrequently fished areas
were dominated by relatively sessile, emergent, high biomass species, while communities in heavily fished
areas were dominated by smaller free-living infaunal taxa. In areas subject to higher levels of natural
disturbance, frequent demersal fishing disturbance produced communities dominated by small polychaetes
CSG 15 (9/01)
1
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
DEFRA
project code
MF0716
and bivalves rather than large bivalves. These shifts in community structure led to reduced benthic production
and biomass in heavily fished areas. However, in the North Sea, the reduction in biomass and production
mainly occurred in size and species groups that were not the main prey of the commercial fishes feeding (sole
and plaice) on the trawling grounds. For most of the smaller infaunal invertebrates eaten by these fishes,
trawling disturbance had no positive or negative effect on production, and the long-term increases in the
biomass of small benthic infauna that have been observed in the North Sea were attributed to the effects of
climate change rather than the long-term effects of trawling disturbance.
The vulnerability of benthic animals to trawling disturbance is size and species dependent. For infaunal
species, large body size was a good predictor of vulnerability, and hence larger bivalves and burrowing
urchins are often locally extirpated in heavily trawled areas. In the North Sea, the frequency of bottom trawling
disturbance had a greater effect on the size structure of the benthic infaunal community than other
environmental variables such as sediment particle size and depth. A size-based model of the vulnerability of
animals to trawling disturbance showed that many larger individuals and species could be extirpated at the
mean levels of trawling effort observed in some southern and central North Sea beam trawling grounds. Since
empirical data show that trawling disturbance has led to reductions in the abundance of many species, but has
not extirpated them, trawling disturbance in real fisheries may be sufficiently patchy to provide refuges that are
unimpacted or infrequently impacted by trawling.
Studies of the impacts of demersal fishing on the dog cockle Glycymeris glycymeris and starfish Asteries
rubens in the Irish Sea, showed that it may be possible to use shell scarring in bivalves or arm damage in
starfish to differentiate between heavily and lightly fished areas of the sea bed. However, the results
suggested that it would be difficult to refine the technique further so that more precise estimates of fishing
effort can be made. The main problem is the difficulty of distinguishing between shell scars caused by fishing
and those caused by natural disturbance. Studies in the North Sea showed that correlations between the
frequency of occurrence of trawl tracks on the seabed (as determined from a sidescan sonar survey) and
estimates of trawling disturbance calculated as the number of sightings of trawlers per unit of search effort by
fishery protection aircraft were poor due to the variable and unknown persistence of trawl tracks on different
sediment types.
While chronic trawling disturbance in the North Sea has led to dramatic reductions in the biomass of infauna
and epifauna, these reductions have not led to marked changes to the mean trophic level of the community, or
in the relationships between the trophic levels of different size classes of epifauna. The trophic structure of
intensively trawled benthic invertebrate communities may be less responsive than community size-structure to
trawling disturbance, thus ensuring the efficient processing of production within those animals that have
sufficiently high population growth rates to withstand the levels of mortality imposed by trawling.
A modelling approach was used to assess the potential effects of large scale temporary area closures (such
as the cod box) on benthic communities. In the case of the cod box, the temporary closure led to the
displacement of trawling effort to areas that were previously unfished or lightly fished and, over the course of
the year, trawling disturbance was more homogeneously distributed than previously. We developed a model of
the impacts of disturbance on benthic community structure and showed that a given level of trawling
disturbance, when distributed homogeneously, would have led to greater reductions in the biomass and
production of the benthic community and the loss of vulnerable species. However, the model could not be
parameterised with real effort data because access to the satellite vessel monitoring data was still being
negotiated at the end of the project.
CSG 15 (9/01)
2
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
DEFRA
project code
MF0716
Scientific report (maximum 20 sides A4)
Can demersal fishing cause long-term changes in benthic community structure
Contents:
1. General Introduction
2. Scientific Objectives
3. Summary of research results in relation to Scientific Objectives
4. Chronic historical fishing disturbance has changed shelf sea benthic community structure
5. Impacts of trawling disturbance on the biomass and production of North Sea benthic communities: with a focus on larger
macrobenthic animals
6. Impacts of trawling disturbance on structure of North Sea benthic communities: with a focus on smaller macrobenthic
animals
7. The effects of different intensities of fishing disturbance on the trophic structure of benthic communities in the North Sea
8. A preliminary assessment of the utility of vessel sightings data (recorded during fishery protection overflights) as a
means of estimating levels of fishing disturbance
9. The effects of the North Sea Cod Recovery Plan on fishing effort distributions and benthic communities
10. Future Work
11. References
12. List of Publications
1.
General Introduction.
Demersal fishing is a source of chronic and widepread disturbance in the marine environment, and DEFRA are increasingly asked to
advise on the impacts of demersal fishing. Concerns about the effects of fishing on the seabed have been raised at national and
European levels, and DEFRA requires a scientific basis on which to assess whether these effects are sustainable. The project ‘Can
demersal fishing cause long-term changes in benthic community structure’ was intended to provide the scientific basis for assessing
whether the impacts of demersal fishing disturbance had long-term deleterious impacts on the marine environment.
The original policy drivers for this research were the Intermediate Ministerial Meeting (IMM, Bergen), the Oslo and Paris Convention
(OSPAR, Sintra), the report of the North Sea Task Force (NSTF), the International Convention on Biological Diversity (Rio,
implemented as UK Biodiversity Action Plan), the FAO Code of conduct for responsible fisheries and the concerns of NGOs,
Conservationists and Fishing Organisations about the impacts of fishing on the marine environment. However, in the course of the
project, the impacts of fishing on the environment became an issue of increasing political concern, and there were many more policy
developments of relevance, such as the EC Green Paper on Future of the Common Fisheries Policy (2001), the formation of the ICES
Advisory Committee for Ecosystems (established 2000), the FSCG paper on Ecosystem Approach to Fisheries Management, the 5th
International Conference on Protection of the North Sea (Bergen 2002), the FAO Reykjavik Conference on Sustainable Fisheries in the
Marine Ecosystem (2001) and the EC paper on the Integration of Environmental Concerns and Sustainable Development into the
Common Fisheries Policy (2001). The research results from MF0716 have been used extensively when providing advice to DEFRA
and other bodies on the impacts of fishing on the marine environment.
2.
Scientific Objectives
The scientific objectives of the project ‘Can demersal fishing cause long-term changes in benthic community structure’ were as follows:
1. To study the effects of different intensities and frequencies of fishing disturbance on different benthic communities in the North and
Celtic Seas (completed on time and in full- see Sections 4,5,6,7)
2. To evaluate which species are most vulnerable to fishing disturbance and relate their abundance to its intensity (completed on time
and in full- see Sections5,6,9)
3. To quantify historical changes in benthic communities by studying areas where historical datasets exist (completed on time and in
full- see Section 4)
4. To identify and evaluate potential species which can be used as direct, fisheries data independent indicators of fishing disturbance
(completed on time and in full- see Section 4 and the final report for DEFRA project MF0714- University of Wales, Bangor)
5. To determine the effects of different intensities of fishing disturbance on the trophic structure of benthic communities in the North
Sea (completed on time and in full- see Section 7)
6. To make a preliminary assessment of the utility of vessel sightings data (recorded during fishery protection overflights) as a means
of estimating levels of fishing disturbance (completed on time and in full- see Section 8)
7. To assess the effects of the North Sea Cod Recovery Plan on fishing effort distributions and benthic communities (completed on
time and in full subject to the restrictions imposed on the use of satellite data- see Section 9)
Objectives 1-4 were the objectives stated in the original CSG7 for MF0716 (1 April 1997 to 31 March 2000), while objectives 5-7 were
added when the project was extended (with the prior agreement of CSG, 1 April 2001 to 31 March 2002).
CSG 15 (9/01)
3
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
Since this project report is limited to a total length of 20 pages, and over 20 full published papers have resulted from the project, we
have reported a selection of our results. The results of the research programmes are reported under general titles, as most research
programmes were intended to provide science in support of two or more objectives. We have appended a full list of publications for
those who require further detail. This more detailed study of the work conducted in preceded by a short summary of the scientific
conclusions that relate to each objective (Section 3).
3. Summary of research results in relation to Scientific Objectives
This short summary of the results that relate to each objective provides a guide to the main conclusions from this project and the report
sections and papers in which the key supporting science has been published (papers are listed in Section 12 ‘List of Publications’).
Objective 1. To study the effects of different intensities and frequencies of fishing disturbance on different benthic
communities in the North and Celtic Seas
The effects of different intensities of fishing disturbance on benthic communities were studied in the Irish and North Seas. Fishing
disturbance was primarily attributed to dredges in the area of the Irish Sea that was studied and to beam trawls in the North Sea. The
intensity of fishing disturbance due to dredges was estimated with fishers logbook records and biological indicators (see '4' below)
while fishing disturbance due to beam trawlers was assessed using fishery protection overflight data.
In the Irish Sea, a spatial comparison of benthic communities at sites subject to low and high levels dredging disturbance around the
Isle of Man demonstrated that chronic disturbance had modified benthic community structure. At infrequently dredged sites, the
benthic communties were dominated by relatively sessile, emergent and high biomass species while at the intensively dredged sites,
the communities were dominated by smaller infauna and there were few 'habitat forming' species to create topographic complexity.
In the central North Sea, a spatial comparison of sites subject to different levels of beam trawling disturbance demonstrated that
several species of infauna had been extirpated at the heavily trawled sites and benthic communities in the most heavily disturbed areas
were increasingly dominated by smaller infauna. Preliminary studies of the relationship between these structural changes and changes
in function of the North Sea benthic community suggested that beam trawling disturbance had limited impacts on the trophic structure
of the benthic community but dramatically reduced productivity.
Since we assessed the impacts of different gears fished in different habitats in the Irish and North Seas, it was not possible to make a
direct comparison of the relative impacts of the gears on benthic community structure. However, in general terms, the changes in the
community that followed intensive disturbance were remarkably similar, with the fauna in the intensively fished areas increasingly
dominated by small free-living species.
Report sections: 4, 5, 6, 7
Key Publications: Kaiser et al. (1998a, b, 1999, 2000), Jennings et al. (2001a, b).
Objective 2. To evaluate which species are most vulnerable to fishing disturbance and relate their abundance to its intensity
Comparisons of benthic communities at North Sea sites subject to different levels of trawling disturbance demonstrated that large body
size was the main factor that made benthic infaunal species vulnerable to fishing disturbance. This is consistent with the idea that
species with larger body size have lower intrinsic rates of natural increase and can withstand lower rates of mortality than smaller
species. Examples of those species that are particularly vulnerable to disturbance would be the quahog Arctica islandica and several
species of burrowing sea urchin. The differential vulnerability of benthic species, rather than intra- and inter-specific effects, was the
primary cause of changes in benthic community structure following trawling.
The effects of trawling on aspects of community structure can be predicted if trawling frequency and size-related vulnerability are
known. Large body size is the best predictor of vulnerability. In the North Sea, we showed that the frequency of bottom trawling
disturbance had a greater effect on the size structure of the fauna in a soft-sediment benthic community than other environmental
variables such as sediment particle size and depth. We modelled the vulnerability of animals to trawling disturbance and, in accordance
with empirical work, showed that many larger species would be extirpated at the mean levels of trawling effort observed in North Sea
beam trawl fisheries. Since empirical data show that many larger species persist at the North Sea scale, we suggest (i) that trawling
disturbance in real fisheries is sufficiently patchy to provide refuges that are unimpacted or infrequently impacted by trawling and (ii)
that some of the largest bivalves may burrow so far into the sediment that trawling induced mortality is reduced.
Report sections: 5, 6, 9
Key Publications: Duplisea et al. (2002), Jennings et al. (2001b), Kaiser et al (2000), Schratzberger et al. (2001).
Objective 3. To quantify historical changes in benthic communities by studying areas where historical datasets exist
The benthic fauna at sites in the English Channel was sampled by Norman Holme in the 1950s and 1960s, prior to the widespread use
of heavy bottom fishing gears such as beam trawls and scallop dredges. Holme's sites were resurveyed in 1998 with similar sampling
4
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
gears to those used in the 1950s and 1960s, to determine whether there had been significant changes in the structure of the benthic
community following the introduction of heavy bottom fishing gears. There were significant changes in the infaunal and epifaunal
communities at Holme's sites. However, it was not possible to link these changes to the impacts of fishing because appropriate data on
the types and distribution of fishing effort were not available.
Report sections: 4
Publications: Kaiser and Spence (2000), Kaiser et al. (2000)
Objective 4. To identify and evaluate potential species which can be used as direct, fisheries data independent indicators of
fishing disturbance
Species that could be used as indicators of fishing disturbance were identified in this project and the associated project MF0714
(University of Wales, Bangor). Starfish and dog cockles were identified as useful indicators of fishing disturbance, because encounters
with fishing gears resulted in arm loss and shell-scarring respectively. The relationship between arm loss in starfish and fishing intensity
was relatively weak and obscured by considerable variance. While counts of lost starfish arms could be used to distinguish 'lightly' and
'heavily' fished areas, they could not be used to provide quantitative estimates of disturbance. The frequency of shell-scarring in dog
cockles also helped to distinguish between 'lightly' and 'heavily' fished areas. However, the main problem was the difficulty of
distinguishing between shell scars caused by fishing and those caused by natural disturbances. With the advent of satellite monitoring
of the main trawling fleets in the North and Irish Seas, further identification of fisheries data independent indicators of fishing
disturbance is unlikely to be needed
Report sections: 4, and covered in detail by M.J. Kaiser in final report for MF0714
Publications: Ramsay et al. (1999, 2000a,b)
Objective 5. To determine the effects of different intensities of fishing disturbance on the trophic structure of benthic
communities in the North Sea
Chronic trawling disturbance in the North Sea has led to dramatic reductions in the biomass of infauna and epifauna, but these
reductions were not reflected in changes to the mean trophic level of the community, or the relationships between the trophic levels of
different sizes of epifauna. The trophic structure of intensively trawled benthic invertebrate communities may be a robust feature of the
North Sea ecosystem, thus ensuring the efficient processing of production within those animals that have sufficiently high intrinsic rates
of population increase to withstand the levels of mortality imposed by trawling.
Report section: 7
Publications: Jennings et al. (2001a, 2002).
Objective 6. To make a preliminary assessment of the utility of vessel sightings data (recorded during fishery protection
overflights) as a means of estimating levels of fishing disturbance
Studies in the North Sea showed that correlations between the frequency of occurrence of trawl tracks on the seabed (as determined
from a sidescan sonar survey) and estimates of trawling disturbance calculated as the number of sightings of trawlers per unit of
search effort by fishery protection aircraft were poor. This was due to the variable and unknown persistence of trawl tracks on different
sediment types.
Report section: 8
Publications: Jennings et al. (2000, 2001a).
Objective 7. To assess the effects of the North Sea Cod Recovery Plan on fishing effort distributions and benthic
communities
A large area of the North Sea was temporarily closed to fishing as part of the North Sea cod recovery plan. This led to the displacement
of trawling effort to areas that were previously unfished or lightly fished, and trawling disturbance was more homogeneously distributed
than in previous years. We developed a model of the impacts of disturbance on benthic community structure and showed that a given
level of trawling disturbance, when distributed homogeneously, would have led to greater reductions in the biomass and production of
the benthic community and the loss of vulnerable species. However, the model could not be parameterised with real effort data
because the SFI are not currently permitting the use of satellite effort data for scientific purposes.
Report section: 9
Publications: Duplisea et al. (2002)
5
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
4. Chronic historical fishing disturbance has changed shelf sea benthic community structure
(Research in support of Objectives 1-4) (M.J. Kaiser, K. Ramsay, C.A. Richardson, F.E. Spence and A.R. Brand)
4.1. Introduction
Bottom fishing using towed nets and dredges is a widespread source of physical disturbance to the continental shelf seas throughout
the world. Consequently, it has been suggested that habitat degradation and ecosystem changes have occurred in intensively fished
areas. However it has been difficult to attribute these changes to fishing effort at a spatial scale that is truly representative of
commercial fishing activities. We compared benthic fauna found in Irish Sea fishing grounds that have been exposed to either high or
low levels of bottom fishing disturbance over the past 10 years. We were able to validate the fishing effort data in some areas from the
occurrence of scars that result from fishing disturbance in the shells of a long-lived bivalve mollusc (see Objective 2). Multivariate
analyses and the response of abundance/biomass curves indicated that chronic fishing has caused a shift from communities
dominated by relatively sessile, emergent, high biomass species to communities dominated by infaunal smaller-bodied fauna.
Removal of emergent fauna has thus degraded the topographic complexity of seabed habitats in areas of high fishing effort. The
communities within these areas currently may be in an alternative stable state.
4.2. Methods
Study locations
We based the present study around the Isle of Man, UK that is the centre of one of the most heavily fished scallop fisheries in Europe.
Fishing effort data has been collected for over ten years for one third of the scallop dredging fleet via logbooks maintained by the
scallop fishers. Fishers are paid to record their effort per 5 x 5 nautical mile box that is subsequently multiplied up to give total effort for
the fleet. We selected five areas subjected to high (47526±14464 m.h.y -1) and five subjected to low (9335±5416 m.h.y-1) fishing effort
based on data collected between 1986-1996 (mean ± 95% CI metres of dredge width deployed x hours fished per 10 y). As expected,
mean effort was significantly different for these areas (t-test, t = -6.29, P<0.002).
Environmental and biological samples
Within each of the ten areas we collected three Day grab samples for sediment particle-size and organic content analysis and three
qualitative, but comparable, infaunal samples using an anchor dredge deployed for 1 min. on the seabed. We used an anchor dredge
as it samples less common large infauna more effectively than smaller grabs and deep corers that would not work in coarse sediment.
Collected sediments were sieved over a 10 mm wire mesh. In addition, three quantitative samples of the epifauna were collected from
each site using a 2-m wide beam trawl towed for a standard duration of 5 min. Small beam trawls are particularly effective for sampling
less common emergent high-biomass epibiota that might be expected to be most vulnerable to the impact of bottom fishing activities.
At sea, large and easily identified species were counted and weighed using motion compensated balances. Shell fragments and stone
material were also sorted from catches and weighed separately as these may provide information about gross habitat characteristics.
In the laboratory faunal samples were sorted and identified to species level whenever possible and the total number of each species
and their biomass (g of flesh dried to constant weight) quantified. Data collected with the beam trawl were standardised to numbers or
biomass 1000 m-2 ascertained by calculating the area of seabed sampled based on the shot and hauled positions of the net determined
from a differential global positioning system on-board ship.
Determination of disturbance history
Where they occurred, bivalves were separated from the samples for analysis of shell scars in the laboratory. However, the long-lived
(>30 yrs) bivalve Glycymeris glycymeris was abundant at only one of the high intensity and three of the low intensity fishing sites.
These animals were collected from the dredge and trawl samples outlined above and their shells examined microscopically in the
laboratory for the incidence of scars caused by fishing activity. Each shell was sectioned from the umbone to the middle of the
posterior margin of the shell. Each of these sections was then polished using fine glass paper and etched in 0.1 N Hydrochloric acid.
Subsequently acetate peels were made of the etched face of the shell cross-section and examined for the incidence of damage and
scarring (see Witbaard & Klein 1994 and Gaspar et al. 1994 for more details). Witbaard and Klein (1994) identified scars that had been
caused by fishing activity from the occurrence of sand grains within the shell matrix that become lodged between the animal’s mantle
tissue and the growing edge of the shell at the time of damage (see also Gaspar et al. 1994). As in other bivalve species, G.
glycymeris record annual growth rings in their shell matrix, hence for each individual bivalve it is possible to determine the frequency of
scarring for the period for which fishing effort data were extracted. Thus bivalve shells have the potential to provide a historical record
of disturbance of the precise piece of seabed sampled. Although G. glycymeris occurred in only 40% of the seabed areas sampled, the
information that they provide at least provides an indication of the validity or otherwise of the fishing effort data. The relationship
between the yearly incidence of shell scars and fishing effort was investigated using regression analysis.
Data analyses
The data collected with the anchor dredge and beam trawl were analysed separately as they were considered to sample distinctly
different components of the benthic community. Anchor dredges are particularly effective at collecting large samples of infauna that
might be considered to be less vulnerable to fishing disturbance than the surface dwelling epifauna that were sampled most effectively
with the beam trawl. The analyses of community data were undertaken using the PRIMER software package (Clarke and Warwick,
1994) and a general linear model with fishing effort and habitat set as the fixed factors.
6
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
4.3. Results
Environmental and biological samples
Multivariate analyses revealed almost identical results for the biomass and abundance data collected from either the anchor dredge or
2-m beam trawl, hence hereafter we will refer mainly to the biomass data (which includes data for the emergent colonial organisms)
and only refer to the abundance data when differences occurred. Cluster analyses revealed that the first dichotomy in the anchor
dredge and 2-m beam trawl data was related to habitat differences as revealed using the BIOENV procedure. This split in the data was
best explained by sediment composition (either gravel or coarse sand habitats) and depth at the different sites for both the anchor
dredge and 2-m beam trawl data (anchor dredge data, R = 0.32, P < 0.001; 2-m beam trawl data, R = 0.49, P < 0.001).
Before undertaking an analysis to test for the effects of fishing disturbance, it was necessary to account for this habitat effect in a
twoway analysis of similarity matrices using the ANOSIM procedure. These analyses revealed that both the habitat and fishing effort
factors were significant for the abundance (ANOSIM, R=0.24, P<0.005) and biomass (R=0.34, P<0.001) data for the 2-m beam trawl
(epifauna) samples. Although similar differences were apparent for the biomass data for the anchor dredge (infauna) samples (R=0.16,
P<0.05) they were not apparent for the abundance data (R=0.09, P>0.05). This suggests that while a similar density of infauna may
occur at both fishing intensities, the average size of the infaunal organisms in the heavily fished area is significantly smaller.
Analysis of the summary data (total number of species, individuals, and the Hill’s N1 and N2 diversity indices) using a GLM revealed no
significant differences for either habitat or fishing effort effects, except for the total number of individuals for the epifauna data for which
there was a significant effect of fishing effort (high intensity, 4856±4666; low intensity, 2312±1696 mean±95% CI no.1000 m -2; F1,26
=5.1, P<0.03).
K-dominance curves were then plotted for the biomass and abundance data for both infaunal and epifaunal samples. In both cases, the
plot for the low fishing effort areas shows that the biomass curve lies above the abundance curve. These curves tend to converge in
the high fishing effort areas which indicates increased levels of physical stress. The K-dominance plots suggest that the communities
in the heavily fished areas are dominated by higher abundances of smaller bodied organisms whereas the less intensely fished areas
are dominated by fewer larger bodied biota. In order to determine which species accounted for the differences between the high and
low intensity fishing areas we analysed the biomass data for the epi- and infauna using the SIMPER procedure (Clarke & Warwick
1994). The SIMPER procedure computes the average dissimilarity between all pairs of inter-group samples (i.e. samples collected from
low versus high intensity fishing areas). This average is then broken down into the separate contributions from each species to the
dissimilarity between each group. Interestingly, this analysis revealed that the biomass of the emergent soft coral Alcyonium digitatum,
the large sea urchin Echinus esculensis, the bivalve G. glycymeris and the gastropod Buccinum undatum was lowest in the areas of
high fishing effort. In contrast, the biomass of brittlestars such as Ophiura albida and Ophiocomina nigra was highest in these areas.
Determination of disturbance history
As predicted the Glycymeris glycymeris collected from the most heavily fished area had a significantly higher incidence of trawl
damage scars per annual growth ring than those collected from the three less intensively fished areas. Although this corroborates the
fishing effort for only a proportion of our study sites, this data provides us with tangible proof of physical disturbance intensity for the
actual pieces of seabed sampled in this study as the bivalves came from the same samples from which the community data were
analysed.
4.4. Discussion
The results of this study provide compelling evidence that chronic fishing (scallop dredging) disturbance has led to detectable changes
in benthic community structure in several different habitats in the Irish Sea. It would be misleading to attribute too much emphasis to
the fact that this is a scallop dredge fishery as any of the large bottom fishing gears such as rock-hopper otter trawls and beam trawls
will have similar effects. Nevertheless, scallop dredges and rock-hopper otter trawls are often fished in areas that support more diverse
and sensitive communities than areas in which beam trawls are operated i.e. stable gravel habitats c.f. shallow sandy substrata
Previous attempts to compare areas subjected to different levels of commercial bottom fishing disturbance have had to assume that the
exact area of seabed sampled is representative of the average disturbance regime in that area. Unlike previous studies, our
examination of shell scars provides confirmation that at least 40% of our samples of the benthic fauna were collected from areas of the
seabed that have been physically disturbed at known different intensities over many years by fishing gear. Large bivalve species, such
as Arctica islandica and G. glycymeris, are relatively sedentary and therefore remain in situ for the majority of their life and their shells
provide us with a historical record of the physical disturbance regime applied to the sediment in which they lived. Our use of shell scars
as an index of relative fishing disturbance has provided us with independent evidence of fishing effort intensity that has been lacking in
previous studies. Ideally, we would have collected G. glycymeris from each of the areas studied and one might speculate that their
absence from the majority of high fishing effort areas is indicative of this activity. Although this represents a draw-back to using this
technique to corroborate fishing effort data we propose that it is the only method that gives a spatially accurate description of
disturbance history.
The benthic communities sampled in this study show responses to chronic disturbance that concur with theoretical predictions
(Lambshead et al., 1983). The abundance-biomass curves for both infaunal and epifaunal samples (Fig. 3) tend to converge in the
high fishing intensity areas where physical disturbance is greatest, whereas the biomass curve lies well above the abundance curve in
the areas that are less intensively disturbed. This indicates that relatively large bodied fauna have been removed by repeated bottom
fishing such that these benthic communities are now dominated by smaller bodied organisms that are presumably less susceptible to
physical disturbance. Fauna such as sessile soft corals and sea urchins that have a fragile test and long-lived bivalves and gastropods
are the most severely affected fauna. Although gastropods might appear to be protected from the effects of physical damage by their
7
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
robust shell, laboratory and field studies have demonstrated that they experience increased predation mortality from starfish as a result
of direct contact with towed fishing gear (Ramsay & Kaiser 1998). Furthermore, the egg masses laid by these gastropods are also
vulnerable to bottom trawling as they are attached to the substratum. The benthic communities in the heavily fished areas tend to be
dominated by fauna that are resilient to physical damage, either through life-history adaptations or their ability to regenerate damaged
body-parts (e.g. starfishes and hermit crabs). Collie et al. (1997) also reported that scavenging organisms tended to dominate
communities found in areas that experienced heavy dredging disturbance. Thus communities dominated by scavenging fauna may be
indicative of areas that experience high levels of physical disturbance.
Our study leaves us in no doubt that in certain areas bottom fishing with towed gears has led to wide-scale changes in benthic habitats
and communities. There now exists a growing body of literature that indicates the pressing need for fisheries management to be
applied at both the species and habitat level (Hall, 1999; Kaiser & De Groot 1999). We conclude that fisheries managers will need to
consider seriously the exclusion of towed bottom fishing gears from some areas of the seabed that support structurally complex
habitats and benthic fauna that may have an important functional role for both non-commercial and commercially important species.
5. Impacts of trawling disturbance on the biomass and production of North Sea benthic communities: with a focus on larger
macrobenthic animals (Research in support of Objectives 1 and 2) (S. Jennings, D.E.Duplisea, T.A. Dinmore, K.J. Warr)
5.1 Introduction
Trawling disturbance is well known to affect the species composition and structure of marine benthic communities, but virtually nothing
is known of the effects of trawling disturbance on size structure and production. We might reasonably expect trawling disturbance to
affect size structure and production, because mortality due to trawling disturbance is positively correlated with body size (Bergman &
van Santbrink, 2000) and because there are close relationships between body size distributions and production (Brey, 1999).
Body size has fundamental significance in determining the function of communities since it determines potential predators and prey,
and is correlated with many aspects of life history (Peters 1983). The distribution of biomass by body size classes in aquatic
ecosystems follows regular patterns that can be described with size spectra and the patterns can be predicted from models of energy
flow from prey to predators. Size spectra have been used to describe the structure of fish and benthic communities and the slope of the
spectrum can provide a broad indication of the intensity of exploitation (Dickie and Kerr, 2000).
The body size of benthic invertebrates may determine their response to trawling disturbance. Trawling causes mortality of many
species because they are crushed directly by the trawl or get caught and have died by the time they are taken on deck and returned to
the sea. Within and among species, mortality is generally size dependent. Thus larger bivalves suffer very high mortality while smaller
bivalves and polychaetes may suffer lower mortality (Bergman & van Santbrink 2000), often because lighter animals are pushed aside
by the pressure wave in front of the net (Gilkinson et al. 1998). Not only are larger individuals likely to suffer higher mortality, but the
mortality rates they can withstand will be lower. This suggests that intensive trawling may favour smaller species and, since these
have higher P:B ratios, they may be more productive and compensate for the loss of production among larger species.
Long-term studies of the benthos in the southern and central North Sea suggest that biomass and production may have increased
(Kröncke et al. 1998). This could be a response to trawling disturbance, climate change or eutrophication (Rijnsdorp & van Leeuwen
1996; Kröncke et al. 1998). Increases in benthic production have been linked to increases in the growth of flatfishes. To some, these
studies have suggested that trawling disturbance is 'farming the sea'; ploughing the seabed to boost production. To others, trawling is
assumed to damage key functional processes. Remarkably, there have been no attempts to look at the effects of trawling on
production.
We compared the size composition and production of benthic invertebrate communities across quantified gradients of trawling
disturbance. Changes in size composition are described using size spectra, and production is predicted from the size spectra using
allometric relationships between body size and the P:B ratio. We test the hypothesis that larger organisms decline in response to
trawling disturbance while smaller ones proliferate. If this hypothesis is correct, then both the slope and intercept of the size spectra
would be positively correlated with trawling disturbance and total production of the community will rise if the increased production of
smaller animals exceeds the loss of production in depleted populations of larger animals. If the hypothesis is invalid, then trawling
disturbance would be positively correlated with the slope of the size spectra, but not with the intercept and the overall production of the
community will fall.
5.2. Methods
Effects of trawling disturbance and environment on benthic community size-structure
We assessed the effects of trawling disturbance and environmental factors on the size structure of the benthic community at 7 sites of
1*1 nautical mile in the Silver Pit, an important beam trawling ground in the central North Sea.
The infaunal invertebrate community at each site was sampled with an anchor dredge during winter and summer. The anchor dredge
samples on an appropriate scale for the study of fishing effects (over areas of m 2) and integrates small scale patchiness of the larger
macrofauna (individual body mass >0.0625g). Three randomly located replicate tows were completed at each site in each season from
the research vessel 'Corystes'. Winter samples were collected from 22 November to 8 December 1999 and summer samples from 1-13
May 2000. A sub-sample of 0.2m3 of sediment was taken from each anchor dredge sample and sieved through 1mm square mesh. All
free-living infaunal species retained by the mesh were removed for processing. All individuals were weighed, either fresh using heave
compensated balances or after preservation in 4% seawater formalin buffered with 3 g l-1 sodium acetate and were assigned to log2
8
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
body size categories. Biomass by size class was reported as the mean for the six replicates at each site. Biomass size spectra were
normalised by dividing the biomass in a given body mass class interval by the width of that class interval.
Trawling disturbance at each site was estimated from records of vessel sightings by fishery protection aircraft as described by
Jennings et al. (2001a), and reported as sightings of beam trawlers per unit of searching effort (SPUE). The mean depth at each site
was calculated from 6 measurements taken at random locations during the deployment of the anchor dredge. Five randomly located
sediment cores were taken at each site, and the sediment and overlying water frozen for subsequent particle size analysis. Particle
size analysis was conducted with a laser particle sizer and sediment characteristics were reported as means by site.
Multidimensional scaling (MDS) was used to investigate the biotic relationships between sites. In this analysis, we treated each of the
infaunal log2 body mass classes from -4 to 7 as if it were a species in a conventional community analysis (Schwinghamer, 1988), and
produced a similarity matrix for the 7 sites using a Bray-Curtis similarity measure.
To examine the relationships between the size structure of the infaunal community, depth, trawling disturbance and sediment
characteristics, we used the procedure of Clarke and Ainsworth (1993). This selects the environmental variables best explaining
community pattern by maximising a rank correlation between environmental and community similarity matrices. The environmental
similarity matrix was calculated using square root transformed mean depth, trawling disturbance (SPUE) and mean sediment diameter,
using a Euclidean distance measure. The community similarity matrix was the matrix underlying the MDS plot. Mean sediment
diameter was used as a surrogate for percentage sand and mud/ clay when calculating the environmental similarity matrix because
there were highly significant correlations between mean sediment diameter and the percentages of sand and mud/ clay. All analyses
were conducted using SAS and PRIMER (Clark and Warwick, 1994) software.
Effects of trawling disturbance on production
We also investigated the impacts of trawling in the Silver Pit and Hills regions of the central North Sea. Seven sites subject to different
levels of trawling disturbance were studied in the Silver Pit and thirteen sites in the Hills. Sites were chosen to cover the range of
fishing intensities from the maximum to the minimum in each region. Each site was a square of 1 nautical mile North-South (1nm=
1853m) by 1nm East-West. Trawling disturbance was determined from records of vessel sightings by fishery protection aircraft as
described by Jennings et al. (2001a).
We sampled the infaunal and epifaunal invertebrate communties in both regions in winter and summer, as described above, to account
for the variations in size structure that result from 'pulses' of recruits growing up the size spectrum. Epibenthic invertebrates were
sampled with a 2m beam trawl fitted with a 1mm mesh liner and infaunal invertebrates with an anchor dredge. We deliberately chose
gears that sampled relatively large areas of seabed, even though the samples they take reflect relative rather than absolute
abundance. Both gears sample on an appropriate scale for the study of fishing effects (over areas of m 2 to 10s m2) and integrate small
scale patchiness of the larger macrofauna (individual body mass >0.0625g) that were the focus of this study. We assumed that the
catchability of different species did not change from site to site, so bias in abundance estimates was consistent. Three randomly
located replicate tows with each gear were completed at each site in each season. The beam trawl was towed for 5 minutes at 1 knot
and the anchor dredge was towed for 1 minute while drifting.
All organisms taken in the beam trawl sample were sorted and free-living epifauna were removed for processing. A subsample of
0.2m3 of sediment was taken from each anchor dredge sample and sieved through 1mm square mesh. All free-living infaunal species
retained by the mesh were removed for processing. All individuals were weighed, either fresh using heave compensated balances or
after preservation in 4% seawater formalin buffered with 3 g l -1 sodium acetate. Most individuals estimated to weigh less than 0.5g
were preserved. We did not apply shrinkage factors to account for weight changes following preservation as these were not available
for the very wide range of species included in our samples, and blotted weight was recorded in each case. After weighing, all infauna
and epifauna were assigned to log2 body size categories. Subsequent conversions between dry mass, ash free dry mass (AFDM), kcal,
kJ and wet mass were made using conversion factors kindly provided by Thomas Brey (pers. comm.).
Allometric (cross species) relationships were used to estimate P:B ratios and production from individual body mass. We calculated the
cross species relationship between P:B and body mass using a subset of the data compiled by Brey (1999). Body size spectra were
calculated for the infaunal and epifaunal communities at each site. Biomass by size class and production by size were calculated as
means for the six replicates from two seasons. Production was calculated from biomass using the allometric relationship determined
from studies of P:B and mean body mass. Total production for the community was given as the sum of production estimates by size
class.
5.3. Results
There was a 27 fold range in beam trawl disturbance (SPUE) among the Silver Pit sites and 10 fold among the Hills sites. We
expressed the level of trawling disturbance as an index, where the lowest level of disturbance was given a value of 1 on a linear scale.
The most intensively trawled site in the Silver Pit was trawled almost 3 times more often than the most frequently trawled site in the
Hills.
Effects of trawling disturbance and environment on benthic community size-structure
The MDS with trawling disturbance superimposed suggested that the size-structure of the infaunal communities was largely influenced
by trawling disturbance, with sites subject to higher levels of disturbance (S4 and S5) clearly separated from those sites where trawling
disturbance was low (S1 and S7). When the BIO-ENV procedure was used to select the environmental variables best explaining the
size structure, the single environmental variable which best grouped the sites in a manner consistent with the size-based community
9
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
patterns was trawling disturbance (pw=0.63). No other environmental variable or combination of environmental variables grouped the
sites as effectively; the best alternative being the combination of trawling disturbance and mean sediment diameter (pw=0.62). This
analysis provided compelling reasons to proceed with a size-based analysis of the effects of trawling on production processes.
Effects of trawling disturbance on production
In the Silver Pit region, there was a significant negative relationship between the total biomass of infauna and trawling disturbance
(F1,5=30.42, p=0.003). When relationships between trawling disturbance and polychaete biomass or bivalve/ spatangoid biomass were
tested separately, that between disturbance and polychaete biomass was not significant (F1,5=0.02, p=0.883) while that between
disturbance and bivalve/ spatangoid biomass was (F 1,5=43.37, p=0.001). However, polychaete biomass at the least disturbed site was
lower than at the sites subject to more frequent disturbance. For epifauna, there was no significant relationship between trawling
disturbance and biomass in the Silver Pit region (F 1,5=0.35, p=0.581).
The production of infauna in the Silver Pit was significantly and negatively related to levels of trawling disturbance (F1,5=130.82,
r2=0.96, p<0.001). When components of the infauna were considered separately, the production of polychaetes did not fall in response
to disturbance (F1,5=0.05, r2=0.01, p=0.831), while that of bivalves and spatangoids did (F1,5=56.06, r2=0.92, p<0.001). The P:B ratio of
the entire infaunal community rose significantly with disturbance (F1,5=168.43, r2=0.97, p<0.001), but this did not compensate for the
loss of overall biomass and production. For epifauna in the Silver Pit region, there was no significant relationship between disturbance
and production (F1,5=0.02, r2=0.01, p=0.900) or between disturbance and mean P:B of the whole community (F 1,5=0.40, r2=0.07,
p=0.555).
In the Hills region, infauna production was not significantly related to trawling disturbance (F 1,11=0.09, r2=0.09, p=0.764), and nor was
spatangoid and bivalve production (F1,11=0.19, r2=0.02, p=0.620). However, polychaete production increased significantly with
disturbance (F1,11=11.71,r2=0.52, p=0.006). The P:B ratio of the entire infaunal community was not significantly related to disturbance
(F1,11=0.06, r2=0.06, p=0.805). For epifauna in the Hills region, there was no significant relationship between disturbance and
production (F1,11=1.32, r2=0.11, p=0.274) and there was no consistent change in the mean P:B of the whole community (F 1,11=0.07,
r2=0.06, p=0.802).
5.4. Discussion
This was the first large scale study of trawling effects on benthic production and size structure across quantified gradients of
disturbance. Trawling disturbance led to reductions in the production of larger infaunal invertebrates in the more heavily trawled and
deeper region. The P:B ratio of the infaunal community rose with increased disturbance, but this reflected the differential loss of larger
individuals rather than the proliferation of smaller ones, and total production fell. Our results are broadly consistent with studies of the
effects of natural disturbance on production. Thus the correlative study of Emerson (1989) suggested that natural disturbance created
by wind stress limited benthic production in shallow areas. Consistent gradients in environmental factors could have explained
differences in the benthic communities among sites, but disturbance rather than environmental factors accounted for size-based
community structure.
Epifauna biomass and production were not significantly related to trawling disturbance. While trawling causes high levels of mortality
among epifaunal species (Lindeboom & de Groot 1998; Bergman & van Santbrick 2000), changes in abundance may not have been
detected because epifaunal mobility was high in relation to the spatial separation of sites subject to different levels of trawling
disturbance. Thus the effects of mortality would have been dissipated across a wide area and could not have been detected by spatial
comparisons. A significant relationship between epifauna biomass and trawling disturbance was recorded in the Silver Pit region
during winter (Jennings et al., 2001a). The Silver Pit region was much more heavily trawled in the October-November period that
preceded the winter survey than the March-April period that preceded the summer survey (K.J. Warr, unpublished). Thus the link
between epifauna biomass and disturbance in winter may have reflected the immediate effects of local fishing mortality.
In the Hills region, there was no significant reduction in the biomass of bivalves and spatangoids with trawling disturbance. This may
have reflected the lower range of trawling intensities in this region, since the decrease in biomass of bivalves and spatangoids was
relatively small across this range of trawling intensities in the Silver Pit region. In addition, the bivalve community in the Hills region was
dominated by fast-burrowing species such as Ensis, that are associated with mobile sands, rather than the more vulnerable Arctica
islandica (Bergman & van Santbrink 2000) that was found in the Silver Pit. Ensis may have been able to bury in advance of beam
trawls. Moreover, natural disturbance due to currents and waves is expected to be higher in the shallower Hills region and the fauna in
that region may be less susceptible to trawling effects because it is already well adapted to natural disturbance (e.g. Hall 1994; Kaiser
& Spencer 1996).
Our results suggest that recent temporal increases in the productivity of smaller polychaetes in the North Sea were not primarily a
response to local disturbance by beam trawls. Rather, we suggest that they were predominantly a response to increased primary
production, that resulted in a greater food supply for benthic fauna (Reid et al. 1998). Increased biomass may be observed among
polychaetes and smaller bivalves, because these species are the only ones that can withstand the high rates of mortality now imposed
by trawling. Since polychaetes are a favoured food of commercially important North Sea flatfishes such as plaice and sole, it is possible
that the recent increases in plaice and sole growth were a response to increased benthic production following increased primary
production, with trawling disturbance favouring species that were a preferred food and periodically exposing them to fish predation
(Millner & Whiting 1996; Rijnsdorp & van Leeuwen 1996).
10
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
Primary production in the central North Sea has increased in recent years. The increase began from 1985-1987, as evidenced by
greater densities of phytoplankton recorded over a longer period of each year (Reid et al. 1998). This coincided with the period when
benthic biomass increased (Kröncke et al., 1998). From 1965 to 1985, however, primary production was low and variable. The increase
in primary production has been attributed to an increase in sea temperature (Reid et al. 1998), as determined by the strength of the
North Atlantic Oscillation. It is possible that increases in primary production during recent years were also driven by nutrient release
following trawling. However, the beam trawling fleet in the North Sea grew most rapidly in the period from 1966 to 1976 (Rijnsdorp &
van Leeuwen 1996; Millner & Whiting 1996), but there were no consistent increases in primary production prior to 1985 (Reid et al.
1998).
Our results suggest that future studies of trawling effects on ecosystem function should focus on the production and dynamics of the
meiofauna and the smallest macrofauna, because these groups, along with bacteria, are the only large groups of species that have
sufficiently fast life cycles to proliferate in intensively trawled areas and process the carbon and nitrogen that cannot be processed by
depleted populations of larger animals. If these studies also show that the rate of production is reduced by disturbance, then trawling
disturbance could have indirect effects on the strength of bentho-pelagic coupling and the rate of detritus accumulation in marine
ecosystems.
6. Impacts of trawling disturbance on structure of North Sea benthic communities: with a focus on smaller macrobenthic
animals (Research in support of Objectives 1 and 2) (S. Jennings, M.D. Nicholson, T.A. Dinmore, K.J. Warr)
6.1. Introduction
Bottom trawling causes chronic and widespread disturbance to soft-sediment communities in shelf seas. The relative impacts of this
disturbance depend on the frequency of trawling and levels of natural disturbance. In general, studies of trawling impacts have
compared community structure at sites subject to different intensities of trawling disturbance; usually based on multivariate analysis of
transformed species-abundance data. These studies have focused on impacts on larger macrofauna and habitat forming species,
primarily because reductions in their abundance and diversity are an important conservation issue and provide habitat for bottomdwelling fishes. Few studies have considered the implications of trawling disturbance on ecosystem processes. Such processes
include the production of the benthic community and its role in supporting the production of fished species.
The studies reported in Section 5 showed that there were six fold reductions in total community production across a gradient of trawling
disturbance. This study focused on a qualitative analysis of larger infauna and the reductions in production were largely due to
reductions in the biomass of large bivalves and spatangoids, species that are very vulnerable to trawls. Multivariate analyses showed
that the reduction in biomass and production of larger animals was due to trawling effects rather than sediment type or depth. The
results were clear for the large bivalves and spatangoids that play a key role in bioturbation and community production, but, for the
small polychaetes and bivalves that support flatfish production, the sampling design and analysis provided little power to determine the
effects of trawling on production.
The aim of this study was to investigate the effects of beam trawling disturbance on the production of the smaller size classes of
benthic infauna that support flatfish production. We use a size-based approach that could be applied to other soft-bottom systems. Our
study was conducted on real fishing grounds with a quantified history of trawling disturbance. We used a generalised additive model to
test for the effect of trawling disturbance and to account for differences in sediment type and depth that could also affect infaunal
production. Because the variation in these predictor variables was not controlled, it is important to quantify how effective the statistical
analyses of these data are likely to be. This is particularly important when interpreting statistical non-significance under a precautionary
regime. We have therefore assessed the statistical power of all our analyses to detect i) linear increases and decreases in infaunal
production and ii) increased production at intermediate levels of disturbance.
6.2. Methods
Our research was conducted in the Silver Pit region of the central North Sea. We studied the production of benthic communities at nine
sites in the Silver Pit. Each site was an area of 5 nautical miles (9265m) North-South by 6 nautical miles East-West (11118m), and
subject to a different level of trawling disturbance. Within each site, we sampled the infaunal community in three boxes of 1 n mile
North-South by 1 n mile East-West (1 n mile = 1853m). Boxes were haphazardly located within sites.
Sample collection and processing
Benthic infauna were sampled with a NIOZ corer. This device takes a circular sediment core of 35.7 cm internal diameter (area of
0.1m2) to a depth of 40 cm. Ten cores were taken from each of the 27 boxes, five from 21 November - 4 December 2000 and five from
5- 18 April 2001, to account for the variations in size structure that result from 'pulses' of recruits growing up the size spectrum. Core
samples were sieved through 1mm square mesh. All infaunal species retained by the mesh were removed for processing and
preserved in 4% seawater formalin buffered with 3 g l -1 sodium acetate. A sediment sample was also taken from each core for particle
size analysis. This was removed with a 5.5 cm diameter perspex tube, and frozen to – 20 C pending analysis. After thawing, sediment
samples were wet sieved through a 500 µm sieve, and the fraction greater than 500 µm was oven dried at 90 °C for 24 hours. This
fraction was then dry sieved at 0.5 phi intervals, down to 1 phi (500 µm) and weighed on a top pan balance (precision= 0.01 g). The
fraction smaller than 500 µm was freeze dried and analysed on a Coulter LS 130 Laser sizer. The laser sizer results were combined
with the dry sieve results to give the full particle size distribution. The mean particle size diameter and sorting coefficient were
calculated from these results.
In the laboratory, infaunal samples were sorted to higher taxonomic categories (from genus to phylum) and individual animals were
weighed (blotted wet weight) to the nearest 0.001 g. Tube forming polychaetes were removed from tubes before weighing. We did not
11
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
apply shrinkage factors to account for weight changes following preservation as these were not available for the very wide range of
species included in our samples. As this study dealt with the production of benthic animals that would be available to support
production at higher trophic levels, we converted wet weight to ash free dry mass (AFDM) using conversion factors kindly provided by
Thomas Brey (pers. comm.). All individuals with a calculated AFDM of >0.00078g were included in the analysis. Inevitably, a proportion
of the animals in our samples were damaged or incomplete. We applied the following rules to damaged animals. First, we tried to
assemble 'complete' animals from the fragments in the sample, and recorded these as a single individual. Second, if fragments of
animal constituted less than 30% of the expected mass of a complete animal, we discarded them.
Trawling disturbance
Mean levels of trawling disturbance at the study sites were determined from records of beam trawler sightings by fishery protection
aircraft. We converted our SPUE data to a mean frequency of beam trawling at each site. We assumed that the average beam trawler
fishing in the Silver Pit will tow two 12m wide beams at a speed of 6 knots (11.1 km hour-1), and that 267264 m2 of seabed will be
disturbed each hour. Therefore, an annual SPUE of 1 (beam trawler always sighted at the site) will equate to the entire area of the site
being trawled, on average, 22.73 times each year. In reality, trawling effort within the site will be patchy and some areas will be trawled
more frequently than others. However, the mean frequency of trawling provides a useful comparative index of disturbance and sidescan observations of trawl tracks within the Silver Pit study sites (where the sediment is sufficiently soft for trawl tracks to persist for a
few days after trawling) suggest that track frequency and SPUE are correlated and that tracks cross the more heavily fished sites in
many directions (T.A. Dinmore & S. Jennings, unpublished data).
Data analyses
Body size spectra were produced for the infaunal communities at each site. Biomass by size class and production by size were
calculated for each box in each site as means of the ten replicates cores from two seasons. Biomass size spectra were normalised by
dividing the biomass in a given body mass class interval by the width of that class interval. The relationship between size (as classes)
and total normalised biomass was described using least squares linear regression. Production was calculated from biomass using an
allometric relationship between P:B and mean AFDM. This was calculated using the subset of Brey's (1999) dataset described in
Section 5. The relationship for the same species, but parameterised for AFDM, was log 10 P:B=-0.431-(0.236*log10B). Total production
for the community was calculated as the sum of production estimates by size class.
Following a preliminary assessment of the data, each effect variable was analysed using a generalised additive model of the form
yi  a0  a1 x1i  a 2 x12i  a3 x2i  a 4 x3i  lo( x 4i )   i
for observation i, where
x1i = mean particle diameter
x2I = sorting coefficient
x3I = depth
x4i = trawling disturbance
lo(.) indicates a loess smoother and
i
is Normally distributed with constant variance.
The smoother was given approximately three degrees of freedom, corresponding to a span of 0.66, and each term was fitted and
tested sequentially at the 5% significance level.
We conducted post-hoc power analyses to assess the power of our analysis to detect a linear trend in the production of small animals
(<0.0625g AFDM) or small polychaetes (<0.0625g AFDM). We calculated power as a function of the percentage change from the mean
response to mean response ± 50% over the range from the minimum to the maximum frequency of trawl disturbance. We also
conducted post-hoc power analyses to assess the power of our analysis to detect a linear trend in the slope and intercept of the
biomass size spectra.
Since non-linear responses to trawl disturbance may arise, we also tested the power of our analyses to detect non-linear trends in the
production of small animals (<0.0625g AFDM) or small polychaetes (<0.0625g AFDM). The most likely forms of non-linear response
are those that reflect increased production at some intermediate level of disturbance where, for example, small animals would benefit
from reduced competition or predation but would have sufficiently high rates of population growth to tolerate mortality due to trawling.
To incorporate a range of possible response scenarios, a family of hypothetical responses were generated using a function of the form
responsei = constant
xi 
x 4i  min( x 4i )
 xia (1  xi ) a where
max( x 4i )  min( x 4i )
is the i’th proportion of trawl disturbance, and the constant scales the maximum of the response-curve to be constant for different
values of a shape parameter, a. When a=0, the function describes a negative linear response and when a=1, a positive linear
response. For intermediate values of a, the function describes higher levels of production at intermediate levels of disturbance.
12
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
6.3. Results
There was a 17.5 fold range in beam trawl disturbance (SPUE) among the Silver Pit study sites, and the trawled habitats were
predominantly sandy or muddy-sand with mean particle size diameters ranging from 0.015 to 0.081 mm. The slopes of the infaunal
biomass size spectra were all significant and negative.
Biomass Size Spectra
The relationships between the slopes and intercepts of the biomass size spectra and the environmental variables and trawling
disturbance, showed that the slopes were significantly related to mean particle diameter (MPD) and MPD 2, and that after the model had
been fitted, there was no evidence for a change in the slopes of the biomass size spectra with trawling disturbance. The post-hoc
power analysis showed that the power to detect a change of ±20% in slope due to disturbance would have been high. The intercepts
were significantly related to the sorting coefficient (SC) and depth, and after the model had been fitted, there was no evidence for a
significant change in the intercepts of the biomass size spectra with trawling disturbance. The power to detect a change of ±50% in the
intercepts with disturbance was also high.
Production
The production (g AFDM m-2 y-1) of all small animals (< 0.0625 g) in the community was significantly related to MPD, MPD 2, SC and
depth, and after these effects were accounted for in the generalised additive model, there was no evidence for a significant effect of
trawling on production. The production (g AFDM m -2 y-1) of small polychaetes (< 0.0625 g) in the community was significantly related to
MPD, MPD2 and depth (p=0.06) (Table 1). After these effects were accounted for in the model, there was no evidence for a significant
effect of trawling on production. The power to detect linear and non-linear responses (as defined by parameter a) of small animal and
polychaetes was high. Thus the post-hoc power analysis showed that there was a minimum 90% probability of detecting a 50% change
in the production of small animals, whether that change was a positive, negative or resulted in increased production at intermediate
levels of disturbance.
Table 1. Analysis of variance for the generalised additive model fitted to relationships between (a) the production of small animals
(AFDM<0.0625g), (b) the production of small polychaetes (AFDM<0.0625g), (c) the slopes of biomass size spectra, (d) the intercepts
of biomass size spectra, environmental variables and trawling disturbance. Terms were added sequentially, first to last.
a. Production of small animals
mean particle diameter
mean particle diameter2
sorting coefficient
depth
residuals
loess (trawling disturbance)
residuals
df
1
1
1
1
22
3.32
18.68
SS
3.527
3.102
1.344
0.515
2.430
0.370
2.060
MS
3.527
3.102
1.344
0.515
0.110
0.111
F
31.94
28.09
12.17
4.66
p
0.000
0.000
0.002
0.042
1.010
0.417
df
1
1
1
1
22
3.32
18.68
SS
0.413
0.250
0.104
0.181
1.051
0.181
0.870
MS
0.413
0.250
0.104
0.181
0.048
0.055
0.047
F
8.64
5.22
2.17
3.78
p
0.008
0.032
0.155
0.065
1.17
0.350
df
1
1
1
1
22
3.32
18.68
SS
0.014
0.014
0.000
0.000
0.007
0.000
0.006
MS
0.014
0.014
0.000
0.000
0.000
0.000
0.000
F
46.93
45.51
0.91
0.09
p
0.000
0.000
0.349
0.769
0.687
0.585
df
1
1
1
1
22
3.32
18.68
SS
0.005
0.014
0.034
0.016
0.097
0.025
0.072
MS
0.005
0.014
0.034
0.016
0.004
0.007
0.004
F
1.21
3.13
7.73
3.66
p
0.283
0.091
0.011
0.069
1.94
0.154
b. Production of polychaetes
mean particle diameter
mean particle diameter2
sorting coefficient
depth
residuals
loess (trawling disturbance)
residuals
c. Slope of biomass size spectra
mean particle diameter
mean particle diameter2
sorting coefficient
depth
residuals
loess (trawling disturbance)
residuals
d. Intercept of biomass size spectra
mean particle diameter
mean particle diameter2
sorting coefficient
depth
residuals
loess (trawling disturbance)
residuals
13
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
6.4. Discussion
Our results suggest that chronic beam trawling disturbance has minimal effects on the production and size structure of small benthic
infauna. This conclusion is likely to be robust because the power of our analyses to detect small negative or positive changes in
production, or increased production at intermediate levels of disturbance, is high. Our results can probably be generalised to other
shallow, trawled, sand and mud habitats dominated by free-living species, but we would expect very different results in areas where the
habitat is more complex, where there are many biogenic species and where there was no previous history of fishing activity. The small
infauna we studied are among the few groups of benthic invertebrates for which biomass and production do not appear to be reduced
by trawling disturbance. Since small infaunal polychaetes are a key source of food for flatfishes, we conclude that beam trawling
disturbance has a relatively minor impact on the food chains that support the production of the primary target species (plaice and sole).
Our results also suggest that beam trawling is not farming the sea (by boosting production at intermediate levels of disturbance), even
though trawling disturbance may increase the availability of small infauna to fishes by exposing them to predation (Ramsay et al. 1997).
Moreover, our results show that the size distribution of small infauna is not markedly affected by trawling, even though the power of our
analyses to detect an effect is high.
While the production of small animals that support commercial fish species was not reduced by trawling disturbance, the role of the
infaunal community will change following chronic trawling disturbance. Polychaetes and small infauna dominate the benthic fauna in
heavily trawled areas, because populations of larger species are greatly depleted, and yet the smaller infauna are much less significant
as bioturbators (Swift 1993). As a result, sediment community function, carbon mineralisation and biogeochemcial fluxes are likely to
be strongly affected by trawling activity because the physical effects of trawling are equivalent to those of an extreme bioturbator, and
yet, unlike bioturbating macrofauna, trawling does not directly contribute to community metabolism (Duplisea et al. 2001).
If the patchiness of trawling effort is maintained at present levels, our study suggests that the production of fish-food on flatfish trawling
grounds is unlikely to be compromised by trawling disturbance. However, trawling disturbance will reduce the biomass of many larger
species and affect biogeochemical processes. Given the power of our analyses, and the lack of evidence for increased the production
of small infauna at intermediate levels of disturbance, recent increases in the production of small benthic infauna in the North Sea are
more likely to reflect increases in primary production following climate change (Kroncke et al. 1998, Reid et al. 1998) and the tolerance
of the small polychaetes and bivalves to trawling disturbance. This conclusion could only have been reached by comparing the spatial
impacts of trawling with existing analyses of temporal trends in the abundance of North Sea infauna and shows why both spatial and
temporal analyses should be considered when making any assessment of the impacts of fishing.
7. The effects of different intensities of fishing disturbance on the trophic structure of benthic communities in the North Sea
(Research in support of Objective 5) (S. Jennings, K.J. Warr, T.A. Dinmore, J.K.Pinnegar, N.V.C. Polunin)
7.1. Introduction
Many studies have described the impacts of bottom trawling on the structure of benthic communities, but these have focused on
changes in the relative abundance of different species in the benthic community, but have not dealt directly with changes in the trophic
structure or function of that community. We might expect trawling to have profound effects on the trophic structure and function of
benthic communities, because certain functional groups, such as large filter-feeding bivalves, are more vulnerable to trawling
disturbance than others (Lindeboom & de Groot 1998).
We compared benthic communities among sites subject to different levels of trawling disturbance in two regions of the North Sea. The
fauna in these regions is dominated by free-living infauna and epifauna living in mobile sediment, typical benthic communities of North
Sea beam trawl grounds. We focus on aggregate responses of the community to trawling rather than those of individual species. We
attempt to link structural changes in the abundance of species groups to those in the trophic structure of the community. This is a
fundamental step towards understanding the impacts of trawling disturbance on patterns of production and energy use in marine
ecosystems. We focus on differences among sites that have been subject to different levels of trawling disturbance for at least 5 years.
We seek to address the following questions: (1) how does trawling disturbance affect the total biomass of infauna and epifauna, (2) is
there evidence for a shift in the infaunal community from one that is dominated by bivalves and spatangoids to one that is dominated by
polychaetes, (3) does trawling disturbance favour species from lower trophic levels and lead to a fall in the trophic level of the
community, (4) does trawling disturbance affect any size-based predator-prey relationships that are seen in the community.
7.2. Methods
The impact of trawling disturbance on the trophic structure of infaunal and epifaunal benthic communities was investigated in two series
of sites subject to different levels of fishing disturbance in the Hills and Silver Pit regions of the North Sea (Sections 5 and 6). The
trawling disturbance at each site was determined from records of vessel sightings by fishery protection aircraft (Sections 5 and 6).
Epibenthic invertebrates were sampled with a 2m beam trawl and infaunal invertebrates with an anchor dredge. Three randomly
located replicate tows of 1 minute duration were also made with an anchor dredge. The sediment collected was emptied on the deck of
the ship and a 0.2m3 sub-sample removed. This was sieved through 1mm square mesh and all infaunal species retained by the mesh
were removed for processing.
All infauna from the replicate anchor dredge samples and epifauna from the replicate beam trawl samples were identified, individually
weighed on heave compensated balances and assigned to log 2 size classes. Hermit crabs (Paguridae) were weighed after removal
from their shells, but animals that secreted their own shells were weighed with the shells intact. To obtain tissue samples for stable
isotope analysis, animals from the same size class in each of the 3 replicate samples of (i) infauna and (ii) epifauna from each site
were combined and homogenised in an electric blender, usually with added water, to produce a thoroughly mixed suspension that
14
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
poured smoothly. Approximately 7 ml of this suspension was frozen to -20° C in a glass vial. Shells were removed from all bivalves
before blending.
The frozen samples of homogenised tissue were freeze dried, and the freeze dried material was crumbled or ground to a fine powder
(particles <60 μm). The 15N composition of the samples was determined using continuous flow isotope ratio mass spectrometry (CFIRMS) (Preston & Owens 1983, Preston 1992). Weighed samples of 1.0 mg ground material were oxidised and the N 2 passed to a
single inlet dual collector mass spectrometer (Automated Nitrogen Carbon Analysis (ANCA) SL 20-20 system). This was a continuous
flow system, so two samples of reference material (an internal standard- in this case homogenised cod Gadus morhua tissue with
similar nitrogen content to the samples we analysed) were analysed after every five tissue samples in order to calibrate the system and
compensate for drift with time (ANCA-SL Dual Isotope v3.4 software).
Ratios of 15N:14N were expressed relative to N2 in air for nitrogen and calculated as:
 15N =(15N:14N sample/15N:14N standard-1)*103
The mean δ15N for the infaunal or epifaunal community at each site was calculated as a weighted mean of δ15N by size class. The SD
for the repeated  15N measurements made with the reference material was 0.1 0/00. Two samples were excluded from the final
analysis. First, an infaunal sample in the 8.1-16.0g size class from site S4 which contained only 0.202% N, below the recommended
tolerance of the ANCA system and significantly below the mean nitrogen content (9.3±2.75%) for other samples in this size class taken
at Silver Pit. Second, an infaunal sample in the 1.1-2.0 g size class from site H13 which contained only 0.435% N, also below the
recommended tolerance of the ANCA system, and significantly below the mean nitrogen content (9.8±1.61%) for other samples in this
size class taken at the Hills sites. We do not know why the N content of these samples was so low.
7.3. Results
All the study sites were trawled. There was a 27-fold range in beam trawl SPUE among the Silver Pit sites and 10-fold among the Hills.
The total biomass (all references to biomass refer to biomass per sample) of infauna and epifauna decreased significantly with trawling
disturbance in the Silver Pit (linear regression: infauna r 2=0.68, F1,5=10.78, p=0.022; epifauna r2=0.60, F1,5=7.34, p=0.042). The slope
of the relationship between infaunal biomass and trawling disturbance (b=0.0376) suggested that there would be an order of magnitude
decrease in infaunal biomass when trawling disturbance was increased 27 fold. For epifauna, the slope (b=0.0162) suggested an
equivalent decrease when trawling disturbance increased 62 fold. Trends in total biomass of infauna and epifauna in relation to fishing
disturbance were not significant at the Hills sites (infauna: F 1,11=0.10 p=0.761; epifauna: F1,11=0.80, p=0.389).
Analysis of the gross composition of the infaunal community at Silver Pit showed that there was a highly significant decrease in
biomass of bivalves and spatangoids with fishing effort (linear regression r 2=0.78; F1,5=17.56, p=0.009) but no significant change in
polychaetes (F1,5=0.03, p=0.865). At the Hills site, there was no significant change in bivalves and spatangoids with effort (F 1,11=0.36;
p=0.560), but an increase in polychaetes (r2=0.53; F1,11=12.26; p=0.005). Changes in the biomass of the species groups at the Silver
Pit sites were reflected in significant changes in the proportions of these species (by biomass) in the community. The proportion of
bivalves and spatangoids decreased significantly (r 2=0.90; F1,5=42.50; p=0.001) while the proportion of polychaetes increased (r 2=0.68;
F1,5=10.82; p=0.022). At the Hills sites, decreases in the proportion of bivalves and spatangoids with trawling disturbance were also
significant (r2=0.39; F1,11=7.00; p=0.023), but increases in the proportion of polychaetes were significant only at p<0.1 (r 2=0.24;
F1,11=3.43; p=0.090). A range of exploratory analyses confirmed that the small depth variations between sites did not account for the
differences in faunal biomass and faunal composition that were observed among sites within either the Silver Pit or Hills regions. Our
sampling confirmed that the community was dominated by free-living species, since they accounted for >96% of infaunal and >98% of
epifaunal biomass at all sites.
There was no significant and positive relationship (p>0.1 in all cases) between δ15N and body size for infauna at any of the Silver Pit or
Hills sites, implying that the largest organisms in this community fed at lower trophic levels. The relationships between δ 15N and body
size for epifauna at the Silver Pit sites and Hills sites were generally positive and the relationships were well described using linear
regression. The relationships were significant at all Silver Pit sites and 10 of 13 Hills sites. The relationships between the slopes and
intercepts of these plots and levels of fishing disturbance were investigated. In no cases were the slopes or intercepts significantly
related to trawling disturbance (linear regression, p>0.05), though intercepts were higher at Silver Pit sites than at Hills sites (Silver Pit:
mean± SD= 11.67± 0.22; Hills: mean± SD= 11.29± 0.26; T test: T=3.52, p=0.003).
Mean δ15N of the sampled infaunal and epifaunal communities were remarkably consistent across sites and were not significantly
related to trawling disturbance. Mean δ15N was significantly higher for epifauna than infauna within the Silver Pit (Infauna: mean δ 15N±
SD= 9.07± 0.92; Epifauna: mean δ15N± SD= 12.27± 0.19; T test: T=9.02, p<0.001) and Hills (Infauna: mean δ 15N± SD= 9.53± 1.05;
Epifauna: mean δ15N± SD= 11.82± 0.55; T test: T=6.95, p<0.001) regions.
7.4. Discussion
Our results suggest that intensive trawling disturbance has led to reductions in the biomass of infauna and epifauna in the Silver Pit,
and dramatic changes in the composition of the infauna. However, these changes are not reflected in the mean trophic level of the
community, or the relationships between the trophic levels of different sizes of infauna. We suggest that the trophic structure of
intensively fished communities has not changed, despite changes in species composition, to ensure the continued processing of
production by those remaining invertebrates that can withstand the levels of mortality imposed by trawling.
15
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
Clearly, factors other than trawling disturbance could have explained the patterns in biomass and trophic structure that we observed.
However, as shown in Section 6, the size structure of the community of larger macrofauna is governed by the effects of trawling
disturbance rather than environment. The effects of trawling on biomass and gross composition of the community were most apparent
at the Silver Pit sites where differences in trawling disturbance exceeded 30 fold and the most intensively trawled site was fished 3
times more intensively than the most intensively fished Hills site. A positive cross-species relationship between body mass and δ15N is
the expected consequence of larger predators eating smaller prey. The relationship between body mass and δ 15N was significant within
the epifaunal communities at most Silver Pit and Hills sites, suggesting that larger epifauna fed on smaller ones. If epifauna only preyed
on other epifauna, then the gradient of the relationship between body mass and δ 15N would indicate the mean ratio of epifaunal
predator: prey body mass. However, epifauna also prey on infauna and fish, and the gradient will be affected by these feeding
interactions. The gradient of the relationship between body mass and δ 15N was not consistently related to the level of trawling
disturbance. Rather, it appears that the trophic position of epifauna in a specific size class is more or less independent of fishing
disturbance. Given that biomass fell in response to fishing disturbance across the Silver Pit sites, we suggest that patterns of energy
flow in the epifaunal community are largely unaffected by the increased rates of mortality caused by trawling disturbance.
Infauna did not show an increase in δ15N with body size. The infaunal community was dominated by polychaetes, bivalves and
spatangoids. The polychaetes are generally small species with a log 2 body mass of <0, while bivalves and echinoderms tend to be
larger but feed at low trophic levels, as shown by the low values of δ15N in large body mass categories. It would seem that two groups
of infauna may exist, a conventional 'food chain' of polychaetes where the larger species and individuals feed on smaller ones
(Schubert & Reise 1986, Beukema 1987) and larger organisms, such as bivalves and spatangoids, that are deposit and filter feeders
that feed at lower trophic levels. As such, there is little or no consumption of the smallest infauna by the largest. Polychaetes may be
consumed directly by infauna and fish, while larger bivalves are only accessible if crushed by trawls or if their siphons can be nibbled
by fish and epifaunal invertebrates..
Despite order of magnitude decreases in biomass of infauna, and a shift from a community dominated by bivalves and spatangoids to
one dominated by polychaetes, the mean trophic level of these communities differed by less than one trophic level between sites and
differences were not linked to levels of fishing disturbance. The lack of changes in the trophic level of the benthos could imply that the
trophic structure of the community is relatively robust in the face of fishing disturbance because species less vulnerable to disturbance
are taking the trophic roles of larger more vulnerable species. It would be very valuable to underatke an explicit study of whether
smaller species with faster life histories begin to fill the trophic functions vacated by larger species with slower life histories because the
latter cannot withstand the high mortality rates imposed by repeated trawling.
8. A preliminary assessment of the utility of vessel sightings data (recorded during fishery protection overflights) as a means
of estimating levels of fishing disturbance (Research in support of Objective 6) (S. Jennings, K.J. Warr and T.A. Dinmore)
8.1. Introduction
Demersal fishing effort data are typically collected on a coarse scale (ICES rectangle) and may be inappropriate for detailed
assessment of the effects of fishing on benthic communities. In order to investigate the finer scale distribution of fishing effort we used
data collected by British Fishery Protection flights in southern and central North Sea, and investigated relationships between trawling
effort as assessed using this data and official effort statistics (larger scale) and sidescan records of trawl marks on the seabed (finer
scale). The fishery protection aircraft provided good coverage of the spatial distribution of fishing effort in the period before the satellite
vessel monitoring system was put into operation. However, satellite vessel monitoring is only suitable for assessing the offshore
distribution of effort, because only those vessels >24m are tracked. Inshore, such vessels are a very small proportion of the total fishing
fleet. Moreover, at the present time, access to satellite data is restricted and it cannot be used for scientific purposes.
8.2 Methods
Estimates of trawling effort from fishery protection overflights
The staff on fishery protection overflights record the location, type and identification number of all vessels sighted and whether they are
fishing or steaming. We have used these data to calculate sightings per unit of searching effort (SPUE) as an index of fishing intensity
by assigning sighted vessels of all nationalities to ICES rectangles and dividing by surveillance effort (number of visits by aircraft) to
those rectangles. These estimates of effort were compared with those recorded in the official scheme. We also used the sightings data
to calculate effort at much smaller scales (nautical mile boxes) that were suitable for the sampling and investigation of fishing effects.
We compared SPUE and sidescan records of trawling disturbance in the Silver Pit. The Silver Pit region was divided into 5nm (9265m)
(North-South) by 6nm (11118m) (East-West) areas. Within each area, the mean number of beam or otter trawl SPUE was calculated.
We used records of all sightings of actively fishing beam and otter trawlers in the Silver Pit and Hills regions from 1 Jan 1994 to 31 Dec
1998. In this period, aircraft visited sub-rectangles in the Silver Pit with a mean frequency of 103.7 to 153 visits year -1, and subrectangles in the Hills with a mean frequency of 121.2-160.2 visits year-1. Trawling effort in the 1nm2 (3.43 km2) study sites within the
30nm2 (103 km2) area was taken as 1/30th of that in the 30nm2 area. This was preferable to calculating SPUE in the 1nm2 study sites
because the error associated with locating a vessel inside or outside such a small area is very large when modern beam trawlers fish at
up to 7 knots (13 km h-1) and can travel 1nm (1853m) in less than 10 minutes. Clearly, our approach assumes that trawling and aircraft
search patterns within the areas are random. We recognise that this assumption will be violated and introduce some bias.
in those sub-rectangles. To compare reported effort with SPUE, we calculated mean SPUE by rectangle (mean SPUE for the four
subrectangles) and tested the significance of any correlation with reported effort.
Mean levels of trawling disturbance at the study sites were determined from records of beam trawler sightings by fishery protection
aircraft. We converted our SPUE data to a mean frequency of beam trawling at each site. We assumed that the average beam trawler
16
Can demersal fishing cause long-term changes in benthic
community structure
Project
title
MAFF
project code
MF0716
fishing in the Silver Pit will tow two 12m wide beams at a speed of 6 knots (11.1 km hour-1), and that 267264 m2 of seabed will be
disturbed each hour. Therefore, an annual SPUE of 1 (beam trawler always sighted at the site) will equate to the entire area of the site
being trawled, on average, 22.73 times each year. In reality, trawling effort within the site will be patchy and some areas will be trawled
more frequently than others.
Measurement of trawling frequency from side-scan images
An estimate of trawling frequency in the Silver Pit area of the North Sea was made during three research cruises in 2000 and 2001.
Estimations were made from sidescaning images obtained by towing ISIS Datasonics SIS 1500 chirp sonar equipment at a speed of 57 knots across 7 study locations (Table 2). At each location, a grid measuring 1*1 nm was scanned.
The digital sidescan-sonar data were all processed using DelphMAP image visualisation software. The sidescan backscatter data were
geoencoded using the recorded GPS position of the vessel, and corrected for layback and slant range.
Each 1nm2 grid surveyed was sub-divided into 81 200m2 boxes. From these 81 boxes, 20 were chosen at random via a simple random
number formula generated in EXCEL. Within each of the chosen boxes, the total length of all visible individual beam trawl tracks was
measured using a measurement screen cursor within the DelphMAP program. To be consistent, track measurements were made
through their centre line. All measurements for an individual 200m 2 box were then totalled to give a single estimate of trawling
frequency per box.
Table 2. NW locations of each 1nm2 grid within the Silver Pit study area
S
Site
Latitude
Longitude
S1
54O 02’N
01O 54’E
S2
54O 01’N
02O 05’E
S3
54 03’N
02O 15’E
S4
54O 01’N
02O 23’E
S5
54 02’N
02O 31’E
S6
54O 03’N
02O 42’E
S7
53 57’N
02O 55’E
O
O
O
8.3. Results
In the southern and central North Sea, patterns of effort shown by the official data broadly corresponded with those based on the
fishery protection overflight SPUE data for both otter and beam trawling. Relationships between reported effort and mean SPUE by
rectangle in the southern and central North Sea were significant (p<0.05), in accordance with the results of analyses conducted in
previous years (1990 to 1995, Jennings et al., 2000).
Relationships between the frequency of tracks recorded with sidescan sonar and the annual mean SPUE data in the seven 1 nautical
mile2 boxes were not significant (Table 3, p>0.05). While the sidescan provided clear records of tracks, their persistence turned out to
be relatively short (a few days based on a single track that we surveyed on several occasions after a trawler had fished- compared with
persistence of weeks and months that has previously been recorded in some muddy areas), and so they reflected very recent patterns
of disturbance rather than the longer-term impacts of fishing. As such, side-scan tracks were not appropriate for validating the SPUE
data.
Table 3. Mean length of trawl track recorded in 20 200m2 boxes at sites S1-S7 during each survey cruise and the mean SPUE for beam
trawlers (*103) in 2000.
Site
S1
S2
S3
S4
S5
S6
S7
Cruise 6/00
169
882
124
180
87
264
58
Cruise 14/00
594
311
166
331
282
53
37
Cruise 3/01
773
711
181
580
340
394
1095
Mean track length/ 200m2
512± 310
635± 293
157± 29
364± 202
236± 132
237± 173
386± 588
SPUE
17.5
74.0
111.1
203.7
307.7
346.1
0.0
8.4 Discussion
Sidescan sonar proved to be an inappropriate tool for validating overflight estimates of the frequency of trawling disturbance because
the persistence of trawl tracks is short, and probably very variable, in the Silver Pit. As a result, our survey reflected effort patterns over
a few days rather than a year. Moreover, displacement of effort from the cod box during cruise 3/01 complicated our analyses because
17
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
it resulted in anomolously high effort at site S7 on cruise 3/01, when the cod box was closed. Unfortunately, our study also coincided
with the period when fishery protection overflights were being reduced because the satellite vessel monitoring system was coming into
operation. The satellite effort data will provide an ideal resource for mapping finescale effort distribution in offshore areas (see for e.g.
Rijnsdorp et al., 2001).
9. The effects of the North Sea Cod Recovery Plan on fishing effort distributions and benthic communities (Research in
support of Objective 7) (T.A. Dinmore, D.E. Duplisea and S.Jennings)
9.1. Introduction
The North Sea cod stock is considered by ICES to be outside of safe biological limits and at risk of collapse. Fishing mortality on this
stock increased steadily throughout the 1960’s and 1970’s reaching a peak in 1972 with 353 000 tonnes landed. Since then landings
have decreased but fishing mortality remains at a very high level. The spawning stock biomass has declined rapidly, falling to an
historic low in the early 1990’s and by 1999 it was estimated to be 70,000 tonnes less than half the safe level. In response to these
concerns the EU Council asked the Commission to establish a recovery plan with the aim of both protecting the cod stock during the
spawning season thus allowing maximum reproductive activity, and deterring discarding and mis-reporting of cod in all fisheries.
The Cod Recovery Plan comprises of 3 stages: (1) closed areas; (2) technical measures and (3) comprehensive proposals for long
term measures. Stage 1 (Commission Regulation (EC) No 259/2001) came into force on 14 February 2001. An area of more than
40,000 square miles of the North Sea, almost a fifth of its entire area, was closed to all fisheries likely to catch cod, for a period of 12
weeks. Pelagic and sandeel fisheries were allowed to continue under observation. The areas closed included some of the main fishing
grounds for the North Sea otter and beam trawl fleets.
The imposition of management systems, such as closed areas, will lead to a redistribution of effort to grounds rarely impacted under
normal fishing regimes. We developed a model (Duplisea et al, 2002) to assess how the production and biomass of benthic fauna
changed in response to different levels of fishing effort, and used this to develop a protocol for assessing the impacts of effort
redistribution on the benthic community.
9.2. Methods
As we have shown in section 6, the mortality of benthic fauna is strongly size dependent, and we have already used empirical data to
demonstrate that the frequency of bottom trawling disturbance in the central North Sea had a greater effect on the size structure of the
fauna in a soft-sediment benthic community than other environmental variables such as sediment particle size and depth. This
analysis provided compelling reasons to predict the impacts of trawling on benthic community structure using a size-based model.
Accordingly, we simulated the impacts of trawling disturbance on a benthic community consisting of thirty seven size classes of
organisms in three broad categories: meiofauna, soft bodied macrofauna and hard bodied macrofauna and ranging in size from 1 μg to
80 g shell free wet weight. The model is described in detail by Duplisea et al (2002), and provides estimates of infaunal biomass and
production, by size class, for given frequencies of trawling disturbance.
We studied the effects of the cod recovery plan on the size-structure, biomass and production of benthic communities in an area from
54° to 57° North and 1° to 6° East, in the central North Sea. We developed a procedure to analyse effort data at scales of 1 x 1nm to
16 x 16nm, and so we trimmed the area of this study box to 176 x 176 nm for analytical purposes. The total area of the study box
exceeds 30 000 square miles. Within the study box depths range from approximately 25m off the Dogger Bank to 100m in the
northwest corner of the study area, with sediment types variable. Sand, sandy gravel and muddy sand dominate on the UK side of the
median line.
We developed an approach for assessing the effects of different frequencies of trawling disturbance (effoort, as derived from satellite
monitoring) on the size structure of a soft-sediment benthic community using the model described by Duplisea et al (2002). The model
contains 37 state variables defined on the basis on body size and faunal group (5 meiofauna, 16 soft-bodied macrofauna and 17 hardbodied macrofauna). The growth of population biomass in each of these compartments was modelled independently using LotkaVolterra competition equations, making the assumptions that (1) soft and hard bodied marcofauna were in competition, (2) meiofauna
did not compete with either of the macrofauna groups. The carrying capacity of each size class was taken as 0.1 x the weight of the
largest organisms in the size class, reducing it by a factor of 600 for meiofauna and 5 for soft-bodied organisms to correspond to
relative biomasses described in the literature (Duplisea et al., 2000)
The simulations were run to a steady state for 1000 time steps with a step size of 30 days for the frequencies of observed trawling
disturbance under two management scenarios; a temporary closed area imposed each year and no such closure. The maximum range
of trawling frequencies considered were from 0 to 15 times year –1. Total biomass and production of animals >0.04g and >1.75g wet
weight year-1 were predicted.
9.3. Results and Discussion
We were unable to apply the satellite effort data to the model as an embargo was placed on the use of satellite data in November 2001.
However, the general behaviour of the model was clear from our simulations. This shows that fishery management measures (such as
the cod box) that do not reduce total effort, but do lead to effort displacement and increased homogeneity of trawling disturbance (e.g.
temporary closed areas), may have adverse effects on the persistence of intermediate and large size classes of benthic fauna. This is
because the opening and closing of an area will increase the homogeneity of the demersal fishing disturbance over the course of a
year.
18
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
10. Future work.
The programme of research funded under MF0716 has effectively addressed most of the key concerns that relate to the impacts of
trawling disturbance in the North Sea, Irish Sea and English Channel. The research that was commissioned has allowed CEFAS to
advise DEFRA on most of they key issues that relate to the impacts of trawling on benthic community structure. The exception is in
understanding the effects of trawling disturbance on biogeochemical process and pelagic-benthic interactions. However, in the wider
context of the developing ecosystem approach to fishery management, there are now more pressing issues where science is needed
to support advice. The relevant issues were identified during the external review of the MF07 programme, and it was concluded that
future research should address the following:
Development of Indicators of Ecological Impact: The aim of this area of research is to develop suitable indicators of ecological impact
of fishing, including EcoQOs. In order to develop appropriate indicators there is a need to better understand the relationships between
beam and otter trawling intensity, diversity, productivity, trophic structure and functional processes in the marine ecosystem. Indicators
should reflect the main properties of the system and will have to be used by DEFRA in a practical sense, for example, to test the
impacts of closed areas or the recovery of the ecosystem following the adoption of mitigating measures. The development of indicators
will require the integration of theoretical and field studies.
The Development of Predictive Models through Case Studies: Through an integrated study of an important North Sea gadoid fishery
develop models which help predict the impact of fishing on the ecosystem. Models must be comprehensively validated with field data.
Use the models to assess the likely response of the ecosystem under different management regimes.
Analysis of Historical Data Sets: It is likely that historical data sets will help to evaluate the long-term impact of beam and otter trawling
on the ecosystem. The aim will be to produce time series which help assess the impact of trawling, taking account of natural variability.
Ideally studies should formulate a priori hypotheses of the impact and should be used to test the validity of EcoQO's and indices of
vulnerability that are proposed for marine species.
Mitigating Studies: It is important to understand how the impact of fishing can be reduced through the adoption of appropriate
management measures. Research could include for example the development of alternative fishing methods which do not involve the
use of heavy trawl gear such as trawl doors, tickler chains, rakes etc, the testing of acoustic equipment to aid target fish identification,
or the assessment of novel fishing methods.
Research in these areas has now been commissioned by DEFRA.
11. References
Bergman, M. J. N. & van Santbrink, J. W. (2000). Mortality in megafaunal benthic populations caused by trawl fisheries on the Dutch
continental shelf in the North Sea in 1994. ICES Journal of Marine Science 57, 1321-1331.
Beukema, J. J. (1987). Influence of the predatory polychaete Nephtys hombergii on the abundance of other polychaetes. Marine
Ecology Progress Series 40, 95-101.
Brey, T. (1999). Growth performance and mortality in aquatic macrobenthic invertebrates. Advances in Marine Biology 35, 153-223.
Clarke, K. R. & Ainsworth, M. (1993). A method of linking multivariate community structure to environmental variables. Marine Ecology
Progress Series 92, 205-219.
Clarke, K. R. & Warwick, R. M. (1994). Change in marine communities: an approach to statistical analysis and interpretation. Natural
Environment Research Council, Plymouth, UK.
Collie, J. S., Escanero, G. A. & Valentine, P. C. (1997). Effects of bottom fishing on the benthic megafauna of Georges Bank. Marine
Ecology Progress Series 155, 159-172.
Kerr, S.R. & Dickie, L.M. (2001). The biomass spectrum. Colombia University Press, New York.
Duplisea, D. E., Jennings, S., Malcolm, S. J., Parker, R. & Sivyer, D. (2001). Modelling the potential impacts of bottom trawl fisheries
on soft sediment biochemistry in the North Sea. Geochemical Transactions 14, 1-6.
Gaspar, M. B., Richardson, C. A. & Monteiro, C. C. (1994). The effects of dredging on shell formation in the razor clam Ensis siliqua
from Barrinha, southern Portugal. Journal of the Marine Biological Association of the United Kingdom 74, 927-938.
Gilkinson, K., Paulin, M., Hurley, S. & Schwinghamer, P. (1998). Impacts of trawl door scouring on infaunal bivalves: results of a
physical trawl door model/ dense sand interaction. Journal of Experimantal Marine Biology and Ecology 224, 291-312.
Hall, S. J. (1994). Physical disturbance and marine benthic communities: life in unconsolidated sediments. Oceanography and Marine
Biology Annual Review 32, 179-239.
Hall, S. J. (1999). The effects of fishing on marine ecosystems and communities. Blackwell Science, Oxford.
Jennings, S., Warr, K. J., Greenstreet, S. P. R. & Cotter, A. J. (2000). Spatial and temporal patterns in North Sea fishing effort. In
Effects of fishing on non-target species and habitats: biological conservation and socio-economic issues (ed. M. J. Kaiser and S. J.
de Groot), pp. 3-14. Blackwell Science, Oxford.
Jennings, S., Pinnegar, J. K., Polunin, N. V. C. & Warr, K. J. (2001). Impacts of trawling disturbance on the trophic structure of benthic
invertebrate communities. Marine Ecology Progress Series 213, 127-142.
Kaiser, M. J. & de Groot, S. J. (2000). The effects of fishing on non-target species and habitats: biological, conservation and socioeconomic issues. Blackwell Science, Oxford.
Kaiser, M. J. & Spencer, B. E. (1996). The effects of beam-trawl disturbance on infaunal communities in different habitats. Journal of
Animal Ecology 65, 348-358.
19
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
Kroncke, I., Dippner, J. W., Heyen, H. & Zeiss, B. (1998). Long-term changes in macrofaunal communities off Norderney (East Frisia,
Germany) in relation to climate variability. Marine Ecology Progress Series 167, 25-36.
Lindeboom, H. J. & de Groot, S. J. (1998). The effects of different types of fisheries on the North Sea and Irsh Sea benthic ecosystems.
Netherlands Institute of Sea Research, Texel.
Millner, R. S. & Whiting, C. L. (1996). Long-term changes in growth and population abundance of sole in the North Sea from 1940 to
the present. ICES Journal of Marine Science 53, 1185-1195.
Peters, R. H. (1983). The ecological implications of body size. Cambridge University Press, Cambridge.
Preston, T. (1992). The measurement of stable isotope natural abundance variations. Plant Cell and Environment 15, 1091-1097.
Preston, T. & Owens, N. J. P. (1983). Interfacing an automatic elemental analyser with an isotope ratio mass spectrometer: the
potential for fully automated total nitrogen and nitrogen-15 analysis. Analyst 108, 971-977.
Ramsay, K., Kaiser, M. J. & Hughes, R. N. (1998). Responses of benthic scavengers to fishing disturbance by towed gears in different
habitats. Journal of Experimental Marine Biology ad Ecology 224, 73-89.
Reid, P. C., Edwards, M., Hunt, H. G. & Warner, A. J. (1998). Phytoplankton change in North Atlantic. Nature 391, 546.
Rijnsdorp, A. D. & van Leeuwen, P. I. (1996). Changes in growth of North Sea plaice since 1950 in relation to density, eutrophication,
beam-trawl effort and temperature. ICES Journal of Marine Science 53, 1199-1213.
Rijnsdorp, A. D., Piet, G. J. & Poos, J. J. (2001). Effort allocation of the Dutch beam trawl fleet in response to a temporary closed area
in the North Sea. International Council for the Exploration of the Seas, Committee Meeting CM 2001/ N: 01.
Schubert, A. & Reise, K. (1986). Predatory effects of Nephtys hombergii on other polychaetes in tidal sediments. Marine Ecology
Progress Series 34, 117-124.
Schwinghamer, P. (1988). Influence of pollution along a natural gradient and in a mesocosm experiment on biomass-size spectra of
benthic communities. Marine Ecology Progress Series 46, 199-206.
Swift, D. J. (1993). The macrobenthic infauna off Sellafield (North-eastern Irish Sea) with special reference to bioturbation. Journal of
the Marine Biological Association of the United Kingdom 73, 143-162.
Witbaard, R. & Klein, R. (1994). Long-term trends on the effects of the southern North Sea beamtrawl fishery on the bivalve mollusc
Arctica islandica L. (Mollusca, bivalvia). ICES Journal of Marine Science 51, 99-105.
12. List of publications
The following peer reviewed scientific papers, book chapters and reviews were produced during the MF0716 research project. These
provide a full account of the research that was conducted.
Duplisea, D. E., Jennings, S., Malcolm, S. J., Parker, R. & Sivyer, D. (2001). Modelling the potential impacts of bottom trawl fisheries
on soft sediment biochemistry in the North Sea. Geochemical Transactions 14, 1-6.
Duplisea, D. E., Warr, K. J., Dinmore, T. A. & Jennings, S. (submitted). A size-based model to predict the impacts of bottom trawling on
benthic community structure. Canadian Journal of Fisheries and Aquatic Sceinces
Jennings, S. & Kaiser, M. J. (1998). The effects of fishing on marine ecosystems. Advances in Marine Biology 34, 201-352.
Jennings, S., Dinmore, T. A., Duplisea, D. E., Warr, K. J. & Lancaster, J. E. (2001a). Trawling disturbance can modify benthic
production processes. Journal of Animal Ecology 70, 459-475.
Jennings, S., Pinnegar, J. K., Polunin, N. V. C. & Warr, K. J. (2001b). Impacts of trawling disturbance on the trophic structure of benthic
invertebrate communities. Marine Ecology Progress Series 213, 127-142.
Jennings, S., Pinnegar, J. K., Polunin, N. V. C. & Warr, K. J. (2002). Linking size-based and trophic analyses of benthic community
structure. Marine Ecology Progress Series 226, 77-85.
Jennings, S., Nicholson, M.D., Dinmore, T.A. & Lancaster, J.E. (submitted) The effects of chronic trawling disturbance on the
production of infaunal communities. Marine Ecology Progress Series
Kaiser, M. J. (1998). Significance of bottom-fishing disturbance. Conservation Biology 12, 1230-1235.
Kaiser, M. J., Armstrong, P. J., Dare, P. J. & Flatt, R. P. (1998a). Benthic communities associated with a heavily fished scallop ground
in the English Channel. Journal of the Marine Biological Association of the United Kingdom 78, 1045-1059.
Kaiser, M. J., Edwards, D. B., Armstrong, P. A., Radford, K., Lough, N. E. L., Flatt, R. P. & Jones, H. D. (1998b). Changes in
megafaunal benthic communities in different habitats after trawling disturbance. ICES Journal of Marine Science 55, 353-361.
Kaiser, M. J., Ramsay, K. & Hughes, R. N. (1998c). Can fisheries influence interspecific competition in sympatric populations of hermit
crabs. Journal of Natural History 32, 521-531.
Kaiser, M. J., Cheney, K., Spence, F. E., Edwards, D. B. & Radford, K. (1999). Fishing effects in northeast Atlantic shelf seas: patterns
in fishing effort, diversity and community structure. VII. The effects of trawling disturbance on the fauna associated with the
tubeheads of serpulid worms. Fisheries Research 40, 195-205.
20
Project
title
Can demersal fishing cause long-term changes in benthic
community structure
MAFF
project code
MF0716
Kaiser, M. J. & de Groot, S. J. (eds) (2000). The effects of fishing on non-target species and habitats: biological, conservation and
socio-economic issues. Blackwell Science, Oxford.
Kaiser, M. J. & Jennings, S. (2002). Harvesting with due care and attention: an ecosystem based perspective on conserving targeted
and non-targeted species. In Conservation of Exploited Species (ed. J. D. Reynolds, G. M. Mace, K. H. Redford and J. G.
Robinson), pp. 343-369. Oxford University Press.
Kaiser, M. J. & Jennings, S. (2002). Ecosystem effects of fishing. In Handbook of Fish and Fisheries (ed. P. J. Hart and J. D.
Reynolds). Blackwell Science, Oxford.
Kaiser, M. J., Ramsay, K., Richardson, C. A., Spence, F. E. & Brand, A. R. (2000). Chronic fishing disturbance has changed shelf sea
benthic community structure. Journal of Animal Ecology 69: 494-503.
Ramsay K., Kaiser M.J., Richardson C.A., Veale L.O., Brand A.R. (2000a) Can shell scars on dog cockles Glycymeris glycymeris L.)
be used as an indicator of fishing disturbance? Journal of Sea Research 43: 167-176.
Ramsay K., Turner J.R., Vize S.J., Richardson C.A. (2000b) A link between predator density and arm loss in the starfish Marthasterias
glacialis and Asterias rubens. Journal of the Marine Biological Association of the United Kingdom 80: 565-566.
Ramsay, K., Kaiser, M. J., Rijnsdorp, A. D., Craeymeersch, J. A. & Ellis, J. (2000c). Impacts of trawling on populations of the
invertebrate scavenger Asterias rubens. In The effects of fishing on non-target species and habitats: biological, conservation and
socioeconomic issues (ed. M. J. Kaiser and S. J. de Groot), pp. 151-162. Blackwell Science, Oxford.
Schratzberger, M., Dinmore, T. A. & Jennings, S. (2002). Impacts of trawling disturbance on the biomass and community structure of
meiofauna. Marine Biology 14: 83-93.
Schratzberger, M & Jennings, S. (submitted) Impacts of chronic trawling disturbance on meiofaunal communities. Marine Biology
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