Download important considerations for the management of demersal fisheries

DEPARTMENT for ENVIRONMENT, FOOD and RURAL AFFAIRS
Research and Development
CSG 15
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
Essential fish habitat: important considerations for the management of
demersal fisheries
DEFRA project code
MF0805
Contractor organisation
and location
Dr Michel J. Kaiser
School of Ocean Sciences
Menai Bridge, Anglesey
Wales LL59 5AB
Total DEFRA project costs
Project start date
£ 302 969
02/01/01
Project end date
31/01/04
Executive summary (maximum 2 sides A4)
Anthropogenic activities such as bottom fishing, aggregate extraction and the construction of offshore wind farms are
capable of changing seabed habitats and hereby altering essential fish habitats such that they are no longer suitable to
sustain the fish and benthic species associated with them. In this research project, we identified areas that consistently
attracted high densities of six species of commercially important fish (whiting, cod, haddock, sole, lemon sole, plaice) over
a period of 9 years using data from the annual national groundfish surveys. These fish distributions were compared with
information gleaned from the fishing industry through questionnaires that asked specific questions about fish distribution
and habitat. According to habitat selection theories such as ‘ideal free distribution’ theory, we postulated that areas that
consistently attract high densities of fish had habitat features that were indicative of habitat quality in terms of provision of
prey or shelter from predation. Whiting and plaice had the greatest number of stations that consistently harboured high
densities of fish compared with all other species considered in this study. This suggests that whiting and plaice may be
less site-selective than, for example, cod and lemon sole that appeared to be much more site specific, although in the
case of the former this may be largely attributed to their low population size at present. We paired each high density
station with another station that consistently supported lower densities of the same species in close proximity and of
comparable depth to enable a comparison of habitat features between such sites.
Sites that harboured consistent high densities of whiting and cod were characterized by a wide spectrum of habitat
types ranging from homogenous muddy sediments to sand to more complex habitats with a large proportion of stones,
shell gravel and sessile epifauna. These stations appeared to be grouped by geographical distance or a gradient in
sediment particle-size rather than by fish density. Although we identified discreet epibenthic assemblages that were linked
with abiotic habitat features, we were unable to detect universal differences between stations of high and lower fish
density in terms of both, their physical and biological habitat components. Nevertheless, for each high – lower density site
pairing, habitat characteristics were significantly different. What this suggests is that habitat features do play a role in the
spatial distribution of fish abundance, but that deterministic habitat features are not consistent on a regional basis, but
vary at a much more localised scale. This has important implications for the scale at which any habitat focussed or
ecosystem based management practices are exercised.
Similarly, we only detected marginal differences in the diet of whiting from high and lower density stations although
there were clearly local differences. From our analysis, it would appear that Irish Sea whiting and cod were capable of
CSG 15 (Rev. 6/02)
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Project
title
Essential fish habitat: important considerations for the
management of demersal fisheries
DEFRA
project code
MF0805
utilising a wide range of habitats with habitat specificity occurring at a more localised scale, a conclusion that was
corroborated by statements from local fishers. Piscivorous species of fish such as large cod and whiting may be less
reliant on specific benthic prey organisms compared with primarily benthivorous fish. However, an accurate interpretation
of our results was complicated by the large geographic scale of the study and the resulting range of habitat types
encountered, all of which were characterized by different benthic communities and ultimately prey organisms.
Adverse weather conditions, unsuitable seabed topography and unexpectedly low catch rates at certain stations
prevented us from sampling sufficient numbers of fish at an adequate number of stations for haddock, cod, sole and
lemon sole. We were therefore unable to draw any firm conclusions about their habitat requirements in terms of feeding
habitat. This highlights a major problem of trying to undertake research on species whose population size is currently so
low that it is now extremely difficult to sample them adequately.
While stomach fullness of whiting was significantly higher at high density stations there were no significant differences
in the overall condition index of fish sampled from high and lower density stations. This suggests that fish condition was
averaged out as the result of local feeding activity that varies across good quality and poorer quality feeding habitats or
alternatively that fish density occurs in relation to the available food resources as predicted by Ideal Free Distribution
theory at a local scale. These findings are important because they imply that relative habitat quality varies on a large
scale, with fish aggregating in the best habitat that is available within a local area (100 km²). Large-scale variation in
habitat quality is evident from the present study and is an important consideration from a fish stock/habitat management
perspective. The regional differences highlight the importance of choosing appropriate sampling scales that can take local
variation into account when describing the diet of a widely distributed species. The importance of local differences in fish
condition may increase their vulnerability to increases in fishing mortality and hence reduce further their reproductive
output.
Very similar trends were found for the three flatfish species in the English Channel when we used the data collected
during our research cruise. There appeared to be no significant differences in the species composition and diets of fish
sampled from high and lower density stations although there were significant local differences. More specific habitat
preferences emerged, however, when a larger-scale dataset on epifauna from groundfish surveys and sediment data
from the British Geological Survey were used in conjunction with our more detailed but smaller-scale data. Stations of
high plaice and sole density, for example, appeared to be generally restricted to relatively shallow areas, with low
salinities that are typical of near-shore conditions whereas high density stations of lemon sole were distinguished by
greater depths, higher salinities and lower temperatures. In terms of sediment preferences, plaice appeared to prefer
sediments with a high sand content and only small quantities of stones and shells whilst they avoided sediments with a
high gravel or mud content. Plaice density was positively correlated with the density of emergent epifauna, which could be
due to the fact that complex habitats provide a multitude of niche spaces for mobile epifauna, that form an important part
of their diet. By contrast, stations of high sole density were found over a wider range of sediment types including
sediments with high mud content and there was a negative correlation between emergent epifauna and sole density,
although this was not significant. This species relies on tactile and chemosensory cues to detect prey via papillae located
on the ventral side of the mouth and it feeds primarily on infaunal polychaetes and molluscs. Since epibenthic
invertebrates do not form an important component of the diet of sole, emergent epifauna may interfere with prey capture
or may simply be auto-correlated with a greater proportion of unsuitable prey-types. High density stations of lemon sole
were more restricted in number and characterized by a very different combination of environmental parameters. They
preferred sediments with a high content of sand and gravel and were found at greater depths, higher salinities and lower
temperatures. High density stations were also distinguished by larger quantities of stones and shells compared with
stations of lower densities of lemon sole. Similar to our results for plaice, there was a positive correlation between lemon
sole density and the biomass of emergent epifauna.
Our results indicate differential vulnerability to habitat changes among different fish species. The largest number of
high density stations was found for plaice and whiting, which indicates that they can draw on a larger total area compared
with more selective species such as lemon sole and cod which had a more limited number of high density sites and may
thus be more vulnerable to modification of these. In other words, species such as whiting and plaice that are less habitat
specific in the adult stage are likely to be more resilient to alteration of proportion of their habitat. Although plaice and
whiting were selective for certain prey species it would appear that they generally had a wide feeding range and would
thus be adaptable to changes in the availability of their prey. The removal of habitat elements that add to structural
complexity may increase the risk of predation for cod and to a lesser extent also for whiting. However, it should be noted
that unlike corals or slow-growing sponges, the epifaunal organisms that contributed to structural complexity at our
stations may be more ephemeral in nature and adapted to life in dynamic environments. However, this may be a result of
previous habitat modification. In terms of specific locations of potential ‘essential fish habitats’ for the species under study,
it would certainly appear that Belfast Lough represented an important nursery ground for cod and that an area offshore
from Strangford Lough and Point Lynas seemed to play an important role for adults of this species. Certain areas off the
French coast (Boulogne) in the English Channel with its large mats of tube worms appeared to be important habitats for
all three species of flatfish, although such habitats are ephemeral in nature. This tube worm habitat appeared to be of
high quality as reflected by large mean body size and abundance of fish and the prevalence of tube worms in the diet.
It is clear from the present study that the scale of national surveys as presently undertaken is too coarse to quantify
gradients in fish habitat quality that occur on a much more localised scale. In order to address this issue adequately, a
much larger-scale regional study would be required undertaken collaboratively with UK Government Agencies.
CSG 15 (Rev. 6/02)
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Project
title
Essential fish habitat: important considerations for the
management of demersal fisheries
DEFRA
project code
MF0805
Scientific report (maximum 20 sides A4)
DEFRA has the responsibility for ensuring that populations of commercially important species are harvested in a
sustainable manner while protecting the marine environment from adverse effects that would lead to a reduction in
biodiversity. Human activities such as bottom fishing, aggregate extraction and the construction of offshore wind parks are
capable of changing seabed habitats by reducing their complexity thereby altering essential fish habitats such that they
are no longer suitable to sustain the fish and benthic species associated with them. This research programme sought to
provide DEFRA with the scientific information that would enable it to assess the habitat requirements of three species of
roundfish and three species of flatfish. Once these requirements have been identified it would be possible to assess the
extent to which commercial species may benefit from protection of their habitat that might otherwise be damaged by
human activities.
1) IDENTIFICATION OF AREAS OF HIGH FISH DENSITIES:
Habitat selection has been described in the context of concepts such as ‘Ideal Free Distribution’ (IFD) and isoleg theory.
Many temperate groundfish species move freely between habitats and are assumed to select areas of the seabed that
best meet their requirements at the time. Hence, they meet some of the basic prerequisites of IFD theory: if all
competitors are equal and free to enter and leave an area, individuals will occupy the most suitable habitat. However,
they will ‘spill’ into less suitable habitats, as populations grow and crowding effects emerge or if habitat quality is altered.
Areas of persistent high fish density could therefore be indicative of consistent high resource value and habitat quality for
a particular species.
In addition to the anticipated dataset from CEFAS beam trawl surveys on fish abundance in different areas of the Irish
Sea and English Channel, we were also able to gain access to a similar dataset from DARD in Northern Ireland which
improved the resolution of the sampling grid and may also provide more accurate data for gadoids as otter trawls were
used in those surveys which are known to be more selective for gadoids.
The development of the final robust method to identify such sites was a long process which involved consultation with
several other scientists over the course of a year. In our first approach (used to identify potential sites in the Irish Sea),
the abundance of each fish species for each station per year was ranked and a mean rank over time (per station)
calculated to identify potential EFHs for future habitat survey. The standard deviation was used to assess the consistency
of the ranking over time. This process yielded a list of candidate stations for each species of fish, from which we selected
a number of sites for further survey (Appendix 17). We paired each station known to support consistently high densities of
fish with a complimentary station known to support lower densities in as close a vicinity as possible and within a similar
water depth to enable comparison between the habitat features of high and lower density stations. Our initial cruise plan
also included stations that were distinguished by high haddock densities in the Solway Firth, which had to be abandoned
due to particularly poor weather conditions in this area during the research cruise.
Ongoing discussion resulted in our final methodology (Appendix 24) which was then applied to identify candidate
sites of EFHs in the English Channel based on CEFAS beam trawl surveys (1990-1998). Fish below the minimum landing
size were excluded from the analysis. To delineate stations with consistently high densities of fish from those that vary
from medium to low densities, the relationship between mean percentage density and its standard deviation was
examined. In short, stations with a high mean percentage fish density over 9 years and a relatively low standard deviation
qualified as candidate sites for potential essential fish habitats. Again, we paired each station with consistently high fish
densities with a complimentary station known to support lower densities in as close a vicinity as possible and within a
similar water depth to enable comparison between the habitat features of high and lower density stations.
For more details of this research readers are referred to our publications in Appendices 17 and 24 .This work was
presented as an oral paper at the winter meeting of the British Ecological Society and as poster at ‘Fishing and Benthic
Habitats’ in Tampa in 2002.
2) IDENTIFICATION OF HABITATS THROUGH CONSULTATION WITH FISHERMEN
Our project was first introduced to the fishing community by publication of an article in ‘Fishing News’ in 2001. A
questionnaire form and maps were designed to which a total of 75 fishers responded in interviews and mail shots.
Interviews were conducted at the fishing exhibition Glasgow 2001 and 2002, Lowestoft Fish Fayre (2001), South Western
Fish Producers’ Organization in Brixham (2001), Fishing Cooperative’s exhibition in Newcastle (2001) and the Fleetwood
Fish Forum (2001). Questionnaires were also sent to various Sea Fisheries Committees, fishers’ associations and
producer organizations. Many of the respondents targeted fish outside the Irish Sea and English Channel or other
species, such that the sample size of responses concerning gadoids in the Irish Sea and flatfish in the English Channel
was relatively small (Appendix 17). Fishers were invited to plot grounds that they perceived as important for their target
species on standard maps. Such grounds may be characterized by particular features or the presence of prey organisms
and therefore harbour high densities of fish and could therefore be indicative of EFHs. The hand-drawn plots were
digitized as a chart in a geographic information system (GIS) suitable for comparison with maps that showed mean ranks
of fish abundance generated from groundfish surveys. A comparison of the areas of seabed highlighted by fishers and the
CSG 15 (Rev. 6/02)
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Project
title
Essential fish habitat: important considerations for the
management of demersal fisheries
DEFRA
project code
MF0805
observations made on groundfish surveys were broadly compatible for all three species of gadoids examined (Appendix
17). Both methods indicated important grounds for cod and whiting off north Wales, the Ribble estuary, Solway Firth,
north of Dublin and Belfast Lough. However, they also highlighted areas that are currently not sampled by groundfish
surveys (Appendix 17).
We developed two new methods to validate fishers’ information. To compare the fishers’ verbal habitat descriptions
(in terms of seabed types) with the occurrence of seabed types in the areas that they plotted on charts, they were overlaid
with sediment maps from the British Geological Survey. We calculated the percentage area covered by mud, sand, hard
grounds and gravel/shingle in areas that had been plotted as important fishing ground for each species of fish. There
were some discrepancies between the ground types that fishers stated as important for each fish species and the
sediment composition of grounds plotted on charts (Appendix 18). Large areas of the cod and haddock fishing grounds
were characterized by sand although this sediment type was mentioned less frequently in questionnaires. Similarly, the
whiting fishing grounds comprised a much higher proportion of gravel/shingle than was stated in questionnaires.
Generally, hard grounds were named as important habitat feature more often than would appear from the features of
plotted fishing grounds which might be explained with differences between the BGS-converted sediment classification and
the categories used by fishers. For example, gravel/shingle may have been termed hard grounds by some respondents.
Also muddy sand (here termed as ‘mud’) may have been classed as sand by some fishers. Alternatively, the resolution of
fishers’ knowledge may exceed the sampling resolution of the BGS data, which rely on interpolation between widely
interspersed sample points.
Fishers were invited to give either their name or the name of their vessel, which enabled us to cross-check the areas
plotted by individual fishers using records of named vessel sightings collated from fishery protection overflights. Using
these data, we calculated the number of sightings for each fishing vessel (whose identity had been disclosed) that
corresponded with the areas plotted by the respective owner. The majority of vessel sightings by aircraft did not match the
areas plotted by fishers (Appendix 18). However, fishing restrictions, adverse weather conditions and seasonal variation
of fish stocks may have forced fishers to operate outside their favoured areas on the (few) occasions that they had been
recorded by aircraft. Fishers provided biological observations that were consistent between several independent sources,
for example the occurrence of haddock over brittlestar beds. We conclude that fishers’ knowledge is a useful supplement
to existing datasets that can better focus more detailed EFH studies. More extensive collaborative research programmes
may not only foster a relationship of mutual trust between fisheries scientists and fishers but also extend our ecological
knowledge considerably.
Our results were presented as a poster at a conference ‘Putting Fishers’ Knowledge to Work’ in Vancouver in August
2001 and at ‘Fishing and Benthic Habitats’ in Tampa in 2002 (Appendices 16 and 18). This research has been published
in Fisheries Research and awaits publication as a peer-reviewed chapter in a book by the American Fisheries Society due
to be published in summer 2004 (Appendices 17 and 18).
3) IDENTIFICATION OF DISCRETE DEMERSAL FISH ASSEMBLAGES
As stated in earlier reports, we did not attempt an analysis of the CEFAS data base for discrete demersal fish
assemblages because Kaiser et al. and Ellis et al. have recently published papers on demersal fish assemblages in the
Irish Sea and English Channel using the same dataset:
Ellis JR, Armstrong MJ, Rogers SI & M Service. 2002. The distribution, structure and diversity of fish assemblages in the
Irish Sea. In: Marine Biodiversity in Ireland and adjacent waters. Ed. by JD Nunn. Ulster Museum, Belfast, 93-107.
Ellis JR, Rogers SI & SM Freeman. 2000. Demersal Assemblages in the Irish Sea, St George’s Channel and Bristol
Channel. Estuar Coast Shelf Sci, 51: 299-315.
Kaiser MJ, Rogers SI & JR Ellis. 1999. Importance of benthic habitat complexity for demersal fish assemblages. In: Fish
habitat: essential fish habitat and rehabilitation. Ed. by LR Benaka. American Fisheries Society, Hartford, Connecticut,
212-223.
4) RELATIONSHIP BETWEEN FISH ASSEMBLAGE, BENTHIC ASSEMBLAGES & HABITAT
a) Irish Sea. Eight study sites were selected on the basis that they consistently harboured high or lower densities of
whiting and cod and may thus be indicative of their habitat requirements. These sites were surveyed using a suite of
benthic sampling techniques during a research cruise in August 2001 (see Appendix 19 for sampling procedure). Owing
to adverse weather conditions we were unable to sample an adequate number of stations characterized by high haddock
densities and collected only very few haddock at the two stations which were assumed to support high densities of
haddock. Furthermore, at one of these stations (W5), we had to discontinue trawling after damaging the net and could
only take two quantitative grab samples as the seabed appeared to be very rough indeed. No analysis was therefore
undertaken for this species.
Our stations were characterized by a wide spectrum of habitat types ranging from homogenous muddy sediments
over sandy to more complex habitats with a large proportion of stones, shell gravel and sessile epifauna (Appendices 1
and 2). Our stations appeared to be grouped by a gradient in sediment particle size or geographic distance rather than by
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Project
title
Essential fish habitat: important considerations for the
management of demersal fisheries
DEFRA
project code
MF0805
fish density. Although we identified discreet epibenthic, infaunal and demersal fish assemblages that were linked with the
habitat features at each station, we were unable to detect universal differences between stations of high and low cod and
whiting density in terms of both, their physical and biotic habitats (Appendices 3 and 19). However, sites of high cod and
whiting density were characterized by a significantly higher epifaunal species diversity (Appendix 19). The species
composition of demersal fish assemblages was strongly correlated with the composition of epifaunal and infaunal
communities (Appendix 3).
The size spectra of fish indicated that some of the stations posed nursery grounds although two structurally complex
habitats off the north Welsh coast, harboured larger cod and whiting (Appendix 19). We calculated the condition index of
fish as it may be indicative of habitat quality. Cod condition was negatively correlated with shell gravel and stones,
whereas whiting condition was negatively correlated with median grain size, total organic carbon content and % silt/clay
(Appendix 19). These results are in disagreement with the general consensus of numerous essential fish habitats studies
from north America which have shown that juvenile cod seek refuge in structurally complex habitats to avoid predation.
While offering shelter, however, more complex habitats may either contain less suitable prey or impede prey detection
and capture leading to a lower condition index of fish in such habitats. Given a choice of habitats in a laboratory
experiment, whiting preferentially spent most time over sand, then gravel and least time over a habitat with emergent
structures (Appendix 15). The introduction of a predator stimulus increased the preference for the sand habitat for large
whiting, whereas small whiting had an increased preference for the habitat with emergent structures. This in agreement
with the fact that whiting from sandy habitats off the Ribble had the highest condition index (Appendix 20). To validate
these results which are based on eight stations only and to increase the spatial scale and hence power, it may proof
useful to calculate the mean condition index of fish per station and year for all the stations sampled by groundfish surveys
and test for a correlation with sediment type as obtainable from the British Geological Survey.
From our analysis, it would appear that Irish Sea whiting and cod are not very habitat-selective, a conclusion that was
corroborated by statements from local fishers (Appendix 17) and the fact that gadoids are known to cover large distances
and undergo large-scale migration. Furthermore, our initial analysis of fish abundance data from groundfish surveys
showed that whiting (and plaice) had the highest number of stations with high means and a low standard deviation which
therefore meet the criterion for possible essential fish habitats (Appendix 24). This suggests that whiting are less siteselective than, for example, cod and lemon sole who had the lowest number of such sites (Appendix 24). Also,
piscivorous fish such as larger cod and whiting may be less reliant on specific benthic prey organisms compared with
primarily benthivorous fish as has been shown for other fish species. However, there are other possible explanations.
Inter-specific competition may have affected habitat choice as habitat selection of North Sea cod is affected by haddock
densities, such that cod occupied less suitable habitats when haddock abundances increased in its prime habitat.
Moreover, one of the fundamental population equilibrium assumptions of IFD theory may no longer apply given the
heavily depleted nature of fish stocks, such that there is insufficient competition among so few fish to lead to such
behaviour in the field at the scale of sampling (IFD may still apply at a local scale). Furthermore, sites distinguished by
high fish densities may not necessarily indicate high habitat quality, as they may constitute sink habitats that were
selected due to intra- and interspecific competition. Practically this could mean that we may have missed important
habitats as a result of our site selection methodology that was informed by bottom trawl surveys and interviews with
fishers. As a result, our station choice was restricted to areas that can be sampled by trawl gear. Although these stations
may have been more suitable for fish than other ‘trawlable’ sites, they may not necessarily constitute the most suitable
habitat for fish. For example, a high density station could have been adjacent to an essential habitat such as a rocky reef
that was saturated with fish, such that some fish spilled over into the ‘second best’ habitat that is amenable to sampling
with trawl gear. This highlights one of the inherent difficulties in the identification of essential fish habitats in the field.
SCUBA divers, sea anglers and commercial fishers that use static gears (e.g. long liners) are able to sample fish from a
wider range of habitats. In future research, it may prove beneficial to base the initial site selection on information from
bottom trawl surveys and trawlers as well as divers, sea anglers and the static-gear sector. From a fisheries manager’s
point of view such an approach may be considered too time-consuming and irrelevant since such habitats are unlikely to
be targeted and thus affected at a large-scale by commercial trawling.
A more detailed analysis and discussion can be found in Appendix 19.
b) English Channel: The relationship between flatfish and benthic habitats/assemblages was explored during a cruise in
the English Channel in August 2002. Fourteen sites of high and lower flatfish density were surveyed using a suite of
benthic sampling techniques (see Appendix 21 for sampling procedure). Stomachs of plaice, sole and lemon sole were
collected during this and a simultaneous CEFAS cruise. We found a significant relationship for plaice and sole between
the fish densities expected based on our analysis of groundfish survey data (Appendix 24) and our own catches of fish
during the cruise (Appendix 25). For lemon sole no correlation was found as it was caught only at two stations. However
these stations were the two stations with the highest lemon sole density during groundfish surveys. These results add
confidence to our choice of stations.
A total of 129 taxonomic entities (28 fish & 101 invertebrates) were recorded from 2-m beam trawl tows. The species
composition of demersal fish was significantly correlated with epifaunal species composition (Appendix 6). Similar to our
results from the Irish Sea, we were unable to detect significant differences overall between high and low density stations
CSG 15 (1/00)
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Project
title
Essential fish habitat: important considerations for the
management of demersal fisheries
DEFRA
project code
MF0805
for plaice, lemon sole and sole, although there were significant differences in the species composition belonging to
different stations (Appendices 6 and 21). This applied to both epi- and infaunal assemblages (Appendix 6). Again, this
could be due to the wide spectrum of habitat types encountered, each of which harboured different assemblages.
CEFAS has regularly taken sub-samples of the benthic fauna from their 4-m beam trawl tows, undertaken during their
annual beam trawl surveys at fixed stations throughout the English Channel. We included this dataset and data from the
British Geological Survey to increase our sampling size for given sediment types and depths in the hope that this would
enable us to delineate general habitat preferences over a wider geographical scale and thus add power to our results.
Details of the analysis and the datasets used can be found in Appendix 25. The results from this larger-scale analysis
indicate that each of the three flatfish species showed slightly different habitat preferences.
Plaice. Stations of high plaice density, for example, appeared to be generally restricted to relatively shallow areas, with
low salinities that are typical of near-shore conditions (Appendix 25). The sediments at these stations were characterized
by a high sand content and had relatively small amounts of stones and shells. Stations ‘avoided’ by plaice, by contrast,
were distinguished by greater depth and lower salinities. Such sediments had either a high gravel or mud content.
Previous researchers have also found plaice primarily over sandy habitats in the Irish Sea. The preference of plaice for
sediments that have a high sand content may have several reasons that are related to their burial behaviour as well as
prey detection and capture efficiency. Although adult flatfish are known to be capable of burial in most sediment types,
plaice may find it easier and hence more energy-efficient to bury in loosely packed sand compared with gravel or compact
mud. Furthermore, a visual predator such as plaice relies on good visibility for prey detection and capture. Sediments with
a high mud content, however, could impair foraging success since turbid water or sediments stirred up by the prey
organism itself may allow it to evade capture and reduce hunting success. Capture of infauna may also be more
successful in sand, as easier to excavate.
Our results also indicate that the biomass of emergent epifauna was positively correlated with plaice density
(Appendix 25). Concurrently, several fishermen stated in interviews that they often caught large quantities of the widely
distributed bryozoan Flustra spp. in areas of high catches of plaice. Recent research from north America also suggests
that some juvenile flatfishes associate with emergent structures such as sponge and other biogenic and bed-form
features of otherwise low-relief habitats. The basis for this relationship may again be linked to the feeding strategy and
prey spectrum of adult plaice. Plaice are known to feed increasingly on epibenthic crustaceans, small fish and
echinoderms with increasing size (‘mobile epifauna’ in Appendix 21). These groups benefit from the structural complexity
presented by emergent epifauna which offers a multitude of niche spaces. So, plaice may not rely on structural complexity
as such but on the prey species resident in such structures. Furthermore, they may use emergent epifauna as additional
protection to distract predators visually. However, high consistent densities of plaice were also found at stations with low
densities of emergent epifauna (Appendix 25). Although beneficial for the habitat selection of plaice, emergent epifauna
may thus not play a crucial role. This indicates that increased prey availability is the prime reason for the positive
correlation between emergent epifauna and plaice density. Adults also feed on infaunal polychaetes (Appendix 21) which
in most cases do not benefit from the presence of emergent epifauna. Large densities of polychaetes may thus
supersede the need for structural complexity.
Sole. Stations of high sole density were found over a much wider range of sediment classes compared with plaice
(Appendix 25). High density stations were found over a wide range of sediment types including sediments with a high
mud content (Appendix 25). No clear sediment preference could therefore be deduced Similar to plaice, however, they
appeared to avoid sediments with a high gravel content. With respect to the other environmental factors analysed, sole
showed similar requirements to those of plaice with a preference for shallow stations, low salinities and high
temperatures, while deeper stations with higher salinities and lower temperatures appeared to be less preferred or
avoided (Appendix 25). At high density stations, shells and stones were only present in small quantities and there was a
negative correlation between emergent epifauna and sole density, although this was not significant (Appendix 25). Hence,
unlike plaice, sole may not benefit from the presence of emergent epifauna. This species relies on tactile and
chemosensory cues to detect prey via papillae located on the ventral side of the mouth and it feeds primarily on infaunal
polychaetes and molluscs. Since epibenthic invertebrates do not form an important component of sole diet emergent
epifauna may also not play a significant role for sole but indeed hinder prey capture. Unlike plaice, sole may rely more
exclusively on immobility, burial and crypsis than upon visual distraction through emergent epifauna to impede predator
detection. The different body shape and colouration/pattern of the two species may also be an indication of these different
strategies.
Lemon sole. Overall, lemon sole were only rarely caught during both our research cruise and nine years of groundfish
surveys. At many stations, they were never recorded throughout the duration of the survey. The distribution of lemon sole
in the English Channel was thus restricted to very few stations (Appendix 24). High density stations of lemon sole were
characterized by a very different combination of environmental parameters compared with those of plaice and sole. They
seemed to prefer sediments with a high content of sand and gravel and were found at greater depths, higher salinities
and lower temperatures (Appendix 25) than the two other species. Most high density stations were thus found in the
deeper areas offshore. These findings are in line with previous research from the North Sea where high densities of
CSG 15 (1/00)
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Project
title
Essential fish habitat: important considerations for the
management of demersal fisheries
DEFRA
project code
MF0805
lemon sole were found in relatively deep waters and over sediments with a high gravel content. In the present study, high
density stations were also distinguished by larger quantities of stones and shells compared with stations of lower or
variable lemon sole density (Appendix 25). Similar to our results for plaice, there appeared to be a positive correlation
between lemon sole density and the biomass of emergent epifauna (Appendix 25). Thus, lemon sole may also benefit in
some way from structural complexity. Like plaice, they have large eyes and rely on vision for prey detection and capture.
Accordingly, adult lemon sole feed on molluscs, echinoderms and sedentary polychaetes which are associated with
coarse sediments.
The combination of the large-scale dataset with our own data corroborated the approach taken to detect stations of high
flatfish density which may be indicative of the habitat requirements of the three flatfish species under investigation.
However, it also highlighted some of the inherent difficulties in studies on essential fish habitats. Stations of high flatfish
density were located throughout the English Channel and characterized by very different benthic assemblages. Indeed,
the large scale of the study made it difficult to infer universal habitat requirements of flatfish. Out of the three species
studied plaice and lemon sole appeared to show a preference for emergent epifauna, that may be removed by activities
that alter habitat structure such as bottom fishing or aggregate extraction. However, certain species contributing to the
biomass of emergent epifauna, such as Flustra, may be ephemeral, sustain high growth rates and may thus be adapted
to life in dynamic environments.
A more detailed analysis and discussion can be found in Appendix 25 which will be submitted for peer-review in due time.
5) STRENGTH OF THE RELATIONSHIP BETWEEN FISH & HABITAT –DIETARY LINKS
As was highlighted during the last progress meeting, whiting was the only gadoid species caught in sufficient numbers for
a meaningful statistical dietary analyses of fish caught at different stations. Similarly, we were unable to sample sufficient
quantities of lemon sole and sufficient numbers of full stomachs of sole in the English Channel, which has limited our
analyses.
Whiting: Fish condition could be considered a useful indicator of nutritional status and habitat quality, as it reflects the
energetic status of the individual. Fish that live in more suitable habitats should be able to forage more efficiently such
that fewer energy reserves have to be allocated to swimming in search for food compared with less suitable habitats. Fish
condition was significantly higher at two stations characterized by sandy sediments located off the Ribble which is
consistent with the fact that whiting condition was negatively correlated with median grain size, total organic carbon
content and % silt/clay (Appendix 19) and that whiting showed a preference for sandy habitats in laboratory experiment
(Appendix 15). However, there were no significant differences overall in the condition index of fish sampled from high and
lower density stations (Appendix 20). There were also no significant differences in selectivity, niche width and the
‘resource value’ of stations of high and low whiting density (Appendix 20). Ideal free distribution (IFD) theory predicts that
individuals distribute themselves between patches in proportion to the available food resources. As all competitors are
assumed equal and free to enter and leave a feeding area, each individual should receive the same pay-off, and feeding
rates should be equal across patches. Hence the condition of whiting sampled from high and lower density areas might
be expected to be similar, as fish distribute themselves according to resource availability. There were also large-scale
differences in feeding quality as indicated by significant differences between the different stations in terms of the
proportion of full stomachs and stomach fullness. Differences in condition index occurred at a large scale between sites,
but did not vary according to fish density (Appendix 20). This suggests that condition index is averaged out as the result
of local feeding activity that varies across good quality and poorer quality feeding habitats (high and lower fish density
sites). These findings are important because they imply that relative habitat quality varies on a large scale, with fish
aggregating in the best habitat that is available within a local area (100 km²). Large-scale variation in habitat quality is
evident from the present study and is an important consideration from a fish stock/habitat management perspective. The
regional differences highlight the importance of choosing appropriate sampling scales that can take local variation into
account when describing the diet of a widely distributed species.
Stomach fullness was determined as it may also give an indication of habitat quality in terms of general preyavailability and was significantly higher at high density stations (Appendix 20). So, it would appear that stomach fullness
may be a better indicator of feeding habitat quality in the short-term. As mentioned above, fish condition, on the other
hand, may be averaged out over time and thus over larger spatial scales especially for very mobile species of fish such as
whiting.
We determined the niche width of fish from different stations as it may give an indication of how closely fish are linked
with the occurrence of specific prey types. A fish with a narrow niche width, for example, may be closely linked with the
occurrence of certain prey types and could thus be a specialist and as such more vulnerable to changes in the prey
availability through changes to the habitat. Fish with a broader niche width, by contrast, may be able to capitalize a larger
range of prey types and therefore be a generalist, which are less at risk to changes in their environment. Niche width
varied with station, and there appeared to be an inverse trend: whiting from many stations with a high species diversity
were characterized by a narrow niche width and vice versa (Appendix 20). However, many of the species found at these
high diversity sites may have been unsuitable prey for whiting. Station W1, for example, was distinguished by emergent
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Project
title
Essential fish habitat: important considerations for the
management of demersal fisheries
DEFRA
project code
MF0805
epifauna (Appendix 19), species that increase structural complexity. Although such environments harbour more prey
organisms as they form micro-habitats, they may also increase searching cost for predators by providing cryptic cover for
prey or harbour unsuitable prey types as has been shown for other fish species in north America. When offered a great
abundance of prey types whiting may also specialize (relative to the number of prey available) on one particular prey type
by developing a search image as has been shown for fifteen-spined stickleback. When the choice is more limited, whiting
may become more generalist in their choice of prey.
On a local scale, it would appear that whiting adopted a mixed feeding strategy with specialization on one prey type
but with individuals also feeding on small quantities of other prey types indicating individual variability and thus a degree
of generalist feeding behaviour (Appendix 20). Although they were highly selective for fish and small mobile epibenthic
crustaceans at most stations, they specialized on echiuran worms and the burrowing shrimp Jaxea nocturna in the
western Irish Sea and on Ensis in the eastern Irish Sea (Appendix 20). This highlights that whiting can also be selective
for benthic prey. These prey groups may serve as dietary supplements. Fish have an energy content which is twice as
high as that of crustaceans. However, our beam trawl samples gave low densities of prey fish species. Our sampling gear
may not have been selective for species such as herring and sandeels and thus lead to underestimates of their density.
Nevertheless, it would appear that whiting have to cover longer distances in search for prey-fish, which may counter some
of the benefits of a high-energy content prey. This might be even more pronounced for patchily distributed and schooling
prey species such as sandeels and could explain the presence of other food types in the diet that are more consistently
available.
There appeared to be marginal yet significant differences in the diet of whiting from high and lower density stations
(Appendix 20). Razor clams (Ensis), fish and mysids were 2-3 times more abundant in stomachs from high density sites,
whereas processid shrimps, echiuran worms and the burrowing mud shrimp Jaxea nocturna were more abundant at lower
density sites (Appendix 20). There also appeared to be a significant correlation between the species composition in the
diet of whiting and environmental samples (Appendix 20). This implies that the diet of whiting from different stations was
affected by the prey available at those stations.
The fact that high densities of whiting were found over a variety of habitat types in the Irish Sea, where they fed on
different prey types and adopted various feeding strategies indicates that they may be less vulnerable to changes in their
habitat than fish with a narrow feeding range. However, the importance of local differences in the condition index of
whiting should not be taken too lightly, given that research on the condition of different stocks of cod in the north Atlantic
has shown that poor condition causes reduced productivity in terms of slow growth and low recruitment potential. Stocks
consisting of individuals in poor condition may thus be susceptible to reduced recruitment at low spawning stock biomass,
while the stocks that consist of fish in good condition might behave more robustly with a higher probability of good
recruitment at low spawning stock biomass. The positive effect of condition on their reproductive potential generally
implies that whiting stocks in good condition in the eastern Irish Sea can sustain higher fishing mortality than those in poor
condition from western parts. These vulnerable stocks are currently under threat as large quantities of whiting are caught
incidentally by an expanding trawl fishery for Nephrops prawns in this region.
The diet and feeding strategy of whiting from different areas varied considerably, even when prey species were
categorized into ecological groups. Such ecologically important local differences become masked when data are pooled
over large geographic scales, e.g. the North Sea. Our results indicate a locally different use of benthic/demersal
resources and emphasize the necessity to study the feeding behaviour of fishes at a localized rather than a regional
scale. This has also important repercussions on the management of such widely distributed species.
This research was presented as an oral paper at ‘Fishing and Benthic Habitats’ in Tampa in 2002. A more detailed
analysis and discussion can be found in Appendix 20.
Cod: Despite considerable efforts during several research cruises, we were unable to sample sufficient numbers of cod at
the majority of stations chosen. Our data were thus not suitable for rigorous statistical comparison between high and
lower fish density stations. Nevertheless, we attempt a qualitative description of the feeding habits of cod from stations
where a reasonable number of individuals could be collected. Although we caught cod at a station off Strangford Lough,
W5, we were unable to take trawl samples at this location, as the seabed topography was hazardous to trawling. It should
be noted, however, that the seabed at this station appeared to be characterized by high structural complexity and
supported large numbers of large cod, which we sampled on a subsequent DARD groundfish survey cruise. However,
except for two grab samples we have no benthic data for comparison with the diet of cod from this site.
Concurring with other studies, cod showed a consistent positive selection for mobile epifauna, such as crabs and
shrimps at all stations (Appendix 10) and the smallest fish (7-10 cm at W1) also fed on mysids (Appendix 9). The
importance of the small mobile epibenthic crustaceans in the diet diminished with increasing fish size until it became
negligible for a mean size of ca 30 cm (Appendix 11). Concurring with other studies, juvenile cod also showed a positive
selectivity for fish. Individuals from the only station that harboured adult cod, W5, had a distinctly piscivorous diet that was
dominated by sandeels and supplemented by mobile epifauna such as hermit crabs, pandalid shrimps and brachyuran
crabs (Appendix 8).
Juvenile cod appeared to avoid polychaetes, brittlestars and amphipods, i.e. species that are characterized by an
infaunal or burrowing life style. Positive selection for a bivalve (Ensis) or a polychaete (Nephtys) was only found at station
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Project
title
Essential fish habitat: important considerations for the
management of demersal fisheries
DEFRA
project code
MF0805
W1 (Appendix 8), which also held the highest density of fish. A likely explanation is that increased intra-specific
competition led to the consumption of otherwise inferior or less preferred prey types.
The niche breadth of cod increased with increasing size up to a length of some 30cm and was lowest for adult fish
(W5 size range 32-55cm), that were primarily piscivorous (Appendix 23). As cod grew larger morphometric changes and a
reduced risk of predation presumably allowed them to feed on a wider range of prey species up to a certain size at which
they can prey successfully on fish and abandon a diverse diet in favour of an energetically more profitable diet of fish. It
should also be noted that the highest niche breadth was found at the station with the highest abiotic habitat complexity
and epifaunal diversity (Appendix 1).
There seemed to be a contradiction between stomach fullness and condition index: while fullness was lowest the
condition index was highest at W5 (Appendix 23). However, fullness may not necessarily reflect the energetic value of the
prey consumed or the energetic cost of foraging. It would appear that adult individuals that had reached a larger size and
therefore preyed on fish had a higher condition index (even with a relatively lower food intake) than those fish with a
higher fullness index which had consumed large quantities of crustaceans. Also, (chitinous) crustaceans have a longer
residence time in fish stomachs than the more rapidly digested fish prey.
The most important epifaunal taxonomic groups, epifaunal diversity and biomass found (Appendix 23) concur with
descriptions of epifaunal assemblages of the northern Irish Sea given by other workers. Species that were very abundant
in grabs and trawls were both ‘selected’ and ‘avoided’ by cod (Appendix 23). It would therefore appear that cod fed on
prey types that occurred in greatest abundances but not in an indiscriminate manner. The apparent avoidance of starfish
and brittlestars and the consistent selectivity for brachyuran crabs is in agreement with other studies which indicate that
cod discriminate food of high energetic value.
All stations where sufficient numbers of fish could be caught for our data analysis (E3, E4, W1) appeared to be
distinguished by structurally complex habitats. Unfortunately, we were unable to take environmental samples at W5 which
harboured predominantly adult fish. However, the reason why we were unable to take samples was that unsuccessful
grabs and one trawl tow, which resulted in a damaged net indicated a very rough and thus structurally complex terrain
indeed. Of these stations, W1 clearly represented a nursery ground (Appendix 19). Previous workers found that the
epifauna of Belfast Lough was characterized (among other species) by shrimps (Crangon, Pandalus) which formed an
important part of the diet of juvenile cod in our study (Appendix 23). The sediments here were characterized by mixed
muddy sand with shells and stones and were less coarse than those from our stations in the eastern Irish Sea (Appendix
1). The seabed harboured emergent bryozoans and hydrozoans, swimming and hermit crabs and appeared to provide
suitable micro-habitats for both its diverse fauna and prey for juvenile cod (Appendix 19). At stations E3 and E4, which
were distinguished by the greatest structural complexity (Appendix 19), habitat complexity was primarily created by the
coarse nature of the sediments and only to a lesser degree by attached, structuring epifauna, such as ascidians or
bryozoans. Here, the epifauna was primarily characterized by slow-moving species such as whelks, sea urchins, sunstars
and queen scallops (Appendix 23). These species are unlikely to provide shelter from predators nor are they energetically
valuable prey or likely to attract and support any of the cod’s preferred prey species. Similarly, other workers have
reported higher numbers of epi- and infaunal taxa in ‘hydroid habitats’ than in ‘non-hydroid habitats’ and showed that
mobile crustaceans (such as crabs and shrimps) were more abundant in cores from areas that were covered with
hydroids. It would appear, therefore, that juvenile cod from biogenically structured habitats can make use of the habitat,
by actively swimming in search for prey between the emergent epifauna as well as seeking shelter and remain stationary
at the base of abiotic structural elements when necessary. This is also corroborated by behavioural experiments and field
studies from northern America. Accordingly, it should be noted that the condition index of cod in our study was highest at
station W1 compared with E3 and E4 despite the fact that they were characterized by the highest abiotic habitat
complexity and epifaunal and infaunal species diversity. The general fitness of the fish, as reflected in the condition index,
can also be associated with the feeding strategy at the various stations. At W1, the wide range of suitable resources
resulted in a cod population composed of generalists, each exploiting a spectrum of overlapping resources under
favourable foraging conditions. At station E3, on the other hand, with fewer preferred prey species, cod opted for a more
individualistic feeding strategy, which might have increased the energetic cost of foraging and contributed to a lower
condition index overall. In conclusion, aspects of “essential fish habitat” can be revealed through dietary analyses in
conjunction with habitat information. Our results corroborate that juvenile cod prefer structured habitats and, furthermore,
indicate that the “quality” of the habitat, i.e. specific, mainly biogenic, elements are more important than structure per se
for determining the overall value of these grounds for cod.
A more detailed analysis and discussion can be found in Appendix 23 which will be submitted as a brief communication to
Fisheries Research.
Haddock: Unfortunately we were unable to sample sufficient numbers of haddock for a meaningful dietary analysis.
Plaice: Stomach contents analysis revealed that there were no significant differences in the diet composition and
condition index of plaice collected from high and low density stations (Appendix 21). Ideal free distribution (IFD) theory
predicts that individuals distribute themselves between patches in proportion to the available food resources. As all
competitors are assumed equal and free to enter and leave a feeding area, each individual should receive the same payCSG 15 (1/00)
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Project
title
Essential fish habitat: important considerations for the
management of demersal fisheries
DEFRA
project code
MF0805
off, and feeding rates should be equal across patches. Hence the condition and stomach fullness of plaice sampled from
high and lower density areas might be expected to be similar, as fish distribute themselves according to resource
availability. However, a greater proportion of the fish from high density stations had full stomachs compared with those
from lower density stations (Appendix 21). There were also large-scale differences in feeding quality as indicated by
significant differences between the different stations in terms of the proportion of full stomachs. Differences in condition
index occurred at a large scale between sites, but did not vary according to fish density (Appendix 21). This suggests that
condition index is averaged out as the result of local feeding activity that varies across good quality and poorer quality
feeding habitats (high and lower fish density sites). These findings are important because they imply that relative habitat
quality varies on a large scale, with fish aggregating in the best habitat that is available within a local area (100 km²).
Large-scale variation in habitat quality is evident from the present study and is an important consideration from a fish
stock/habitat management perspective. The regional differences highlight the importance of choosing appropriate
sampling scales that can take local variation into account when describing the diet of a widely distributed species.
One difficulty of the practical application of IFD theory lies in the fact that we do not know at what scale their
mechanisms operate, and hence it is difficult to chose appropriate sampling scales. The large geographic scale of the
study was chosen because we sought general trends between areas of high and lower fish density. However, the
resulting range of habitat types that were encountered complicate the interpretation of our results. The habitats under
investigation included areas of a homogeneous sandy nature and of variously mixed sandy sediments with large
proportion of shell gravel or stones, all of which were characterized by different assemblages and prey species (Appendix
21). We were therefore able to detect differences in the assemblage structure between individual stations but not between
high and lower density sites. If we had restricted our sampling to a smaller area and replicated sampling over distinct
habitat types we may have been more likely to discern specific habitat preferences. However, such results would have
had little power as only applicable to a restricted geographical area and type of habitat. This highlights once again one of
the inherent challenges in the experimental design of studies on essential fish habitat .
The diets of plaice from various stations differed and appeared to be driven by local prey availability (Appendix 21).
Although plaice had a broad feeding range, they appeared to prefer hard-shelled and soft-bodied infauna and emergent
tube worms, namely Lanice conchilega, Lagis koreni, Nephtys, Ensis arcuatus and other bivalves (Appendix 21). They fed
on these prey groups disproportionately to their environmental densities at various stations which indicates a preference
for these organisms (Appendix 21). The niche breadth of plaice from high density stations was significantly higher whilst
the densities of potential prey organisms were significantly lower implying that fish became more opportunistic when
resources were low (Appendix 21). This implies that plaice are adaptable to changes of the prey composition in their
habitat that may occur after habitat disturbance. Indeed, a shift in the diet of North Sea plaice since the beginning of the
20th century from a dominance of bivalves to polychaetes has been ascribed to a dramatic increase in fishing effort and
eutrophication.
A more detailed analysis and discussion can be found in Appendix 21.
Sole: Unfortunately we were unable to sample sufficient numbers of sole with full stomachs for a meaningful dietary
analysis. Our data were thus not suitable for statistically rigorous comparison between high and lower fish density
stations. Nevertheless, we attempt a qualitative description of the feeding habits of sole based on the pooled stomach
contents data from all stations. The high incidence of unidentifiable (digested) remains in sole stomachs and of empty
stomachs in general (n=39 of 110 ≈ 64%) could be due to the fact that trawling could only be conducted during day-light
hours. However, sole are known to feed nocturnally and as the day progressed, trawls may therefore have increasingly
caught sole with empty stomachs or prey organisms in a very advanced stage of digestion. Unfortunately, it was not
possible to conduct trawling at night onboard RV Prince Madog.
For the analysis of the diet of sole, all stomachs of sole from different stations had to be pooled as the total number of
full stomachs with identifiable prey organisms was too low (n=30) for an analysis of the diet of sole caught at different
stations or from different size classes. Similar to plaice, the tube worms Lanice conchilega and Chaetopterus, brittlestars
(Amphiura) and the polychaete worm Nepthys formed an important part of the diet of sole. An analysis of the feeding
strategy indicated that individual fish had specialized on different prey types, and each food category had been consumed
only by a limited fraction of sole. However, this may be due to the fact that for this analysis the stomach contents data of
sole from different stations were pooled. Such results would be expected if the diet of the few individuals of sole with full
stomachs caught at each station was very different, which could be expected if the local prey availability varied. This also
highlights the effect of pooling of stomach contents data of fish caught from large geographical areas, e.g. ICES divisions
or rectangles.
Lemon sole: Unfortunately, we were unable to sample sufficient numbers (n=15) of lemon sole for a meaningful dietary
analysis. In summary, the majority lemon sole were caught off Boulogne at station SE1 (n=8). 33% of these stomachs
were empty. The condition index ranged from 1.23 to 1.44 (mean±SE: 1.36±0.016) and stomach fullness ranged from
0.011 to 0.76 (mean±SE: 0.24±0.075).
6) ASSESSMENT OF POTENTIAL IMPACTS OF HABITAT DEGRADATION FOR DIFFERENT SPECIES OF FISH
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Project
title
Essential fish habitat: important considerations for the
management of demersal fisheries
DEFRA
project code
MF0805
Cod: Certain life-history stages (e.g. juveniles) are strongly associated with complex seabed habitat usually in inshore
areas as indicated in other studies (US, Canada, Scotland, Sweden). Adult life-history stages are far more general in their
use of habitat, although survey and fishers’ data suggests consistent associations with specific habitat features such as
brittlestar beds. The latter are vulnerable to excessive physical disturbance by bottom trawling, hence restrictions on the
type of gear that should be used in such areas might be advisable.
Haddock: Insufficient data to make recommendations.
Whiting: Wide-ranging use of different habitats although evidence for habitat preferences at a local (ICES sub rectangle)
scale. Potential for adverse local population effects if specific habitat is degraded, although unlikely to translate into
cumulative population decline unless habitat degradation occurs on a regional basis.
Plaice: Wide-ranging use of different habitats although general preference for coarse sand environment with some
epifauna. Biogenic (tube worm mats) habitats were of high quality with strong evidence of heavy prey predation of
dominant fauna. These habitats may be ephemeral and are also sensitive to excess bottom disturbance. Further
information required on the longevity of such habitats. Restrictions on the type of gear used on such habitats might be
advisable.
Lemon sole: Strong affinity for coarse sedimentary habitats with structuring fauna. Elimination of habitat is likely to affect
population size. Activities such as aggregate dredging are most likely to have the greatest long-term ecological effect on
this species.
Sole: Soft-sediment habitats of low complexity. The latter habitats are highly resilient to physical disturbance, however
negative effects of fishing on production could affect carrying capacity of system for sole by reducing supply of adequate
food. At low levels of fishing there is the potential for positive effect on production of appropriate food organisms.
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