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Monitoring biodiversity in the North Sea using groundfish surveys


1) To establish the value of existing national ground-fish surveys of the North Sea as monitors of fish and epibenthic biodiversity.
2) Where possible, to link trends in fish and epibenthic biodiversity for selected non-commercial species as revealed by these surveys with trends in fishing effort and methods, and/or environmental and ecological factors.
3) To try out and recommend practical methods for enhancing ground-fish surveys so that they better monitor fish and epibenthic biodiversity without disrupting their primary functions in the management and assessment of commercial fish stocks. Where necessary, to produce taxonomic aids suitable for use on survey vessels using the recommended methods.
4) To recommend protocols for the reporting and assessment of trends in biodiversity in order to monitor the well-being of North Sea fish and epibenthic ecosystems in relation to commercial fishing.


Several nations undertake ground-fish surveys in the North Sea. This 3-year project is intended to show how value may be added to these surveys by monitoring biodiversity of non-commercial species in support of the Convention on Biological Diversity (Rio de Janeiro, 1992). The project is divided into four tasks. Firstly, existing ground-fish survey data on non-commercial fish, and where possible, epibenthic invertebrates, were analysed for information on biodiversity. It was concluded, as expected, that fish survey trawls are not good sampling tools for monitoring biodiversity. Secondly, a 2-metre beam trawl constructed from steel was tested at stations throughout the North Sea, and found to be a rugged and dependable survey tool for epibenthic biodiversity. Attempts are now being made to standardise towing distances. Thirdly, taxonomic material is being prepared to assist scientists on ground-fish surveys to readily identify benthic species caught by the beam trawl. This is taking the form of a computerised database with photographs. A fourth task, to be addressed in the final year of the project is the development of protocols for the reporting and assessment of trends in biodiversity so that the results of any future monitoring can be as relevant and accessible as possible.

1. Analyses of existing national ground-fish survey data for biodiversity data about the North Sea were undertaken by each national partner, and presented at a project symposium held at Aberdeen, Scotland in March 1997. Papers by specialists in related fields also came forward. The collected papers will be submitted to the European Commission as a project report, and a selection of them is also being refereed for publication. A general conclusion of the work was that ground-fish surveys using commercial trawling gear catch only a sub sample of the marine fauna, and additional gear must be deployed when monitoring biodiversity is the objective. A 2-metre, lightweight, steel beam trawl catches additional fish and epibenthic invertebrates, and was recommended as the supplementary gear.
2. Fishing effort by beam and otter trawlers in the North Sea from all fishing nations except Belgium and France has been compiled regionally (by ICES rectangle) for the years 1990-1995, and for Norway, Scotland, England and Wales for 1977-1989. These data will permit variations in trawling effort to be compared with variations in epibenthic communities and demersal fish.
3. The 2-metre beam trawl has been extensively fished on a wide range of substrate types in the North Sea, and found to be a rugged and dependable device for sampling the epifauna. A device to monitor the time spent actually fishing on the bottom has satisfactorily undergone preliminary trials, and is being developed further to permit the distance trawled to be standardised, an important consideration when assessing the occurrence of rare species. Trials have also been made with benthic grabs but these are less satisfactory because of variable performance on different substrates, the smallness of the sample, and the time and expertise taken to process the sample. This may mean that the infauna are not routinely sampled by a biodiversity monitoring programme as part of a ground-fish survey. In addition, equipment to assess the sea floor acoustically has been tested. These are likely to provide a useful supplement to trawl samples. A photographic archive of over 100 epibenthic species has been assembled for preparation of a taxonomic guide. The photos are being edited electronically and put into a computerised database to assist rapid identifications at sea with a minimum of specialised knowledge.
4. A working group has been arranged for April 1998 to test possible methods of analysing and reporting epibenthic biodiversity data. The data recently collected with the survey trawl by several project participants will be used.

Experience in the first two years of this three year project has indicated that monitoring of epibenthic biodiversity during national ground-fish surveys in the North Sea is definitely feasible. Additional costs would be minimal. The epibenthic environment is, of course, only part of the marine system, and a recommendation to monitor the species present there as part of a ground-fish survey would not be meant to deny the importance to biodiversity of the benthic infauna, the plankton, the pelagic fish, marine mammals, the sea birds, etc. The problem with including these components in a monitoring programme is that they require special facilities and/or large amounts of time, which may not be available on all ground-fish surveys. The epibenthos also have particular relevance to studies of fish and fishing because they are directly vulnerable to heavy trawling gear, and because they form a component of fish diets.

An environmental concern about the North Sea is that many epibenthic species are opportunistic scavengers feeding on the living material broken or exposed by trawls, or on the fish caught and subsequently discarded from the fishing vessels. A time-series of epibenthic monitoring data obtained from a good spread of localities in the North Sea, as would be possible using ground-fish surveys, could help us to assess this concern, particularly if good figures on the locations of trawling effort can be obtained for the same time periods. The data collected so far on fishing effort by ICES rectangle reflect a valuable new international effort. Finer resolution geographically may be obtainable from national enforcement activities such as aerial spotting, but so far data from only one country have been obtained.

There are undoubtedly several other factors apart from trawling effort which help to shape epibenthic communities. A regular and complete series of monitoring data could provide the raw material for many academic and applied studies of these factors. One interesting study already begun in association with this project concerns the influence of life history strategies of different species on their present-day survival in the North Sea. However, in addition to the needs of scientists, a biodiversity monitoring programme should also provide information in summarised form to fisheries managers, other marine authorities, and to interested members of the public. It is these people who will decide whether something should be done to alter any ecologically unsatisfactory situations, which come to light. The challenge will be to capture and hold their attention with some kind of annual report of biodiversity monitoring data.


Albert, O.T. (1993). Ecology of some small demersal fish of the Northeastern North Sea and Skagerrak. DSc Thesis, Dept. of Fisheries and Marine Biology, University of Bergen, Norway.

Anonymous (1994). Report of the working group on the assessment of demersal stocks in the North Sea and Skagerrak. ICES C.M. 1994/Assess:6.

Bergstad, O.A. (1990). Ecology of the fishes of the Norwegian Deep: distribution and species assemblages. Netherl. J. Sea Res., 25, 237-266.

Duineveld, G.C.A. & van Noort, G.J. (1990). Geographical variation in the epifauna of the southern North Sea and adjacent regions. ICES CM 1990/Mini: 9, 11 pp (mimeo).

Dyer, M.F., Fry, W.G., Fry, P.D. & Cranmer, G.J. (1983). Benthic regions within the North Sea. J. Mar. Biol.Ass. UK., 63, 683-693.

Frauenheim, K., Neumann, V., Thiel, H. and Turkay, M. (1989). The distribution of the larger epifauna during summer and winter in the North Sea and its suitability for environmental monitoring. Senckenbergiana marit., 20, 101-118.

Glenmarec, M. (1973). The benthic communities of the European North Atlantic Shelf. Oceanogr. Mar. Biol. Ann. Rev., 11, 263-289.

Greenstreet, S.P.R. (1992). Changes in the fish community of the North Sea between 1972 and 1991. Working paper, ICES Study Group on Ecosystems Effects of Fishing Activities, April, 1992.

Greenstreet, S.P.R. and Hall, S.J. (1994). Examining fish community structure in the north-western North Sea: a preliminary analysis. Working paper, ICES STudy Group on Ecosystems Effects of Fishing Activities, April, 1994.

Norse, E.A. (1993). Global marine biological diversity. Center for Marine Conservation, Island Press, 1718 Connecticut Av, Washington D.C., 383pp..

Rees, H.L. (1994). Surveys of the epifauna in UK and adjacent waters. In Aquatic Environment Monitoring Report 1994, Directorate of Fisheries Research, Burnham on Crouch, Essex, England (in press).


1. Nine papers from the symposium held in Aberdeen in March 97 are being refereed for publication in Fisheries Research.
2. "The effects of fishing on marine ecosystems" by S. Jennings and M. Kaiser, has been prepared and accepted for publication by Advances in Marine Biology.
3. "Life history correlates of responses to fisheries exploitation" by S. Jennings, J. Reynolds and S. Mills, has been prepared and accepted for publication in Proceedings of the Royal Society.


1. Symposium of project participants and other European scientists with related interests held in the Marine Laboratory, Aberdeen, on 18 and 19 March 1997.
2. Nine papers from this symposium are being refereed for publication in Fisheries Research.
3. "The effects of fishing on marine ecosystems" by S. Jennings and M. Kaiser, has been prepared and accepted for publication by Advances in Marine Biology.
4. "Life history correlates of responses to fisheries exploitation" by S Jennings, J Reynolds and S Mills, has been prepared and accepted for publication in Proceedings of the Royal Society.

The principle outputs from the project, concerning sampling protocols, taxonomic aids, and reporting protocols, when finalised, will be circulated to other ground-fish survey countries.

New political significance was given to biodiversity by the Convention on Biological Diversity signed by heads of state at Rio de Janeiro in 1992. Since fishing in the North Sea under the Common Fisheries Policy has been intense in recent years, and several commercial stocks are reported to be overexploited (Anon, 1994), significant mortalities of non-target and rare species are likely to have occurred as well. Changes in the balance of species could have led to a radically altered ecosystem, particularly on and near the sea bottom, parts of which are towed over repeatedly each year by heavy trawling gear. A recent review of global marine biological diversity (Norse, 1993) endorses this point, and states that there has been "astoundingly little study of the effects of intense fishing pressure on sustainability" (of the marine ecosystem).

Several national ground-fish surveys are conducted in the North Sea, and are likely to continue in some form for the foreseeable future. Aside from their main purpose for estimating abundance indices of commercial fish, they provide records of the occurrence of a variety of non-commercial fish and, in some cases, epibenthic (i.e. living on the sea bottom) invertebrates which can be caught by the commercial-type trawling gear (usually of the GOV design) fitted with small mesh codend liners which is used on these surveys. The species collected form a subset of the full range of species present in these groups in the North Sea, but existing data extend over many years (back to the 1920s for one set). Studies of fish species were reported by Green-street (1992) and Green-street & Hall (1994).

Using small beam trawls, grabs, and/or sonic devices capable of identifying characteristics of the sea bottom, it is possible that future information on biodiversity coming from ground-fish surveys could be considerably enhanced without necessarily disrupting normal working practices or adding significantly to costs. It is necessary that special gear be deployable quickly and easily, and that resources for taxonomic identification are available. Provided that the inconvenience and extra costs of monitoring biodiversity could be kept small, countries participating in this project could consider augmenting their surveys for this new purpose.

Rees (1994) recently conducted epibenthic surveys in the North Sea and elsewhere around England and Wales using a 2-metre beam trawl with a 10 mm square mesh belly, 3 mm mesh codend liner and three tickler chains. More than 300 species were found, considerably more than would have been taken by a GOV or similar commercial trawling gear. Comparable but more intensive surveys of the North Sea epifauna have been reported by, for example, Dyer et al. (1983), Frauenheim et al. (1989) and Duineveld and van Noort (1990). Rees (1994) reports that despite differences in sampling methodology, all have identified a broad division between a "northern" and "southern" fauna approximately along the northern edge of the Dogger Bank. This division was similar to that proposed by Glenmarec (1973) for the North Sea, based upon thermal stability of the water-column.

Fish communities in the North Sea, as revealed by Scottish trawl surveys, were studied by Green-street (1992, 1994). Differences in diversity and size spectra were found among three north western regions near Scotland, and between the periods 1929-1956 and 1975-1993. This implies that existing ground-fish survey data can provide useful information on biodiversity of fish, and that fuller analyses of a wider range of data will be worthwhile. Examples of special ecological studies of fish communities in the North Sea have been published by Bergstad (1990) and Albert (1993).

Funding Scheme

CSC - Cost-sharing contracts


Pakefield Road
Lowestoft, Suffolk
United Kingdom

Participants (5)

Bundesforschungsanstalt fur Fischerei
12,Palmaille 9
22767 Hamburg
Danish Institute of Fisheries Research
101,North Sea Centre
9850 Hirtshals
Fisheries Research Services
United Kingdom
Victoria Road
AB11 9DB Aberdeen
5024 Bergen
University of East Anglia
United Kingdom
University Plain
NR4 7TJ Norwich