1. 60º Introduction and Background ù The Barents Sea covers an area of about 1.4 x 10 6 km 2, with an average depth of 230 m. ù Climatic variations depend mainly on inflowing Atlantic water and variation in ice extension from year to year may exceed 500 km. ù Annual catches have ranged from about 1 to 3 million tonnes and the main commercial species are capelin, herring, cod, haddock and deep sea shrimp. ù There are strong species interactions among these and other species and in later years recruitment, growth, and mortality have varied considerably for some of these species. ù Commercial species have been heavily exploited through most of the last 20 years and in this study we are using data from standard demersal trawl surveys to get an impression of the impact of exploitation on the ecosystem, including non-target fish species. Material ù Demersal surveys have been conducted annually in February (since 1981) and in July-August (since 1995). Trawl stations taken in February 1996 and July-August 1996 are shown in the Figures to the right. The July-August survey covers a larger area and includes the mature part of the cod stock which is not covered in February due to spawning migrations. ù Stomach samples of cod from the years 1988-1998 were also used in this study. Analysis ù All results exept the cod/capelin/temperature figure below are based on swept area estimates ICES/SCOR Symposium on Ecosystem Effects of Fishing Montpellier, France, 15-19 March 1999 THE ABILITY OF STANDARD DEMERSAL SURVEYS TO EVALUATE THE FISHERIES IMPACT ON THE BARENTS SEA ECOSYSTEM by Knut Korsbrekke and Sigbjørn Mehl Institute of Marine Research, P.O.Box 1870 Nordnes N-5024 Bergen, Norway 2 key species in the Barents Sea: Some ecosystem metrices Treating the cod as an observer Species richness The estimated biomass of cod in the period 1985-1998 peaked at more than 2.4 million tonnes in 1993 which followed an impressive recovery from the the all time low of 0.8 million tonnes in 1988. Individual growth of cod has been closely related to the variations in capelin biomass. A large inflow of Atlantic water starting in 1989 led to an increase in temperature and plankton production. The increase in temperature can be seen in the mean temperatures of 4 hydrographical sections, first in the warmest areas in the west and later in the coldest eastern part of the sea. Based on trawl survey data, the ecosystem metrices Hills N1 and N2 and Species evenness were calculated. Results from the February survey are shown with solid lines, while the short time series of results from July-August are given with dotted lines. No clear trends can be seen, but after 1993 there seems to be less “noise” in the data due to improved standardisation of trawl handling and instrumentation. The “noise” in the early 1990’s could also be related to the strong yearclasses produced by some species (due to better temperature and feeding conditions). The July-August series are too short for any comparisons, but in the two last years they also show an upwards pointing trend. Using stomach contents from cod as samples from the ecosystem (February survey) and calculating the ecosystem metrices Hills N1 and N2 and Species evenness, these results are apparent. There are some clear trends. The low values in 1991-1995 are caused by a domination of capelin in the stomachs. When capelin are not present cod switch to other prey and utilises a higher number of species. There is a time lag in this process; the capelin were quite abundant in 1989, but the increase had no effect on the metrices until 1991. And after the capelin depletion in 1994, the change in metrices was observed in 1996. Generally a higher number of prey species could be identified in the cod stomachs than the total number of species caught in the trawl. The overall stomach contents were very low in 1988, which could explain the low number of species observed. The low number of species observed in stomachs in 1993 could be caused by the dominance of capelin as a prey item. The figure clearly demonstrates the inabillity of the trawl to sample more than a small fraction of the species in the ecosystem. The slightly increasing trend could be caused by the technical staff gaining more experience throughout this period. Size spectrum The slopes of the size spectra (below and over 50 cm) are shown together with the estimated fishing mortality. The minimum landing size for cod is 47 cm and the selectivity of the trawls indicate that little effect of fishing should be observed below 50 cm. The flat size spectrum above 50 cm in 1988 changed as the strong 1983 yearclass of cod entered the size range in 1989 and 1990. High growth and low fishing mortality led to a higher number of larger individuals up to 1994. The 1990 yearclass was also quite strong, but was not able to affect the size spectrum in a similar way. This could be caused by a high fishing effort targeting fish around 50 cm. CONCLUSIONS The standard demersal sampling trawl can only observe a modest fraction of the ecosystem. The effect of reducing the fishing effort in 1989-1992 is difficult to quantify and interpret due to the concurrent changes in environmental conditions. The high fishing mortalities in the latest years have perhaps, caused a reduction in the slope of the size spectrum, but could also be due to low recruitment flattening the spectrum. The strong fluctuations in environmentaly dependent recruitment makes interpretation of changes in the size spectra difficult. The cod can not be trusted as an observer of the ecosystem.