Norwegian combined acoustic and bottom trawl surveys for demersal fish in the Barents Sea during winter
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A combined acoustic and bottom trawl survey for demersal fish in the Barents Sea has been conducted annually since 1981. The main aim is to map the spatial distribution and obtain indices of abundance (numbers at age) of cod, haddock, redfish and Greenland halibut. In addition other biological information and CTD-data are collected. The survey covers an area of about 150,000 square nautical miles, and is carried out with 3 vessels and about 25 scientific staff for a period of 25-30 days. The abundance indices and weight at age data are used directly in the stock assessments in ICES. Stomach data of cod are together with other information used to estimate the Northeast Arctic cod stock's consumption of important prey species, e.g. capelin, cod and haddock. This information is further used in the assessment of these species. The tuning diagnostics indicate that the indices from the survey are the most reliable among the tuning data, and the assessments of Northeast Arctic cod and haddock are thus heavily dependent on the results of the survey. Since the management relies heavily on the assessments, the surveys can have a direct effect on the TAC's. Several changes in survey methodology aiming at increased reliability of the results have been introduced. Some of these changes had large effects on the index level. The time series have therefore been adjusted in order to maintain and improve comparability. However, it is suggested that also in the adjusted series, most recent years index/stock ratios for cod are higher than those from the first 10 years period; a matter that should be taken into account in the stock assessments.
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