Evaluation of the Norwegian shrimp surveys conducted in the Barents Sea and the Svalbard area 1980- 1997

Abstract

Trawl surveys for shrimp (Pandalus borealis) have been conducted annually in the Barents Sea and the Svalbard area since 1980 by Norwegian research institutes. The surveys have been conducted in the period April to September for a duration of 25 to 97 days each year. Until 1992 the Institute of Marine Research in Bergen (IMR) carried out the investigations, and thereafter the Norwegian Institute of Fisheries and Aquaculture Ltd. (Fiskeriforskning) in Tromsø has been doing the work. All together four different vessels have been used .The trawl used for sampling, a 1800 meshes modified commercial trawl, has been the same throughout the period, although with modifications of both the ground gear and the rigging. The modifications have had a considerable influence on sampling performance over time. The study area in Svalbard is stratified according to depth and latitude while the study area in the Barents Sea is stratified based on geographical areas of assumed homogenous density of shrimp together with a subjective division of the area into fishlng grounds. Three different approaches to survey design have been used - random stratified trawl stations, fixed trawl stations in a modified regular grid within a stratification of the area and fixed trawl stations in a depth stratified system. A study has been undertaken to caIculate an optimum sampling density with respect to a desired level of precision. The survey data are primarily used to calculate indices of shrimp abundance, both for the whole area and for a set of sub areas. Data on carapace length distribution is used to separate age groups and to study size and growth of the year classes in different sub areas.