Near-bottom sampling with bottom and pelagic trawls

The Institute of Marine Research, Bergen, has carried out combined bottom trawl and acoustic surveys for cod and haddock in the Barents Sea - Svalbard area since 1981. These investigations have demonstrated clearly that the currently applied survey methods considerably underestimate the young age groups of cod. For haddock the survey abundance estimates have been considerably higher than those used in the stock assessment. Part of the problem has been shown to be be low bottom trawl efficiency for small fish. It has also been indicated that the pelagic trawl has low efficiency for cod. In this paper variation in length and species composition from samples taken with a bottom trawl and with a pelagic trawl on and off bottom are analyzed. Differences in sampling efficiency and the problems connected with combining data from different trawls during routine surveys are discussed