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1999. How vertical fish distribution may affect survey results. -ICES Journal of Marine Science, 56: 345-360.At a selected location in the Barents Sea, acoustic observations were made and bottom-and pelagic-trawl data were collected over a 10-day period. A large proportion of fish were found in the acoustic bottom dead zone. Only during a few hours in the daytime were high acoustic values obtained, mainly consisting of large haddock ascending from the bottom. Small haddock and redfish dominated the acoustic recordings at night. The bottom-trawl catches showed greater variability and higher average catch rates during the day than at night, but the diel variations were relatively less pronounced than those of the acoustic recordings. The largest reductions in catch rates from day to night were observed in small haddock and redfish. This was consistent with the observation that these species were pelagic at night. The acoustic observations and the bottom-trawl catch rates were found to be correlated with diel cycles in observed light level and semi-diel cycles in current speed. The results are interpreted in terms of the variable availability and efficiency of the bottom trawl and of the variable availability of the echosounder. 1999 International Council for the Exploration of the Sea

A full-scale fishing experiment on north-east Arctic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus), involving a trawler, a longliner and a gillnetter was conducted in order to determine how the length distributions and life history parameters of the catches were affected by gear type. Relative selectivity was analysed for catches taken when the boats fished simultaneously in the same area, and when the skippers were allowed to fish under conditions as close as possible to commercial operation. Trawl and longline-caught cod had similar length distributions, but cod caught by trawl were on average 2.3 cm smaller than longline-caught cod. Cod caught by longline had a lower condition factor and length at age. The longliner caught smaller haddock than the trawler when the boats fished in the same area. When the skippers were allowed to change fishing ground, the mean length of cod increased in the trawl catches and became larger than the mean length in the longline catches. The mean length of haddock in the longline catches increased by 2.8 cm and became larger than that of the trawl catches. Gillnet catches consisted almost solely of large cod. Mean length of gillnet caught fish in 186 mm nets decreased from 86.2 cm to 82.3 cm even if the boat did not change area. Separate selectivity experiments for trawl and gillnet were conducted, and the parameters for the mesh selection are presented. To find the functional form of the longline selectivity we used relative catch proportion in each 5 cm length-group for longline compared to trawl and gillnet, whose selectivities were assumed to be known. When compared to that of the gillnet, the plots indicate that the longline selectivity curve takes the form of a unimodal distribution. When longline catches was compared to trawl catches none of the suggested models seemed to fit, and to conclude anything about the form of the longline selection curve is not possible. International Council for the Exploration of the Sea