Vertical distribution of cod, haddock and redfish; Impact on bottom trawl and acoustic surveys in the Barents Sea
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At a selected location in the Barents Sea acoustic observations, bottom- and pelagic trawl catch data were collected over a 10 day period. A large proportion of the fish were in the acoustic dead zone during the sampling period. Only during a few hours in the daytime high acoustic values were obtained. According to the pelagic trawl hauls these recordings consisted of large haddock ascending from the bottom, while small haddock and small redfish dominated the acoustic recordings at night. The bottom trawl catches showed higher variability and higher average catch rates during the day than at night, but the diurnal variations were relatively less than those of the acoustic recordings. The largest reduction in catch rates from day to night was observed for small haddock and redfish. This is consistent with the observation that these were found pelagicly during night. The acoustic observations and the bottom trawl catch rates were found to be correlated with diurnal cycles in observed light level and semidiurnal cycles in current speed. The results are interpreted in terms of variable catchability of the bottom trawl and variable availability for the echo sounder.