Migratory behaviour of north-east Arctic cod, studied by use of data storage tags

To understand the variation in availability of demersal fish to bottom traw ls and to evaluate the reliability of the target strength used in acoustic surveys, more knowledge about individual fish behaviour, such as the range, speed and repeatability of the vertical movements, is required. In this pa per, data storage tags have been used to study vertical movements of indivi dually tagged cod (Gadus morhua L.). In general, the short term vertical mo vements were less than 10 m. Rapid movements of up to 250 m within 2 h inte rval were recorded sporadically. During restricted periods some of the fish did move vertically with a distinct diel rhythm. Calculations of the time needed to equalise the pressure to remain at a constant buoyant state indic ated that the fish probably were negatively buoyant most of the time. Howev er, the direct effect of the vertical movements on the acoustical target st rength could not be quantified. Seasonal differences in mean depth and ambi ent temperature were observed. As a result of horizontal and vertical migra tions, north-east Arctic cod experience higher temperatures in winter and s pring than in summer and autumn. (C) 2000 Elsevier Science B.V. All rights reserved.