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 trawls 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 paper, data storage tags have been used to study vertical movements of individually tagged cod ( Gadus morhua L.). In general, the short term vertical movements were less than 10 m. Rapid movements of up to 250 m within 2 h interval 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 indicated that the fish probably were negatively buoyant most of the time. However, the direct effect of the vertical movements on the acoustical target strength could not be quantified. Seasonal differences in mean depth and ambient temperature were observed. As a result of horizontal and vertical migrations, north-east Arctic cod experience higher temperatures in winter and spring than in summer and autumn.