Preliminary description of swimming activity and estimation of swimming speed of saithe (Pollachius virens) at one location in the North Sea

Abstract

Individual saithe were tracked with a split-beam echosounder, while the vessel was drifting, in the area around Eigersundbank in the North Sea and their swimming speed estimated. The average swimming speed was approximately 4 and 1 body lengths per second for small saithe (20–30 cm) and saithe >70 cm, respectively, and a significant inverse relationship between length of the saithe and swimming speed was found. There was clear evidence of diurnal variation in swimming speed of small saithe, as the swimming speed was significantly higher during night (18–06 h) than during day (06–18 h). The number of observations on saithe >70 cm was too small to compare day and night swimming speeds. Although the results indicate higher swimming speeds of saithe in the demersal layer compared to pelagic saithe, significant differences were not found. The duration of acoustic observation time was 36–56 min per 4-h sampling interval during the 24-h cycle. A total of 278 series of saithe were selected, which, according to selection criteria, were accepted as representing tracking of single fish over two pings or more. The species identify of the targets tracked acoustically was verified by trawling in the layers investigated.