Hydroacoustic measurement of swimming speed of North Sea saithe in the field

Saithe Pollachius virens , tracked diurnally with a split-beam echosounder, showed no relationship between size and swimming speed. The average and the median swimming speeds were 1·05 m s^?1(±0·09 m s^?1) and 0·93 m s^?1, respectively. However, ping-to-ping speeds up to 3·34 m s^?1 were measured for 25–29 cm fish, whose swimming speeds were significantly higher at night (1·08 m s^?1) than during the day (0·72 m s^?1). The high average swimming speed could be related to the foraging or streaming part of the population and not to potential weakness of the methodology. However, the uncertainty of target location increased with depth and resulted in calculated average swimming speeds of 0·15 m s^?1 even for a stationary target. With increasing swimming speed the average error decreased to 0 m s^?1 for speeds >0·34 m s^?1. Species identity was verified by trawling in a pelagic layer and on the bottom.