Vertical distribution and swimming speed of pelagic fishes in winter and summer monitored in situ by acoustic target tracking

We studied the swimming speed and vertical distribution of fish under ice from March to April and in open water in August. Fish were surveyed with a downward facing stationary 120 kHz split beam echo-sounder in a boreal lake. A commercial trace tracking software was used to estimate the depth, speed and size of fish. Most fish in the study area were smelt (Osmerus eperlanus) and vendace (Coregonus albula). In March and April, fish were detected under the ice around the clock, whereas in August only a few fish were detected during daytime. Fish were typically detected between 12 and 19 m depth and they swam from 5 to 50 cm s–1 i.e. 0.5 to 3 body lengths s–1. The effect of sunrise and sunset on the under-ice swimming depth and speed became more marked with increasing day length. In summer, the effect on depth was not so evident. In March–April there was a shift towards day activity, which persisted in August. Changing illumination seemed to account for most variation in the swimming depth and speed of fish.