Swimming activity of perch, Perca fluviatilis, in relation to temperature, day-length and consumption

The swimming activity of a Baltic population of perch Perca fluviatilis was studied by gillnet fishing during a 13 year period in an experimental enclosure receiving cooling water from a nuclear power plant. A distinct seasonal rhythm was observed, correlated with temperature and day-length except for a peak during the spawning period in spring. The timing of the spring activity peak was not affected by the large differences in temperature between years and seemed to be more synchronized by day-length. Swimming activity was strongly related to consumption predicted by a bioenergetics model. The hypothesis that fish abundance may influence the cost of swimming through social interactions could not be supported by the studies, as growth rate remained unchanged although fish abundance increased considerably.