Seasonal migration in cyprinid fish: Causes and consequences

During a number of years we have observed that cyprinid fish migrate out of shallow, eutrophic lakes into streams and wetlands during winter. We have earlier shown that such migrations may have substantial impact on lake ecosystem dynamics. In this project we investigate the causes and consequences of this migration. We hypothesized that fish migrate from the lake when the cost/benefit ratio is higher in the lake than in the streams. Cost is here defined as predation rate (µ) and benefit as potential growth rate (g). In spring, resources, and hence growth, increase in the lake and fish should migrate back when µ/g is lower in the lake than in the stream. To investigate the seasonal migration patterns in detail, we PIT-tagged more than 4000 fish in the lake, including planktivores and piscivores. Their movements were continually monitored for four consecutive years by recording stations placed in the inlet/outlet streams. A large proportion of the tagged roach and silver bream moved out of the lake in autumn and stayed in the streams until early spring. Piscivorous pike and perch did not migrate. Migration patterns followed predictions from theoretical modeling in autumn, whereas in spring fish migrated out into the lake later than predicted. This may be due to resource tracking as zooplankton still were at low levels in the lake at that time. The decision of individual fish to migrate or not appeared to be related to size and experience, i.e. larger individuals migrated more than small individuals and fish that migrated one winter were more likely to migrate the following winters.