Seasonal partial migration of cyprinids - Causes and consequences

Partial migration is a common phenomenon in animals but has mostly been studied in birds. However, partial migration of cyprinid fish from lakes into streams during winter is a wide-spread phenomenon. Despite the potential trophic effects from an absence of dominant planktivores from the lake during large parts of the winter, only few studies have described this phenomenon. This thesis considers various aspects of the seasonal migration of cyprinids. The major aim of the thesis is to explain when, how and why the fish migrates and what possible effects the migration has on the lake ecosystem. I have, together with my co-workers, found that the winter migration differs between different species of cyprinids and that only very few of the predatory fish follow their prey into the streams. The timing of the migration coincides with a drastic increase in the ratio between the predation risk and the potential growth rate for the cyprinids. This also provides a mechanistic explanation for the migration: fish migrate to decrease their mortality risk during winter when potential growth rate is low, since predation risk is lower in the streams than in the lakes, despite lower food abundance in the streams. However, as only a fraction of the population migrates, it is obviously not beneficial for all individuals to migrate. I have, by experimentally feeding fish in the lake prior to migration, found that well fed fish in high condition migrate more than poorly fed fish in low condition. This difference is likely caused by low amount of energy reserves in poorly fed fish, which would not last for a long stay in the streams, where food abundance is low. Instead, these low condition fish accept the higher predation risk in the lake where food abundance is higher. Lastly, I have through mathematical modeling showed that the absence of fish from the lake during winter may have major effects on the lower trophic levels in the lake, especially during spring. For example, spring clear-water phases are more likely to occur when a high percentage of the fish population is migrating and when they stay away for a long time. This may be important for the establishment of macrophytes in lakes and may thus affect the stability and transitions between alternative stable states in lakes