Site Fidelity, Depth Preference and Vertical Movement Patterns of Lentic and Lotic European Catfish <i>Silurus glanis</i>

ABSTRACT To study variables that may trigger the dispersion of the currently expanding European catfish in its native distribution limits, 42 individuals were captured from Upper Lake Constance (lentic) and 42 from two streaming tributaries (lotic), fitted with pressure and temperature loggers, and released at the site of capture (control: lotic or lentic) or into a new environment (lotic‐to‐lentic or lentic‐to‐lotic). Twenty‐seven fish were recaptured within 662 days. Lentic and lotic fish showed similar seasonal vertical migration patterns, descending in winter and ascending in summer to warm shallow water. European catfish descending to the depth of the lotic river‐bed decreased in vertical movement activity, whereas lentic European catfish descending to deeper layers in the lake remained active with vertical movements. This may relate to the differences between these habitats in European catfish's hunting and social behaviour. Of control released fish, 14 out of 17 were recaptured near to the original catch site. The remaining three control released fish were lotic fish and had dispersed to the lake (lentic). Of translocated fish, all eight recaptured lentic‐to‐lotic fish returned to the lake, while the two recaptured lotic‐to‐lentic translocated fish remained in the lake. The overall higher dispersal towards and retention in the lake suggest the lentic habitat to be strictly favoured over the lotic habitat. The results support quick adaptation of European catfish to a cold‐water lake that was few decades ago still mostly unexploited by the species. Conservation and fisheries management should take such fast adaptations in an apex predator into account.