From dusk till dawn - analysing the seasonal behaviour of stocked and resident European catfish Silurus glanis in a natural lake by means of high-resolution acoustic

European catfish (Silurus glanis) is a highly valued species for specialised recreational anglers due to its large body size. Thus it is often supported by stocking. However, little is known about behavioural effects of stocking, and similarly there is a knowledge gap of the behavioural response of catfish to environmental variables in lakes. In this study, 11 resident and 22 stocked catfish individuals were tracked over six months (July to January) in each of two study years using a high-resolution acoustic telemetry system fully covering a 25-ha natural lake. Significantly larger home ranges for stocked individuals were found, showcasing systematic differences in space use, but not in swimming activity, among stocked and resident fish. Home ranges also increased with decreasing dissolved oxygen concentration and chlorophyll a contents, suggesting that environmental challenges and food availability also drove the space use of catfish. Activity and swimming velocity were highest during dusk, decreased throughout the night and were lowest during daytime, most likely reflecting foraging bouts during crepuscular periods. Compared to abiotic and biotic variables, including being translocated to a new environment, the total length (size range 79-166 cm) had little effect on the behaviours measured in the wild. In conclusion, both being in a novel ecosystem and environmental effects jointly affect the behaviour expressed by European catfish, largely independent of body length.