Behaviour of the Apex Predator European Catfish (<i>Silurus glanis</i>) on a Recently Invaded Reservoir

ABSTRACT The European catfish ( Silurus glanis L.) is a large, non‐native, invasive freshwater species known for its high fecundity, long lifespan and significant predatory potential, possibly threatening native fish communities. While some studies exist on its spatial use, most focus on its native range and lack detailed circadian and annual activity patterns. This study uses acoustic biotelemetry to track 10 adult fish in the Tagus River's Belver reservoir, employing transmitters with 3D accelerometer and pressure sensors to record activity and depth over a year. Generalised additive models identify environmental variables influencing activity and depth patterns. The European catfish is active year‐round, with peak activity in summer and minimal in autumn. It occupies shallower depths in spring and summer (median depths of 2.6 and 4.8 m, respectively) and deeper waters in autumn and winter (median depths of 9.8 and 10.8 m, respectively). Circadian vertical movement patterns exhibit high seasonal and individual variability. Significant environmental predictors of the European catfish activity include river flow, water temperature and the photoperiod, while the predictors of depth use are river flow and the photoperiod. These insights can aid in developing effective control measures, such as optimising fishing efforts spatially and temporally to enhance the efficiency of removing this invasive fish.