Spitzenprädator Wels - wie stark beeinträchtigen große Individuen die Passierbarkeit von Fischaufstiegsanlagen?

In the river Rhine system - probably favoured by climate change and increasing cooling water discharges - a significant increase in the population of wels catfish (Silurus glanis) and a continuous spatial expansion has been recorded for around 30 years. Until about 5 years ago, catches and observations were limited to the Rhine and its major tributaries. For some years now, however, catfish measuring 1-2 m have also been increasingly recorded in the lower reaches of medium-sized tributaries. Direct evidence (catches, camera recordings) and indirect evidence (prey fish with characteristic bite wounds) of catfish in and around fishways are increasingly coming into focus. Exorbitant predation-related losses of salmon were documented in a fishway in the French Garonne. At Iffezheim (Rhine), there are clear indications of restrictions on the passability of the fish pass for salmon, among others, due to catfish predation pressure. Echo sounder investigations have shown aggregations of very large catfish (>180 cm) in the tailwaters of the Koblenz (Moselle) and Kostheim (Main) barrages. It must be assumed that catfish have a considerable (and probably increasing) negative influence on the passability of fishways in large rivers as a result of behavioural adaptations. Comprehensive monitoring and adapted management measures are required to ensure the passability of large fishways.