Low European catfish predation on silver eels in the Loire River watershed

• European catfish predation on silver eels was low during downstream migration. • Predation occurred only in the dammed river, not in the free-flowing system. • Hydropower infrastructure may indirectly increase eel predation risk. Since the late 19th century, North Atlantic diadromous fish species have greatly declined in abundance. Among them, the European eel Anguilla anguilla has been classified as Critically Endangered by the IUCN since 2008, making its conservation a major priority. The species is threatened by overfishing, pollution, climate change, migration barriers and marine predation, yet freshwater predation remains poorly understood despite its potential importance for conservation management. Following the introduction of a new predator in French rivers, the European catfish Silurus glanis , its impact on migrating silver eels was assessed. Eighty silver eels were tagged with acoustic tags containing predation sensors and released in the Loire River watershed (France). Forty were released in the obstructed Vienne River, and forty in the free-flowing Creuse River. A predation rate of 10% on the Vienne River and 0% on the Creuse River was observed. Mortality or injuries from hydroelectric dam may partly explain predation in the Vienne River. Therefore, the impact of European catfish predation on European eel at the silver stage is low compared to other diadromous species. This is likely due to the high swimming speeds observed for tagged individuals as well as the low feeding activity for European catfish during migration period. These results are a positive indication for the critical pre-reproductive silver eel stage. However, further studies across additional watersheds and earlier life stages, particularly glass and yellow eels are needed, as upstream migration timing and prolonged spatial overlap may increase their vulnerability to European catfish.