Does the non-native European catfish Silurus glanis threaten French river fish populations?

1. The European catfish, Silurus glanis, was widely introduced in western Europe, and it has now established self-sustaining populations in numerous large rivers of western France. Using data collected from surveys conducted by the French National Agency for Water and Aquatic Environment (Onema) from 1989 to 2010 in more than 500 sites throughout the country (10636 electrofishing surveys), we investigated the potential impact of the European catfish on fish communities in French rivers. In the first part of the analysis, we compared trends observed before and after the European catfish was established at given sites (before-after analysis). Species richness, evenness and diversity decreased significantly after the European catfish was established at 1.4%, 1.4% and 5.8% of the sites, respectively. Total fish biomass and density decreased significantly at 6.6% and 2.9% of the sites, respectively. In the second part of the analysis, we compared sites with European catfish against sites lacking European catfish during the same period (with-without analysis). Fish species richness was significantly higher in sites with European catfish. No significant differences in fish diversity, evenness, total fish biomass or density were found between sites with or without European catfish. While our results indicate that the European catfish may in a few cases impact fish communities or populations, it does not appear to be responsible for a countrywide collapse in fish assemblages.