Impact of invasive European catfish (Silurus glanis) on the fish community of Torrejón reservoir (Central Spain) during a 11-year monitoring study

The European catfish ( Silurus glanis) has been described as an invasive species exerting a relevant impact on the native fauna of the ecosystems where it is introduced. However, the lack of long-term data and the low catchability of this species with traditional methods have often made it difficult to evaluate its real impact. The main objective of this study was to investigate the effect of the invasive European catfish on the fish community of the Torrejón reservoir (Tagus River, Spain) using both direct fishing and indirect hydroacoustic methods. This study is the result of eleven years of monitoring. The results evidence the impact of European catfish on the reservoir fish assemblage, especially on the Iberian barbel ( Luciobarbus bocagei ) which significantly decreased its abundance and biomass from 2010 to 2020. The size structure of the fish assemblages in the reservoir allowed the use of hydroacoustic methodology to discriminate and monitor the population of European catfish. Throughout the 11 years of study, statistically significant differences were identified in the abundance of European catfish as a function of the reservoir´s area (dam - tail axis) and of the limnological period (summer vs. winter), while no significant differences were found in relation to bathymetry (surface - bottom axis). We can conclude that S. glanis has currently established in the Torrejón reservoir leading to the decrease of the Iberian barbel population, the only autochthonous species that persisted in the reservoir and dominated the fish community before the appearance of European catfish.