Effect of the Introduction of Catfish (Silurus glanis) on the Native Fish Fauna in the Torrejón Reservoir (Cáceres, Spain) ^†

Abstract

The introduction of exotic fish species is a global ecological and conservation problem that has caused the reduction and extinction of numerous native species. In Spain, the introduction of exotic freshwater fish is one of the main factors threatening the survival of their native fish species. One such species that recently arrived in Spanish freshwaters is the catfish (Silurus glanis). This study was conducted from 2010 until 2021 in the Torrejón reservoir in the Tagus basin. The results obtained show that the introduction of catfish has led to a significant decrease in the abundance of fish in the reservoir, as well as a clear alteration in the fish assemblage, causing changes in the proportion of species and their biomass. The common barbel (Luciobarbus bocagei), the only native species that still maintained an abundant population in the reservoir, has clearly been the most disadvantaged species. The size structure of the species present in the reservoir allows us to differentiate adult catfish specimens from the rest of the species using hydroacoustic methods and thus be able to estimate their density, biomass, and spatio-temporal distribution. Based on the results obtained, we can affirm that the species is active in the reservoir throughout the year and that, despite its fame as a “bottom dweller”, catfish use surface habitats more than previously thought. It has shown a great capacity of adaptation to exploit the new environments and resources, together with a great tolerance against adverse conditions, such as low oxygen concentration (even anoxia) or high concentrations of ammonium, that are characteristic of the hypolimnion of Torrejón during the stratification period. The insights are an important contribution both to proving the effect of the introduction of catfish in native fish populations and to the implementation of alternative tools, such as hydroacoustic methodologies, in future management programs for this exotic species in our freshwater ecosystems.