Predicting the effects of reintroducing a native predator (European eel, Anguilla anguilla) into a freshwater community dominated by alien species using a multidisciplinary approach

Multiple introductions of alien species can lead to the formation of new and diverse communities with diverse interactions, particularly in isolated freshwater ecosystems. In Lake Arreo (currently known as Caicedo-Yuso), located in Basque Country, Northern Spain, the introduction of several North American species (largemouth bass Micropterus salmoides, pumpkinseed Lepomis gibbosus and the red swamp crayfish Procambarus clarkii) and the common carp Cyprinus carpio has resulted in a unique community composed mainly of alien species. Previous efforts to eradicate them by intensive fishing/trapping have failed. Reintroduction of native predators could represent a complementary method, with an added biodiversity value. The reintroduction of the European eel Anguilla anguilla could lead to an increased predation on crayfish as shown by previous studies, but also affects the abundance of juvenile fish. To investigate the current situation of Lake Arreo, stable isotope analyses were conducted using 15 muscle tissue samples per each fish and crayfish species, while stomach contents of the same species were analysed. Additionally, samples from the common reed Phragmites australis, the trophically lowest food source available for fish and crayfish species, were collected and used as baseline for the isotope analysis. To investigate the usefulness of stable isotopes to predict the effects of species reintroductions on present species communities, available stable isotope and diet data from A. anguilla in a German freshwater lake with a similar species composition were retrieved and included in the Arreo community analysis. While results from both, dietary and stable isotope analyses, indicate high interactions among alien species with P. clarkii having a central position, the modelled reintroduction of A. anguilla shows to possibly affect recruits of alien fish species as well as an increased feeding of M. salmoides on reintroduced eels.