LAUSR

The introduction of non-native fish into the Teuchitlan River, in west-central Mexico, was suggested as a key factor behind the change in the fish assemblage, and even in the extinction of endemic species. The characterization of the fish assemblage and the relationship between fish abundance and the anthropogenic conditions were evaluated over 2-year period. Fish assemblage structure, species replacement, and dominance were examined along a longitudinal environmental gradient. Physical and chemical parameters of the water were recorded. The relationship between fish abundance and abiotic factors was inferred from multivariate analysis. The results showed that the non-native poecilid  Pseudoxiphophorus bimaculatus was dominant, and the abundance and biomass of non-native species exceeded 50% in all sites and seasons. Species diversity and native fish abundance were greater in upstream sites, which presented higher dissolved oxygen and lower nitrates. The native ichthyofauna of the Teuchitlan River has been replaced by non-native species. This decline of the native fish species seems to be due not only to the interaction with non-native species but also to the multiple human impacts on the lotic system.