1. North-eastern Spain is a hot spot for the introduction of alien fish species, and its native fish fauna is one of the most endangered worldwide. We used an extensive data set from 2002 to 2003 and historical information from the area to characterize fish diversity and establish conservation priorities in river catchments.
2. Diversity indices were used to characterize fish diversity at the basin scale. An index of conservation status was applied for each species, which considers the occurrence, abundance and endemicity of each taxon. We used indirect ordination methods to test the relationship among basin features and to identify those variables most correlated with each other. To identify physical, biotic and environmental characteristics that seem to make a basin particularly susceptible to invasion, we performed a step-wise multiple regression to examine the relationship between the number of native, translocated and introduced fish species (including the original native species richness of each basin), and landscape variables.
3. Over a period of approximately 50 years, the mean range size of native fish species has decreased by 60%. The greatest decline occurred in Gasterosteus gymnurus, Anguilla anguilla and Salaria fluviatilis, for which species over 75% of the original distribution area has been lost. The species with the highest conservation index were Gasterosteus gymnurus and Salaria fluviatilis.
4. Basin area and the catchment type explained 70% of variation in native species richness, whereas the number of dams and basin area accounted for more than 80% of variation in the number of introduced species.
5. The original native species richness and the number of introduced species at basin scale were not related, and thus there was no evidence of “biotic resistance” to invasion. The restoration of natural hydrologic processes and the development of specific management tools to protect native species, such as the prioritization of areas for fish conservation and the eradication of local populations of exotic species, are required to restore native fish fauna in these catchments.