Effects of flow regulation on the establishment of alien fish species: A community structure approach to biological validation of environmental flows

The lower Ebro River (Catalonia, Spain) has a fish community dominated by alien species in both richness (55%) and abundance (78%). This river stretch is regulated by many dams and its flow was severely reduced by increasing water uses. We found strong evidence that the success of establishment and dispersal of alien fish species in this Mediterranean large river is enhanced by flow reduction through decreased water flow velocity in the littoral zone. Results show that when water flow is below 0.40 m/s, the probability that a fish community is dominated by alien species is higher than 50%, according to a logistic regression model that achieved 91% of well classified cases when predicting alien species dominance over natives. This relationship was used to perform an ecologically-based validation (biological validation) of 12 environmental flows proposed by several authors using different hydrologically-based methods. An HEC-RAS model previously developed and validated by the Catalan Water Agency (ACA) was used to simulate water velocity in the littoral zone. Results show that most of the proposed environmental flows (i.e., 89, 100 and 122 m3/s mean annual flow) favor a fish community structure dominated by alien species. The only biologically-validated environmental flows (227, 302 and 398 m3/s for dry, medium and wet years, respectively) were those based on data from the Sacramento model and calculated with the RVANGPRP method.