Does longitudinal connectivity loss affect the distribution of freshwater fish?

Loss of natural river network connectivity is presumed to be one of the more generalized and important human-induced alterations in natural environments and is frequently perceived as one of the main causes of the decline of freshwater fish species.The purpose of the present study was to ascertain the impact of barriers on the distribution of freshwater fish species with distinct life histories. In this study 196 sites in three river basins in Western Iberia were sampled and analyzed for the presence of barriers. Three alternative analytical approaches based on Generalized Linear Models (GLM) were used to test the contribution of connectivity-related variables to species distribution: (1) explore whether connectivity-related variables are included in the best-fitting distribution models; (2) use models calibrated at non-disturbed sites to compute deviations from model predictions made at sites with connectivity-related disturbances; and (3) use a hierarchical partitioning approach, in which the improvement of model fit due to the inclusion of connectivity as a predictor variable is assessed using all possible variable combinations.The results indicate a general lack of influence of barriers on freshwater fish species distributions. The effects of environment and human pressures exceeded the isolated effect of connectivity losses. Further studies based on experimental designs that are more specifically directed at this specific issue are needed in order to fully understand the effects of barriers on species and communities. A more thorough assessment of the effects of connectivity on fish is crucial to the implementation of adequate restoration actions that are in turn needed to achieve the goals of the European Water Framework Directive.