Testing predictions of changes in fish abundance and community structure after flow restoration in four reaches of a large river (French Rhone)

Projects that restore river flows can be considered as in situ experiments and should be used to test predictions of the effects of flow changes on fish populations and communities. However, flow restoration projects often lack appropriate monitoring and replication. The Rhone restoration project has included repeated flow changes, in four bypassed reaches of the river, where the increase in minimum daily flow varied from minimal change to a tenfold increase. Fish communities (>55000 individuals of 36 species) were electrofished at nine sites in the main channels of the four bypassed reaches, for 2-9years before and for 5-10years after the flow restoration. An instream hydraulic habitat model, published before restoration and based on observations of fish microhabitat preferences in independent reaches, was applied to the bypassed reaches to predict density changes for 14 species that accounted for 94% of the total fish abundance. In the two bypassed reaches where minimum flow was considerably increased (fivefold and tenfold), the abundance of species preferring fast-flowing and deep microhabitats increased by factors of 1.9 and 2.4, respectively, whereas the abundance of other species strongly decreased. Predicted changes in density made using the habitat model for these two reaches agreed with the observations at several sites and involved several fish species. In contrast, in the two bypassed reaches where flow changes were less, the observed changes in density were weak and less related to the model predictions. Hydraulic habitat models predicted changes of fish populations and the predictions also explained observed community responses to the changed flows. Ten years after the first flow restoration, our results suggest that the Rhone restoration generated perennial changes of the fish community structure, reversing community patterns that were observed prior to the flow restoration.