Temporal fish community responses to two cascade run-of-river dams in the Madeira River, Amazon basin

We examined responses from shore fish assemblages and bottom channel fish assemblages before and after 2 cascade run-of-river dams in the Madeira River, in the Amazon. Those dams were installed in a river corridor where historical factors, represented by the presence of the Teotonio and Jirau falls, were the best predictors of the fish fauna assemblage composition. Jirau and Santo Antonio were the first dams constructed with Kaplan horizontal bulb turbines in the Amazon. These bulb turbines have the ability to operate by forming small reservoirs known as run-of-river, which should minimize impacts on fish assemblages. Overall, there were major short-term impacts in shore fish assemblages by increasing species richness, abundance, and biomass in experimental catches. Shore fish assemblages dissimilarities before and after the dams' closure were also noticed and were related to an increase in water temperature and dissolved oxygen. However, the historical factor represented by Teotonio Fall is still the best predictor of fish assemblage dissimilarities, combined with effects of dams' closure. Bottom channel fish assemblage dissimilarities can be explained only by dams closure and are related especially to changes in fish species abundance inside the reservoirs. Our study revealed an increase in native opportunistic fish species and changes in fish assemblage structure at local scale. Kaplan horizontal bulb turbines employed in run-of-river dams seem to be less deleterious than vertical axis turbines typically used in accumulation reservoirs, and should be preferred in the face of burgeoning new hydroelectricity development plans for rivers across the Amazon basin.