Functional responses of fisheries to hydropower dams in the Amazonian Floodplain of the Madeira River

Tropical river fisheries support food security for millions of people but are increasingly threatened by hydropower development. How dams affect these fisheries remains poorly known in most regions. Here, we used a functional traits approach to evaluate the extent to which compositions of fishery yields in the Madeira River Basin, the largest sub-basin in the Amazon, respond to dam construction. We also explored how dams affected the monetary value of yields and fishing-based income of the communities. We collected fishing data in 17 locations distributed over 300 km across upstream, reservoir and downstream zones during pre-and post-dam construction periods. We interviewed 711 fishers from 13 communities to assess fishing income during pre- and post-dam periods. Catch-per-unit effort (CPUE) declined significantly, that is, by 37%, after dam construction. Multivariate analysis yielded six species clusters according to trait syndromes related to life history, migration, swimming performance/habitat-use and economic value that were associated with the environmental data characteristic of pre- and post-dam periods. Comparison of CPUE of each cluster indicated that large species with periodic life-history strategy and regional or long-distance migratory behaviour were most affected by dam construction, with CPUE declining by, on average, 31%. Declines in yields and shifts in functional composition of the fishery yields resulted in average decline of 21% in the monetary value of functional clusters and 30% in fishing income. Synthesis and applications. Our study indicates that the implementation of the dams affected the functional composition of yields and reduced catches, negatively affecting the fishing-based income of communities in the Madeira River. These results imply that hydropower expansion will cause detrimental effects for fisheries and the livelihoods they sustain. Our results underscore the urgent need for considering alternative sources of renewable energy (e.g. solar power and in-stream turbines) to avoid irreversible socio-environmental damages of large dam projects. In river reaches where dams are already in operation or under construction, minimizing impacts will require improving operational protocols to reduce hydrological alterations and developing research and technology to improve the functionality of fish passages. In these locations, addressing losses in fishery value and fishing-based income will also require the implementation of fair compensation measures. Maintaining fish production requires conserving flow pulses and free-flowing rivers and tributaries critical for completing life cycles of fish species with vulnerable traits.