To support eco-friendly hydropower planning in developing regions, we propose a spatial optimization model for locating dams to balance tradeoffs between hydropower generation and migratory fish species richness. Our model incorporates two special features. First, it is tailored to the dispersal of tropical migratory fishes, which require long, unimpeded river stretches to complete their life-cycle. To model fish with this type of dispersal pattern, we introduce the concept of a river pathway, which represents a novel way to describe river connectivity. Second, it combines decisions about dam placement and removal, thus facilitating opportunities for hydropower offsetting. We apply our model to the São Francisco River basin, Brazil, an area of hydropower-freshwater biodiversity conflict. We find that dams have reduced weighted migratory fish richness 51% compared to a pre-dam baseline. We also find that even limited dam removal has the potential to significantly enhance fish biodiversity. Offsetting the removal of a single dam by the optimal siting of new dams could increase fish richness by 25% above the current average.Moving forward, optimizing new dam sites to increase hydropower by 20%, rather than selecting the fewest number of dams, could reduce fish species losses by 89%. If decisions about locating new dams are combined with dam removal, then a win-win can even be achieved with 20% greater hydropower and 19% higher species richness. Regardless of hydropower targets and dam removal options, a key observation is that optimal sites for dams are mostly located in the upper reaches of the basin rather than along the main stem of the São Francisco River or its main tributaries.

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evaluation of the spatial and temporal distribution of fish in the upstream proximity of a dam in a Neotropical reservoir.