Small hydropower plants (SHPs) are proliferating globally, but their cumulative threat to blocking migratory fish and the fisheries that these fish sustain has been underappreciated when compared with large hydropower plants (LHPs). Here, we quantified the trade-offs between hydroelectric generation capacity and the impacts on river connectivity for thousands of current and projected future dams across Brazil. SHPs are the main source of river fragmentation, resulting in average connectivity losses of fourfold greater than LHPs. Fragmentation by SHPs is projected to increase by 21% in the future, and two-thirds of the 191 migratory species assessed occupy basins that will experience greater connectivity losses due to SHPs than LHPs. A Pareto frontier analysis identified future dam portfolios that could halve the number of hydropower plants that are required to deliver the same energy-generation capacity compared with the least-favourable solutions, while simultaneously resulting in lower river fragmentation and protecting numerous undammed basins. Our results highlight the need for strategic planning that considers the unprecedented growth and cumulative effects of SHPs.

小型水力发电厂（SHP）在全球范围内激增，但与大型水力发电厂（LHP）相比，它们对阻塞gr游鱼类和这些鱼类维持的渔业的累积威胁却没有得到重视。在这里，我们量化了巴西全国成千上万个目前和计划中的大坝在水力发电能力和对河流连通性的影响之间的权衡。小水电是河流支离破碎的主要来源，导致平均连通性损失是小水电的四倍。SHP的碎片化预计在未来会增加21％，在评估的191个迁徙物种中，三分之二的盆地由于SHP而比LHP遭受更大的连通性损失。帕累托前沿分析确定了未来的大坝资产组合，与最不有利的解决方案相比，可以提供相同发电量的水电站数量减少一半，同时导致河流破碎化程度降低，并保护了许多未筑坝的盆地。我们的结果强调了战略规划的必要性，要考虑小水电的空前增长和累积效应。