Data from: Hydropower impacts on reservoir fish populations are modified by environmental variation

Global transition towards renewable energy production has increased the demand for new and more flexible hydropower operations. Before management and stakeholders can make informed choices on potential mitigations, it is essential to understand how the hydropower reservoir ecosystems respond to water level regulation (WLR) impacts that are likely modified by the reservoirs' abiotic and biotic characteristics. Yet, most reservoir studies have been case-specific, which hampers large-scale planning, evaluation and mitigation actions across various reservoir ecosystems. Here, we investigated how the effect of the magnitude, frequency and duration of WLR on fish populations varies along environmental gradients. We used biomass, density, size, condition and maturation of brown trout (Salmo trutta L.) in Norwegian hydropower reservoirs as a measure of ecosystem response, and tested for interacting effects of WLR and lake morphometry, climatic conditions and fish community structure. Our results showed that environmental drivers modified the responses of brown trout populations to different WLR patterns. Specifically, brown trout biomass and density increased with WLR magnitude particularly in large and complex-shaped reservoirs, but the positive relationships were only evident in reservoirs with no other fish species. Moreover, increasing WLR frequency was associated with increased brown trout density but decreased condition of individuals within the populations. WLR duration had no significant impacts on brown trout, and the mean weight and maturation length of brown trout showed no significant response to any WLR metrics. Our study demonstrates that local environmental characteristics and the biotic community strongly modify the hydropower-induced WLR impacts on reservoir fishes and ecosystems, and that there are no one-size-fits-all solutions to mitigate environmental impacts. This knowledge is vital for sustainable planning, management and mitigation of hydropower operations that need to meet the increasing worldwide demand for both renewable energy and ecosystem services delivered by freshwaters.

全球向可再生能源生产的转型进程，使得对新型且更具灵活性的水电运营的需求持续攀升。在管理者与利益相关者能够针对潜在减缓措施做出科学决策之前，我们亟需明晰水电水库生态系统如何响应水位调控（water level regulation, WLR）带来的影响——而这类影响很可能会因水库的非生物与生物特征发生改变。然而，当前多数水库相关研究均为针对单一案例的研究，这阻碍了跨不同水库生态系统的大规模规划、评估与减缓行动的开展。本研究聚焦于水位调控的强度、频次与持续时长对鱼类种群的影响如何随环境梯度发生变化。我们以挪威水电水库中的褐鳟（Salmo trutta L.）的生物量、种群密度、体型、健康状况与成熟情况作为生态系统响应的衡量指标，并检验了水位调控与湖形特征、气候条件以及鱼类群落结构之间的交互效应。研究结果表明，环境驱动因子会改变褐鳟种群对不同水位调控模式的响应。具体而言，褐鳟的生物量与种群密度随水位调控强度提升而增长，这一现象在大型且形态复杂的水库中尤为显著，但这种正相关关系仅在无其他鱼类共存的水库中得以体现。此外，水位调控频次的提升与褐鳟种群密度升高相关，但却会导致种群内个体的健康状况下降。水位调控的持续时长对褐鳟无显著影响，且褐鳟的平均体重与成熟体长也未对任一水位调控指标呈现出显著响应。本研究表明，本地环境特征与生物群落会显著调控水电活动引发的水位调控对水库鱼类与生态系统的影响，且不存在能够缓解环境影响的一刀切解决方案。这一研究结论对于可持续规划、管理与减缓水电运营的环境影响至关重要——而此类水电运营需满足全球范围内日益增长的可再生能源需求，同时也要保障淡水生态系统所提供的生态服务功能。