Human activities frequently create structures that alter the connectivity among habitats or act as barriers to the natural movement of animals. Movement allows individuals to access different habitats, connect life history stages, and maintain genetic diversity. Here, we evaluated whether run‐of‐river (RoR) hydropower projects, an emerging renewable energy source in British Columbia, interrupt the longitudinal connectivity among larval stream amphibians, by altering larval rearing densities, with possible repercussions on growth and survival. In three watersheds, we tested for differences in the average upstream and downstream density of larval coastal tailed frog (Ascaphus truei), as well as changes to their longitudinal distribution upstream of the dams, as would be expected if RoR dams or their headponds act as barriers to the natural downstream drift of larvae. We found a 60% decrease in larval densities downstream compared to upstream of dams, consistent with RoR dams interrupting the natural pattern of downstream A. truei drift. Larval densities in the first 10 m above RoR headponds were 3 times higher compared to 100 m upstream, and when expressed in terms of relative abundance, we find a similar pattern, with between 2.5 and 3 times more larvae in the first 10 m above of the headpond than expected if larvae followed a uniform distribution. Our results are consistent with the hypothesis that RoR dams alter the spatial connectivity of A. truei larvae, leading to an accumulation of larvae directly above the dam, with unknown consequences for larval growth and survival. Our findings suggest caution is warranted when interpreting before–after monitoring studies that are often used to evaluate the impact of dams, whereby we find that reductions in downstream densities could be due to interruptions of downstream movement as opposed to direct mortality.

中文翻译：

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跨河水坝是溪流两栖动物运动的障碍

人类活动经常创造出改变栖息地之间连通性或阻碍动物自然运动的结构。运动使个人能够进入不同的栖息地，连接生活史各阶段，并保持遗传多样性。在这里，我们评估了不列颠哥伦比亚省新兴的可再生能源“过流”（RoR）水电项目是否通过改变幼体饲养密度来中断幼体两栖动物之间的纵向连通性，从而对生长和生存产生影响。在三个集水区中，我们测试了幼体沿海尾蛙（Ascaphus truei）的平均上游和下游密度的差异），以及在大坝上游的纵向分布的变化，如果RoR大坝或它们的头顶充当幼虫自然向下游漂移的屏障，这是可以预期的。我们发现，与大坝上游相比，下游幼虫密度降低了60％，这与RoR大坝打断了下游A. truei漂移的自然形态相一致。RoR头上方10 m的幼虫密度是上游100 m的3倍，以相对丰度表示时，我们发现了类似的模式，在RoR头上方10 m的幼虫多2.5至3倍。如果幼虫遵循均匀分布，则比预期的要大。我们的研究结果与RoR坝改变了A. truei的空间连通性的假设是一致的幼虫，导致幼虫在大坝上方堆积，对幼虫的生长和存活产生未知的后果。我们的研究结果表明，在解释通常用于评估大坝影响的监测研究前后，应谨慎行事，从而发现下游密度的降低可能是由于下游活动的中断而不是直接死亡率所致。