Many river systems have been artificially modified for various reasons worldwide. Although simplified riverbeds due to artificial modifications often alter the species compositions and interactions, methods for mitigating habitat degradation in such rivers are underdeveloped. In this study, we evaluated the effects of tree branches on the survival of juvenile masu salmon Oncorhynchus masou under predation risk by piscivorous fish (White-spotted charr Salvelinus leucomaenis) in artificially simplified environments. We found that 0.4 m² or more of tree branches per 1.0 m² of artificial concrete pond doubled the survival of juvenile salmon for 20 days. In addition, body sizes of predator and prey did not have a significant effect on salmon survival, whereas the number of branches did in this experiment. These results indicate that adding structural complexity can mitigate predation pressure by piscivorous fish in artificially simplified environments. This study provides important guidelines for the management and restoration of suitable habitats for stream salmonids in artificially simplified environments where the main predators are piscivorous fish.

由于世界范围内的各种原因，许多河流系统已被人为修改。尽管由于人工改造而简化的河床经常会改变物种的组成和相互作用，但缓解此类河流中栖息地退化的方法仍未开发出来。在这项研究中，我们在人工简化的环境中，评估了树枝对食肉鱼（白斑鲑Salvelinus leucomaenis的捕食风险）的捕食作用对幼年马苏鲑Oncorhynchus masou存活的影响。我们发现每1.0 m ²有0.4 m ²或更多的树枝人工混凝土池塘的放养使幼鲑在20天的生存时间翻了一番。此外，捕食者和猎物的体型对鲑鱼的存活率没有显着影响，而在该实验中分支的数量确实对鲑鱼的存活率有显着影响。这些结果表明，在人为简化的环境中，增加结构的复杂性可以减轻食肉鱼的捕食压力。这项研究为人为简化的环境（食肉鱼类为食肉鱼）在人工简化的环境中管理和恢复适宜流栖鲑鱼的栖息地提供了重要指导方针。