Artificially increased habitat complexity reduces predation-induced mortality for juvenile salmon in simplified environments

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 m 2 or more of tree branches per 1.0 m 2 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.