Experimental evaluation of predation of stocked salmon by riparian wildlife: effects of prey size and predator behaviours

Predation after release is one of the major concerns of hatchery fish conservation and propagation. However, the relationships between the size of hatchery fish, the predator species and their behaviours in natural environments are largely unknown. To understand these relationships, we conducted predation experiments in outdoor tanks and a seminatural stream with exposure to local predators. Masu salmon (Oncorhynchus masou) of two different size classes were used as experimental prey fish in the present study. Camera trap data showed that grey herons (Ardea cinerea) were the primary predator in the experimental system, and that most herons used shallow areas in the morning or evening while feeding. Increasing the density of stocked salmon led to increases in the number of occurrences of grey heron. More importantly, predation by grey herons resulted in a significantly lower survival rate of larger salmon compared with smaller salmon. The results indicate that it is important to understand local predators, adjust the optimum body size of hatchery fish at release and choose the appropriate stocking site and time of day for maximising the effectiveness of fish stocking.