Predation after release is one of the major concerns of hatchery fish conservation and propagation. However, the relationship among the size of hatchery fish, the predator species, and their behaviors in natural environments is largely unknown. To understand the relationship, we conducted predation experiments in outdoor tanks and a semi-natural stream with exposure to local predators. Two different ranges of fork lengths of masu salmon (Oncorhynchus masou) were examined as prey sizes. Camera trap data showed that grey herons (Ardea cinerea) were the primary predator animal in the system, and that most herons utilized shallow areas in the morning or evening. Increasing the density of stocked salmon brought in more grey herons. More importantly, predation by grey herons resulted in the survival rate of larger salmon being significantly lower than that of the smaller salmon. Our results suggest 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 maximizing the effectiveness of fish stocking.