Predation of stocked salmon by riparian wildlife in semi-natural stream

Abstract

Predation after release is one of the major concerns of hatchery fish 11 conservation and propagation. However, the relationship among the size of hatchery fish, the 12 predator species, and their behaviors in natural environments is largely unknown. To understand 13 the relationship, we conducted predation experiments in outdoor tanks and a semi-natural stream 14 with exposure to local predators. Two different ranges of fork lengths of masu salmon 15 (Oncorhynchus masou) were examined as prey sizes. Camera trap data showed that grey herons 16 (Ardea cinerea) were the primary predator animal in the system, and that most herons utilized 17 shallow areas in the morning or evening. Increasing the density of stocked salmon brought in 18 more grey herons. More importantly, predation by grey herons resulted in the survival rate of 19