Experimental observation of the prior residence advantage in masu salmon in an experimental channel

Stocking of hatchery-reared salmonids is still a common management strategy to supplement natural resources and improve recreational fishing catch. However, concerns have been raised about the effectiveness of stocking as there is evidence that stocked fish may replace prior resident fish or fail to settle at the intended stocking sites, resulting in downstream emigration, and may subsequently die. This study evaluated whether stocked masu salmon (Oncorhynchus masou masou), a species widely stocked in Japanese streams, displace prior resident fish or emigrate from the stocked site. Experimental stocking trials were conducted in an artificial channel with multiple pools where factors potentially affecting the interactions between residents and intruders can be controlled. We found that stocked intruder fish were more likely to move out of the artificial channel compared to prior resident fish. Our findings indicate that stocking of masu salmon may be ineffective in streams with prior resident fish and few available habitats, as most stocked fish are unable to settle at the site stocked. To optimize stocking efficiency for species that show prior residence advantage, such as masu salmon, stocking should only occur in situations where suitable, unoccupied habitats are available and where site fidelity of stocked fish can be expected.