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.