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If the release of hatchery-reared juveniles exceeds the carrying capacity of the envi- ronment, the growth rate of wild fish may decrease or hatchery fish might displace wild fish because of competition for prey and space, or cannibalism. However, limited evidence is available to confirm these ecological effects. Therefore, we used census-marking experiments to investigate the ecological interactions between hatchery and wild fish in the large piscivorous species Japan- ese Spanish mackerel (JSM) Scomberomorus niphonius in the Seto Inland Sea. We analysed the relationship between the mean body weight and population size of Age 0 JSM, and the relation- ship between the biomass of juvenile Japanese anchovy Engraulis japonicus, which is the princi- pal prey fish of JSM, and that of Age 0 JSM. We also estimated the spawner−recruit relationship. We measured the body sizes of 5008 JSM juveniles during July and December between 1999 and 2005 in the eastern Seto Inland Sea, and 551 hatchery fish were identified in the sample based on examinations of marked otoliths. There was a negative correlation between the mean body weight and population size of Age 0 fish, which clearly demonstrated the density-dependent growth of Age 0 JSM. The ~35% variation in the biomass of Age 0 JSM was explained by the biomass of prey fish. Hatchery fish tended to be larger than wild fish, and they had better growth perform- ance. Our analysis showed that hatchery fish reduced the growth rate of wild fish and displaced the wild fish in terms of biomass when hatchery fish stocking exceeded the carrying capacity of the environment.

Ecological interactions between hatchery and wild fish: a case study based on the highly piscivorous Japanese Spanish mackerel