Interspecific Interaction Between Wild and Hatchery Salmonid Fry

Although multiple species of salmonids inhabit streams sympatrically under the effect of both intra- and interspecific interactions, primarily competition, previous studies mostly focused on intraspecific interactions and studies on interspecific interactions are very rare. I therefore studied the effects of hatchery chum salmon fry on wild chum and masu salmon fry by enclosure experiments and field survey. The enclosure experiments demonstrated that hatchery chum salmon decreased wild chum salmon foraging efficiency and growth. These results suggest that hatchery chum salmon decreased survival of wild chum salmon, and may influence seaward migration of wild chum salmon because high condition factor appeared to trigger migration onset. Conversely, masu salmon was not affected by hatchery or wild chum salmon. Instead, masu salmon showed an intensive reduction in growth rate with increasing conspecific density. In a natural stream, however, stomach fullness of wild masu salmon fry decreased after explosion of hatchery chum salmon fry by stocking. Given these results, an extreme amount of hatchery fry stocking may influence wild fry regardless of competitive dominant hierarchy. Therefore, stocking procedures which mitigate the impact of hatchery fry on wild fry should be advanced for the effective management of both hatchery and wild salmonids.