Effects of body size and predators on intracohort competition in wild and domesticated juvenile salmon in a stream

Competition between masu salmon (Oncorhynchus masou) of wild and aquacultur e origin was investigated. Fry were individually marked and released in str eam enclosures with and without a piscivorous predator. The aim was to asse ss the effects of predators and salmon body size on survival and growth of the two types of fish under natural conditions. The presence of predaceous Japanese huchen (Hucho perryi) resulted in lower mean growth rates of survi ving fry. Relatively large fish survived and grew better than relatively sm all fish in the absence of predators, bur not in their presence. This proba bly indicates an indirect effect of predation risk on within-cohort competi tive behavior among salmon juveniles, with larger fish forced to give up th eir position as superior competitors. Domesticated fish survived in larger numbers and grew much faster than wild fish, irrespective of predator prese nce. Comparison with similar field studies revealed a pattern that the pre- experimental environment influenced the outcome of competition between wild and domesticated juvenile salmon. Domesticated fish were superior competit ors even in the absence of an initial size advantage, which commonly gives a further advantage to hatchery-raised fish in natural streams. Therefore, caution dictates to avoid the release or escape of such fish into the wild.