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

Abstract

Competition between masu salmon (Oncorhynchus masou) of wild and aquaculture origin was investigated. Fry were individually marked and released in stream enclosures with and without a piscivorous predator. The aim was to assess 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 surviving fry. Relatively large fish survived and grew better than relatively small fish in the absence of predators, but not in their presence. This probably indicates an indirect effect of predation risk on within‐cohort competitive behavior among salmon juveniles, with larger fish forced to give up their position as superior competitors. Domesticated fish survived in larger numbers and grew much faster than wild fish, irrespective of predator presence. 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 competitors 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.