Mechanisms Affecting Competition Between Hatchery-Reared and Wild Juvenile Anadromous Pacific Salmonids in Fresh Water

Avoiding negative effects of competition from released hatchery salmonids on wild fish is a primary concern for recovery efforts and fisheries management. Factors affecting competition among juvenile salmon include: (1) whether competition is intra- or interspecific, (2) duration of freshwater cohabitation of hatchery and wild fish, (3) relative body size, (4) prior residence, (5) environmentally induced developmental differences, and (6) fish density.  The intensity of intraspecific competition is expected to be greater than interspecific because of greater niche overlap between hatchery and wild fish of the same species. Competition is expected to increase with prolonged freshwater cohabitation of hatchery and wild fish. Hatchery smolts are often larger than wild, and larger fish are usually superior competitors. However, wild fish have the advantage of prior residence when defending territories and resources in natural streams. Hatchery-induced developmental differences are variable and can favor both hatchery and wild fish. The density of the composite fish population (wild + hatchery) in relation to habitat carrying capacity likely exerts the greatest influence on competition. The extent of competition and relative competitive ability of wild and hatchery fish can be determined by additive and substitutive experimental designs, respectively, and the limited body of substitutive experiments suggests that the relative competitive ability of hatchery and wild fish is approximately equal using growth as the endpoint. Conducting substitutive experiments becomes difficult as the spatial and temporal scales increase. Large-scale experiments comparing supplemented and control reaches or streams hold some promise for quantifying the effects of released hatchery fish on wild fish behavior, growth and survival.