THE NUMBER OF HATCHERY FISH ON THE SPAWNING

GROUNDS REDUCES NATURAL PRODUCTION

Chilcote, Mark, W., 2003. Relationship between natural productivity and the frequency of wild fish in mixed spawning populations of wild and hatchery steelhead (Oncorhynchus mykiss). Can J. Fish. Aquat. Sci. 60: 1057-1067.

Abstract: “The proportion of wild fish in 12 mixed populations of hatchery and wild steelhead (Oncorhynchus mykiss) was evaluated for its relationship to mean and intrinsic measures of population productivity. The population mean of In (recruits /spawner) was used to represent mean productivity. Intrinsic productivity was represented by values for the Ricker a parameter as estimated from fits of spawner and recruit data. Significant regressions (p <0.001) were found between both measures of productivity and the proportion of wild fish in the spawning population (Pw). The slopes of the two regressions were not significantly different (p = 0.55) and defined a relationship suggesting that a spawning population comprised of equal numbers of hatchery and wild fish would produce 63% fewer recruits per spawner than one comprised entirely of wild fish. Study findings were not sensitive to likely levels of data error or confounded by extraneous habitat correlation with Pw as a critical variable. For natural populations, removal rather than addition of hatchery fish may be the most effective strategy to improve productivity and resilience.

Quotes from the text: “Evidence from a number of studies…suggests that hatchery-reared fish may be less fit than wild fish under natural spawning and stream-rearing conditions. In this study, measures of population productivity were used to indirectly examine the possibility of differential reproductive success between wild and hatchery fish in 12 populations of steelhead. This examination was based on the assumption that if reproductive differences exist, productivity should vary in relation to the relative frequency of wild and hatchery fish in the population. For example, if differences exist, it would be expected that natural populations consisting entirely of wild fish would be more productive in terms of recruits produced per spawner, than those consisting of both hatchery and wild fish. Therefore, the objectives of this study were twofold: first, to look for indirect evidence of reproductive differences between wild and hatchery fish and, second, to assess the influence of naturally spawning hatchery fish on overall population productivity.

“Those steelhead populations with a high frequency of wild fish tended to be more productive than those with a lower frequency of wild fish (more hatchery fish). It appears, at least for these populations that among population differences in productivity were influenced by the relative mixture of wild and hatchery fish in the spawning population. If one were to infer that this relationship holds for all salmonids, then the possible management implications are fourfold. First, in conducting conservation status assessments, interpretation of past recruitment performance must incorporate the depressing effect of naturally spawning hatchery fish. Second. It is critical that fish conservation monitoring and evaluation efforts track the relative frequency of wild and hatchery fish in natural spawning populations. Third, strategies that use hatchery fish to rebuild depressed wild populations may be counterproductive because they are likely to cause a loss in overall population productivity and thereby cancel the recruitment benefit from adding hatchery spawners to the population. Fourth, an effective method to increase the productivity of natural populations and associated conservation benefits may be to minimize the frequency of hatchery fish in natural spawning populations.”