Relationship between natural productivity and the frequency of wild fish in mixed spawning populations of wild and hatchery steelhead (Oncorhynchus mykiss)

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 ln(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 (P-w). 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 P-w. Population status assessments and conservation monitoring efforts should include P-w 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.