Identification and Contribution of Wild and Hatchery Steelhead Stocks in Lake Michigan Tributaries

We developed and validated a simple and accurate assignment rule, based on one quantified scale characteristic, for the identification of wild and hatchery stocks of Great Lakes steelhead Oncorhynchus mykiss (formerly Salmo gairdneri). A ratio between the first winter and spring growth rates, inferred from scale circulus patterns, met the criteria of being both distinct between wild and hatchery adults and consistent among life stages, years, and geographic locations within respective wild and hatchery groups. This rule was applied to samples of unknown-origin adult steelhead collected in seven rivers in the northwestern Lower Peninsula of Michigan during 1983–1984, to estimate the percentage of wild fish in each river and its variance. Adults returning during these periods were representative of pre-1983 hatchery production in Michigan, when yearling parr was the typical life stage stocked. The percentage of wild fish in the adult population was estimated to be 100% for an unstocked trout river, 93% for four stocked trout rivers, and 60% for two stocked marginal trout rivers. This method should be applicable in other regions with similar winter temperature regimes and, possibly, to other species that are reared a full year in a hatchery.