Supplementation is a widespread response to the declining runs of anadromous salmonids in the Pacific Northwest. A common type of supplementation is the intentional release of adult hatchery fish to spawn naturally (outplanting) but this method has seldom been evaluated. Our objective was to quantify the juvenile steelhead (Oncorhynchus mykiss) produc tion from the adult outplants during a 14 year period in two streams. Although densities of juvenile steelhead were highly variable, outplanting status (supplemented versus not supplemented) explained a significant proportion of the variance for the age-1 densities but not for the age-2 densities. We used a simulation model to predict smolt production and adult returns given the observed juvenile age-1 densities from each adult cohort we outplanted. In general, predicted smolt production was greater during the mid 1990s and lower after 1999, despite the fact that more females were stocked into the study streams after 1999. Given the SAR rates measured during the study period and plausible over-winter survival rates in the study streams, we predicted that the observed juvenile production would produce few adults and would not result in a self-sustaining population. This conclusion was corroborated by adult return data. VVe found no evidence that adult outplanting increased wild population levels, i.e., there was no demographic boost in adult spawners. Further, the differences between lire two study streams showed that supplementation programs should carefully assess each target stream.
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Vol. 86 • No. 3