Retrospective Analysis of a Natural-Origin Steelhead Population's Response to Exclusion of Hatchery Fish

We conducted a retrospective analysis with Upper Clackamas River steelhead population census data to determine the cause of a notable decline in natural-origin winter steelhead spawner abundance during adult return years 1972-1998. It was asserted that out-of-basin hatchery summer steelhead directly competed with native juvenile winter steelhead for rearing habitat, thereby causing the decline in winter steelhead abundance. If this casual mechanism were accurate, a population increase would be expected to occur following hatchery fish exclusion (1999). However, we found that hatchery summer steelhead did not affect winter steelhead returns, and winter steelhead abundance in the Upper Clackamas River did not rebound to levels observed during the years preceding hatchery stocking. Instead, fluctuations in winter steelhead abundance were correlated with other regional winter steelhead stocks. There is strong covariation between productivity of contiguous steelhead populations, and this covariance declines with increasing distance between watersheds. Therefore, the decline in abundance of natural-origin steelhead in the Upper Clackamas River (1972-1998) was principally driven by survival rates common to steelhead populations in the Lower Columbia/ Willamette River region. Our analysis provides evidence that summer steelhead hatchery programs in the Clackamas Basin can coexist with natural-origin winter steelhead populations without impairing winter steelhead productivity.