A Natural‐Origin Steelhead Population's Response to Exclusion of Hatchery Fish

Abstract

It is asserted that reduction or elimination of hatchery stocking will increase natural‐origin salmon Oncorhynchus spp. and steelhead O. mykiss production. We conducted an analysis of steelhead population census data (1958–2017) to determine whether elimination of summer steelhead stocking in the upper Clackamas River in 1998 increased the productivity of natural‐origin winter steelhead. A Bayesian state–space stock–recruitment model was fitted to the adult steelhead data set, and productivity was estimated as a function of hatchery‐origin spawner abundance as well as other environmental factors. When used as a predictive variable in our model, the abundance of hatchery summer steelhead spawners (1972–2001) did not have a negative effect on winter steelhead recruitment. However, spill at North Fork Dam (the gateway to the upper Clackamas River basin) and the Pacific Decadal Oscillation (an index of ocean conditions) were both negatively associated with winter steelhead recruitment. Moreover, winter steelhead abundance in the upper Clackamas River basin failed to rebound to abundances observed in years prior to the hatchery program, and fluctuations in winter steelhead abundance were correlated with those of other regional winter steelhead stocks. Our assessment underscores the need for studies that (1) directly quantify the effects of hatchery fish on the production of natural‐origin salmon and steelhead, (2) empirically test published theories about mechanisms of hatchery fish impacts on natural‐origin populations, and (3) document population responses to major changes in hatchery programs.

An integrated population model for estimating the relative effects of natural and anthropogenic factors on a threatened population of Pacific trout

“…2009), and Chinook salmon (Levin, Zabel & Williams 2001). Our results provide further evidence that large releases of hatchery-reared juvenile steelhead have had a negative effect on productivity of wild steelhead, although we note some researchers have used an approach similar to ours and found no hatchery effect on productivity (Courter et al . 2019; Nelson et al .…”

An integrated population model for estimating the relative effects of natural and anthropogenic factors on a threatened population of steelhead trout

“…Nevertheless, research in other river systems points to negative ecological effects of hatchery fish on wild Pacific salmon, including populations of coho salmon (Buhle et al., 2009) and Chinook salmon (Levin et al., 2001). Our results provide further evidence that large releases of hatchery‐reared juvenile steelhead have had a negative effect on productivity of wild steelhead, although we note some researchers have used an approach similar to ours and found no hatchery effect on productivity (Courter et al., 2019; Nelson et al., 2019). Although we provide evidence for a correlation between hatchery releases and wild steelhead productivity, we did not demonstrate causation nor identify a causal mechanism.…”

“…Redds or spawning nests constructed before approximately mid‐March were not counted to avoid inclusion of any naturally spawning hatchery‐origin steelhead, which were purposely bred for earlier spawn timing, or coho salmon in the abundance estimates (cf. Courter et al., 2019). Fisheries biologists in the Skagit River basin generally consider the escapement estimates to be conservative; it is more likely that escapement is underestimated than overestimated because unobserved spawning sites would serve to increase abundance.…”

Hatchery propagation did not reduce natural steelhead productivity relative to habitat conditions and predation in a mid-Columbia River subbasin

“…Clackamas River winter-run steelhead, as originally documented by Kostow and Zhou (2006) and recently amended by Courter et al (2019), also provide a valuable case study of populationlevel effects of hatchery fish on natural-origin anadromous salmonids. Productivity of winter-run steelhead in the Clackamas River was not impaired by the abundance of hatchery-origin summer-run steelhead spawner escapement.…”

“…-Imperfect fish counts and age data can result in biased recruitment estimates (Zabel and Levin 2002;Sanz-Aguilar et al 2016). Therefore, to estimate observation error and process error, as well as provide measures of uncertainty for our recruitment estimates, we used a Ricker stock-recruitment model within a Bayesian state-space framework adapted from Fleischman et al (2013) and Courter et al (2019) (Equation 1) 1 . With this approach, interannual variation in recruitment was driven by productivity covariates (e.g., ocean conditions), density dependence, and observation error in age composition and spawning escapement.…”