Population supplementation programs that release captive-bred offspring into the wild to boost the size of endangered populations are now in place for many species. The use of hatcheries for supplementing salmonid populations has become particularly popular. Nevertheless, whether such programs actually increase the size of wild populations remains unclear, and predictions that supplementation fish drag down the fitness of wild fish remain untested. To address these issues, we performed DNA-based parentage analyses on almost complete samples of anadromous steelhead (Oncorhynchus mykiss) in the Hood River in Oregon (U.S.A.). Steelhead from a supplementation hatchery (reared in a supplementation hatchery and then allowed to spawn naturally in the wild) had reproductive success indistinguishable from that of wild fish. In contrast, fish from a traditional hatchery (nonlocal origin, multiple generations in hatcheries) breeding in the same river showed significantly lower fitness than wild fish. In addition, crosses between wild fish and supplementation fish were as reproductively successful as those between wild parents. Thus, there was no sign that supplementation fish drag down the fitness of wild fish by breeding with them for a single generation. On the other hand, crosses between hatchery fish of either type (traditional or supplementation) were less fit than expected, suggesting a possible interaction effect. These are the first data to show that a supplementation program with native brood stock can provide a single-generation boost to the size of a natural steelhead population without obvious short-term fitness costs. The long-term effects of population supplementation remain untested.

Efforts to rehabilitate threatened summer-run (SR) and winter-run (WR) populations of steelhead Oncorhynchus mykiss in the Hood River, Oregon, include operation of two conservation hatchery programs. Annual collection of discrete broodstocks relies on identification of SR and WR fish based on return time, reproductive state, and other physical and behavioral characteristics. An increased likelihood of misidentification occurs in the spring, when SR and WR return times overlap. The resulting interbreeding between ecotypes in the hatchery imposes serious genetic risks to wild SR and WR steelhead populations. We characterized life history differences between Hood River SR and WR steelhead and evaluated genetic structure using a suite of microsatellite loci. Significant differences in life history, including age at return and fork length, were corroborated by spatially influenced genetic structure among SR and WR fish; SR fish spawn in the west fork of the Hood River, while WR fish spawn in the east and middle forks. Temporal and spatial distinctions persist among Hood River steelhead ecotypes despite past operation of segregated hatchery programs utilizing out-of-basin stocks. We evaluated the feasibility of using real-time genetic assignment tests to differentiate SR from WR among candidate broodstock fish captured downstream of spawning grounds. Among adult steelhead of known origin, we observed 91.5% assignment accuracy for WR fish and 77.7% accuracy for SR fish. A rapid-response protocol was subsequently developed that provides hatchery managers with genetic assignment of broodstock to ecotypes within 24 h after interrogation of returning fish.

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