Evaluating the feasibility of reestablishing a coho salmon population in the Yakima River, Washington

Historical returns of coho salmon Oncorhynchus kisutch to the Yakima River basin were estimated to range from 45,000 to 100,000 fish annually but declined to zero by the 1980s after decades of overexploitation of fishery, water, and habitat resources. In 1996, the Yakama Nation and cooperators initiated a project to determine the feasibility of reestablishing a naturally spawning coho salmon population in the Yakima River. The project explored the feasibility of successful coho salmon recolonization in the Yakima River by introducing stocks that had been reared in hatcheries for multiple generations. After 10-20 years of outplanting, we compared data for adult returns of known natural origin (i.e., returns from parents that spawned in the wild) and returns from hatchery releases. We found that fish of natural origin returned at a significantly larger size than those of hatchery origin. The mean egg mass and mean egg size of natural-origin females were greater than those of hatchery-origin females, but the differences were statistically significant for only one of three sample years. Natural-origin adults returned 2-9 d later and spawned 5 d later than their hatchery-origin counterparts. Preliminary indices of smolt-to-adult survival for natural-origin fish were 3.5-17.0 times the survival indices of hatchery-origin fish. The number of returns to the historical spawning habitats in upriver areas generally increased. Spawning surveys demonstrated the existence of robust and sustainable spawning aggregates in various locations in the basin. Hatchery releases from the local brood source (Yakima River returns) had significantly higher smolt-to-smolt survival than releases from out-of-basin (non-Yakima River) hatchery broodstock, but some of these observed differences in survival may be partially attributable to differences in smolt size. We concluded that hatchery-origin coho salmon with a legacy of as many as 10-30 generations of hatchery influence demonstrated an ability to reestablish themselves in the Yakima River (i.e. as a naturalized, normative population) after as few as 3-5 generations of outplanting in the wild.