An evaluation of the influence of stock origin and out-migration history on the disease susceptibility and survival of juvenile Chinook salmon.

Various methods have been developed to mitigate the adverse effects of the Federal Columbia River Power System on juvenile Pacific salmon out-migrating through the Columbia River basin. In this study, we found that hatchery-reared spring Chinook salmon Oncorhynchus tshawytscha in the river are in varying degrees of health, which may affect delayed mortality and the assessment of the effectiveness of management actions to recover listed stocks (e.g., barging fish downstream versus leaving fish in the river). A laboratory disease challenge with Listonella anguillarum was completed on fish from Rapid River Hatchery and Dworshak National Fish Hatchery (NFH) with different out-migration histories: (1) transported by barge, (2) removed from the river before barging, or (3) left to travel in-river. Barged fish from Rapid River Hatchery experienced less mortality than fish from Dworshak NFH. No statistical differences were found between the hatcheries with fish that had in-river out-migration histories. We suggest that the stressors and low survival associated with out-migration through the hydropower system eliminated any differences that could have been present. However, 18-25% of the fish that were barged or collected before barging died in the laboratory before the disease challenge, compared with less than 2% of those that traveled in-river. Owing to disproportionate prechallenge mortality, the disease-challenged populations may have been biased; thus, they were also considered together with the prechallenge mortalities. The synthesis of prechallenge and disease-challenged mortalities and health characteristics evaluated during out-migration indicated that the benefit of barging was not consistent between the hatcheries. This finding agrees with adult survival and delayed mortality estimates for the individual hatcheries determined from adult returns. The results suggest that the health status of fish and their history before entering the hydropower system (hatchery of origin and out-migration path) are critical variables affecting the conclusions drawn from studies that evaluate mitigation strategies.

Disease Susceptibility of Hatchery Snake River Spring-Summer Chinook Salmon with Different Juvenile Migration Histories in the Columbia River.

Various methods have been developed to mitigate the effects of dams on juvenile Pacific salmon Oncorhynchus spp. migrating to the Pacific Ocean through the Columbia River basin. In this study, we examined the health of hatchery Snake River spring and summer Chinook salmon relative to two mitigating strategies: dam bypass and transportation (e.g., barging). The health of out-migrants was assessed in terms of the difference in the incidence of mortality among fish, categorically grouped into no-bypass, bypass, and transportation life histories, in response to challenge with the marine pathogen Listonella anguillarum during seawater holding. These three life histories were defined as follows: (1) fish that were not detected at any of the juvenile bypass systems above Bonneville Dam were classified as having a no-bypass life history; (2) fish that were detected at one or more juvenile bypass systems above Bonneville Dam were classified as having a bypass life history; and (3) fish that were barged were classified as having the transportation life history. Barged fish were found to be less susceptible to L. anguillarum than in-river fish-whether bypassed or not-which suggests that transportation may help mitigate the adverse health effects of the hydropower system of the Columbia River basin on Snake River spring-summer Chinook salmon. The findings of this study are not necessarily transferable to other out-migrant stocks in the Columbia River basin, given that only one evolutionarily significant unit, that is, Snake River spring-summer Chinook salmon, was used in this study.