Differential susceptibility of Yukon River and Salish Sea stocks of Chinook salmon Oncorhynchus tshawytscha to ichthyophoniasis

Preliminary evidence suggests that Chinook salmon Oncorhynchus tshawytscha from the Yukon River may be more susceptible to Ichthyophonus sp. infections than Chinook from stocks further south. To investigate this hypothesis in a controlled environment, we experimentally challenged juvenile Chinook from the Yukon River and from the Salish Sea with Ichthyophonus sp. and evaluated mortality, infection prevalence and infection load over time. We found that juvenile Chinook salmon from a Yukon River stock were more susceptible to ichthyophoniasis than were those from a Salish Sea stock. After feeding with tissues from infected Pacific herring Clupea pallasii, Chinook salmon from both stocks became infected. The infection was persistent and progressive in Yukon River stock fish, where infections sometimes progressed to mortality, and histological examinations revealed parasite dissemination and proliferation throughout the host tissues. In Salish Seaorigin fish, however, infections were largely transient; host mortalities were rare, and parasite stages were largely cleared from most tissues after 3-4 wk. Susceptibility differences were evidenced by greater cumulative mortality, infection prevalence, parasite density, proportion of fish demonstrating a cellular response, and intensity of the cellular response among fish from the Yukon River stock. These observed differences between Chinook salmon stocks were consistent when parasite exposures occurred in both freshwater and seawater. These results support the hypothesis that a longer-standing host-pathogen relationship, resulting in decreased disease susceptibility, exists among Salish Sea Chinook salmon than among Yukon River conspecifics.

Automated summary

The study showed that juvenile Chinook salmon from the Yukon River are more susceptible to Ichthyophonus sp. infections compared to those from the Salish Sea, leading to higher mortality and infection rates. Infection in Yukon River stock fish was persistent and progressed rapidly, while infections in Salish Sea-origin fish were largely transient and cleared relatively quickly. These results suggest that a longer-standing host-pathogen relationship exists among Salish Sea Chinook salmon, resulting in decreased disease susceptibility compared to Yukon River conspecifics.