Effects of Ichthyophonus on Chinook Salmon Reproductive Success in the Yukon River Drainage

In recent years, Chinook salmon (Oncorhynchus tshawytscha) returns to the Yukon River have been inadequate.  As a result, fisheries managers have had to enforce unprecedented restrictions on Chinook salmon harvest for subsistence users and commercial fishermen, including a no-buy policy for Chinook salmon during chum salmon (Oncorhynchus keta) commercial openings and reductions in subsistence fishing time. The reasons for these run failures remain poorly understood, but Ichthyophonus, a protozoan parasite, is known to adversely affect stamina of adults during their spawning migration. The effects of Ichthyophonus on salmon reproductive success have not been evaluated and therefore, escapement quality, not the number of fishes escaping onto the spawning grounds, may have to be taken into account in spawner-recruit models. In summer 2010, Chinook salmon (n=51) were collected from the Salcha River, a tributary in the Yukon River drainage, to determine the prevalence and effects of Ichthyophonus on hatching success and offspring quality.  Eggs and milt were collected in the field, and gametes of “healthy” and Ichthyophonus-infected parents were cross-fertilized and transported to the University of Alaska Fairbanks. Infection with Ichthyophonus was determined by visual inspection of clinical signs in the field and confirmed by culture of cardiac muscle.  Fertilized ova were raised to the yolk-sac stage and dead eggs were enumerated and removed daily.  The average survival of eggs to hatching was 41% for offspring of “healthy”, Ichthyophonus-negative parents, while the average survival of eggs to hatching for the offspring of Ichthyophonus-positive parents was 24%. Proximate composition (%water, %lipid, %crude protein) and morphometric parameters (length, weight, height of yolk-sac) of yolk-sac larvae were not statistically different between “healthy” and infected crosses, indicating that once hatched, offspring have the same composition regardless of the health status of the parent. Overall, the prevalence of Ichthyophonus in Salcha River Chinook salmon was 8%, similar to other locations along the Yukon River in 2010.  Understanding the effect of pathogens, such as Ichthyophonus, on survival and growth of juveniles hatching from gametes of infected parents is critical to foster pro-active management of all salmon life stages.