Temperature Effect on Pre-Spawn Mortality of Ichthyophonus-Infected Chinook Salmon in the Yukon River

Through field observations and controlled laboratory studies it is possible to elucidate the effect(s) of a single variable on a host-parasite relationship, such as the impact of fluctuating temperature in the Yukon River on Ichthyophonus sp. and its Chinook salmon host. Field studies have shown that up to 40% of salmon entering the Yukon River on their annual spawning migration are infected with Ichthyophonus, and this level of infection remains constant over the entire migration, while overt disease increases. However, among post-spawn fish only 10-12% are Ichthyophonus-positive, raising the question of what happened to the remaining 25-30% of infected fish. We examined water temperature as a variable to explain this pre-spawn loss of infected fish. During the first half of the annual run, salmon encounter water temperatures <15 oC and exhibit few signs of disease, while those in the second half of the run are exposed to temperatures at or greater than 20 oC and exhibit higher levels of disease. Controlled laboratory studies have shown that Ichthyophonus-infected trout experience little mortality when maintained at temperatures below 15 oC, but above this temperature mortality reaches 100%. Likewise, time-to-death decreased as temperature increased from 10 oC to 20 oC. Other studies demonstrated significantly reduced stamina in Ichthyophonus-infected trout compared with healthy fish, when forced to swim against a current. Increased water temperature significantly increased the difference between infected and healthy fish. Based on field observations and controlled studies, we postulated that the loss of Ichthyophonus-infected Chinook salmon prior to reaching their natal streams resulted from increased mortality, early mortality and decreased stamina, all of which prevented severely infected fish from reaching their destination.