Physiological effects of the parasite ichthyophonus on spawning chinook salmon and their offspring in a Yukon River tributary

Abstract

In recent years, Chinook salmon Oncorhynchus tshawytscha returns to the Yukon River, Alaska, have been substantially reduced. In summer 2010-2012, spawning Chinook salmon (n=51, 32, and 23, respectively) were collected from the Salcha River, a tributary of the Yukon River, to determine the effects of Ichthyophonus, a protozoan parasite, on salmon reproductive success. Eggs and milt from Ichthyophonus-infected and non-infected parents were collected in 2010 and cross-fertilized to investigate offspring survival and potential second-generation effects induced by the parasite. Proximate composition analysis of adult muscle, eggs, and alevins, and blood chemistry analysis of adult blood plasma and alevin whole body homogenates were analyzed to explore potential differences between Ichthyophonus-infected and non-infected salmon. Ichthyophonus infection prevalence was 7.8, 6.3, and 8.3 % in 2010, 2011, and 2012, respectively. Egg lipid content was significantly higher in eggs from Ichthyophonus-infected females, compared to eggs from Ichthyophonus-negative females. Survival of fertilized eggs to hatching was not significantly different between offspring from Ichthyophonus-infected parents (Mean±1SD: 24.4±29.8 % survival) and non-infected parents (41.0±24.8 % survival). Proximate composition (% lipid, % protein, kJ/g) of muscle from spawning adult salmon also did not differ, nor did total body composition or morphology of alevins produced by either Ichthyophonus-infected or non-infected parents. We found no significant differences in blood plasma cortisol concentrations (a stress indicator) between Ichthyophonus-positive and negative adults or their offspring. There were also no significant differences in blood chemistry parameters indicative of tissue damage between Ichthyophonus-positive and Ichthyophonus-negative adults or resulting alevins, with the exception of aspartate aminotransferase, which was unexpectedly higher in plasma of Ichthyophonus-negative adults. Overall, infection with Ichthyophonus does not appear to impact the spawning ability or spawning success of Chinook salmon in the Salcha River.