Ichthyophonus In Chinook Salmon - the Marine Face of a Freshwater Problem

Ichthyophonus hoferi is a marine-derived parasite of various teleost species, including Chinook salmon (Oncorhynchus tshawytscha). Salmon stocks in Western Alaska are declining for yet unknown reasons, and involvement of disease in these declines cannot be ruled out either due to pathogen-induced mortality, reduced fecundity, or the inability of Chinook salmon to successfully migrate and spawn in tributaries. Chinook salmon is a staple of Alaska’s fishery and low abundance has led to economic hardships for communities dependent upon them. Ichthyophonus undergoes cyclic prevalence changes in the freshwater phase of Chinook salmon that are likely correlated to marine ocean temperatures and conditions, yet little is known about the parasite in salmonids during their marine phase. Collaboration with seafood processors in Dutch Harbor made it possible to obtain samples from Chinook salmon by-caught in the ‘A’-season of the Pollock (Theragra chalcogramma) fishery in 2010. A total of 393 samples were collected ranging in total length from less than 250 mm to 939 mm with 59.5% of fish being female. Presence/absence of Ichthyophonus 18S rDNA was determined using PCR. Prevalence of Ichthyophonus in salmon cardiac tissue was 1% (4 of 393) and is an order of magnitude lower than prevalence of the parasite in Chinook salmon entering the Yukon River. Metabolomic profiles of infected fish compared to size- and gender-matched “healthly” individuals reveal a number of metabolites and associated metabolic and immune processes that are differentiated between these two groups.