Parasite-associated mortality of juvenile Pacific salmon caused by the trematode Nanophyetus salmincola during early marine residence

The potential effect of the freshwater trematode Nanophyetus salmincola on early marine survival of Pacific salmon was assessed by monitoring the prevalence and intensity of metacercarial infection in yearling coho salmon Oncorhynchus kisutch, and yearling and subyearling Chinook salmon O. tshawytscha caught off Oregon and Washington during May, June, and September of 1999 to 2002. Annual prevalences of N. salmincola infection in yearling coho salmon were 62 to 78% and were significantly greater each year than in both yearling and subyearling Chinook salmon (19.3 to 53.8% and 40.5 to 53.5%, respectively). Yearling coho salmon also had significantly higher intensities of infection (from approximately 2-fold to 12-fold) than yearling and subyearling Chinook salmon. Prevalences and intensities in coho salmon caught in September were significantly lower (by approximately 21%) than in coho salmon caught in May or June in 3 of the 4 years. Variance to mean ratios of parasite abundance in coho salmon were also lowest in September, suggesting parasite-associated host mortality during early ocean residence. There was no evidence for a seasonal decline in infection in yearling or subyearling Chinook salmon. Infection intensities, but not prevalences, were significantly greater in naturally produced (wild) coho salmon than in hatchery produced coho salmon and could be due to differences in exposure to the trematode. Highly infected naturally produced coho salmon were not caught in September. This study suggests that coho salmon with high intensities of N. salmincola may not survive early marine residence, and that disease processes need to be considered as a factor affecting marine survival of juvenile salmon.