Variation in Prevalence of Nanophyetus salmincola, a Parasite Tag Indicating U.S. Northwest Origin, in Steelhead Trout (Oncorhynchus mykiss) Caught in the Central North Pacific Ocean

Infections with the parasite "tag" Nanophyetus salmincola indicated that in 1986 and 1987, steelhead trout (Oncorhynchus mykiss) from the U.S. Northwest (Washington to northern California, including Idaho) were relatively abundant in the central North Pacific Ocean as far west as 165°E, about 5000–5500 km from their area of origin. Infection prevalence (the frequency of the presence of infection) was estimated annually for specimens stratified by 5°-longitude areas. East–west and north–south trends in prevalence occurred in 1986 but not in 1987. Correlation analysis indicated no between-year correlation in the relative magnitude of prevalence in areas sampled in both 1986 and 1987 (r = 0.017). Confidence intervals (90%) for overall annual prevalence in the central North Pacific Ocean steelhead population were 29.9–61.3% (point estimate = 45.4%) for 1986 and 43.3–63.0% (point estimate = 53.2%) for 1987.