Prevalence of Kudoa thyrsites in sexually mature and immature pen-reared Atlantic salmon (Salmo salar) in British Columbia, Canada

The prevalence of Kudoa thyrsites was determined for 17 harvests of Atlantic salmon (Salmo salar) from 7 fish farms in British Columbia, Canada. Fish were divided into 2 categories, sexually immature and sexually mature (grilse), using criteria typically employed at processing plants (external morphology). K. thyrsites was, with the exception of a few harvests, undetectable in the immature fish sampled between January and April 1995. In contrast, the prevalence of K. thyrsites in grilse sampled from the same harvests ranged from 0 to 76.9%. On average, sexually mature fish harvested between January and April 1995 were 13 times as likely to be infected with K. thyrsites as their sexually immature counterparts. In May, the prevalence of K. thyrsites increased in the sexually immature fish populations. A similar increase was not observed in the sexually mature fish from the same harvests. Examination of scale rings revealed that this apparent increase in prevalence of K. thyrsites in sexually immature fish was partially due to the misclassification of 'reconditioned' grilse (fish that have undergone sexual maturation and then lost their external sexual characteristics) as immature fish at the processing plants.