Long-term changes in parasites of sockeye salmon (Oncorhynchus nerka) smolts

Abstract

Parasites were examined in 41 annual samples of sockeye salmon (Oncorhynchus nerka) juveniles originating from lakes in British Columbia: Chilko and Shuswap. Nine species of parasitic helminths and two species of parasitic Crustacea were recovered from various organs. The number of species ranged from three to nine per year. Most (94%) parasites belonged to three tapeworm species, identified as core to the parasite community because they also infected the most hosts. Two of these cestode species are probably maintained by resident freshwater hosts in the lakes. Positive numerical associations were observed between four parasite species pairs. Parasite communities in both lakes were composed of similar species, with the exception of three rare Chilko Lake species that were never recovered from Shuswap Lake. Parasites were often as variable within stock (temporally) as between stocks (geographically), except for the prevalence of Eubothrium sp. and Neoechinorhynchus sp. and mean annual intensity of Proteocephalus sp. and encysted digenetic metacercariae, which were greater at Chilko Lake than at Shuswap Lake. Discrimination between the two lakes based on their parasite community is possible only in extreme cases of infection and thus is of limited use to fisheries officers attempting to distinguish between these stocks.