Influence of Four Species of Helminth Parasites on Orientation of Seaward Migrating Sockeye Salmon (Oncorhynchus nerka) Smolts

Abstract

Each of four helminth parasite species, including the cestodes Eubothrium salvelini, Diphyllobothrium dendriticum, and Proteocephalus sp. and the nematode Philonema oncorhynchi, was found to influence the orientation of seaward-migrating sockeye salmon (Oncorhynchus nerka) smolts from the Great Central Lake area, Vancouver Island, British Columbia. Smolts placed in circular tanks were allowed to exit into evenly spaced bucket traps, and fish from five such runs were measured, sexed, and examined for the presence of parasites. Fish were categorized on the basis of infection status ("+" vs. "−") for each of the four parasite species found. The +/− groups did not differ for any of the parasite categories with respect to either bucket distributions or overall orientation directions. However, "−" groups did exhibit consistently greater departure from random distribution than their "+" counterparts. This finding suggests interference of parasites with smolt orientation, which could in turn have important implications for smolt survival. This finding also suggests that parasite infection may account to some extent for the considerable variability observed in the migratory behaviour of smolts.