Effect of the "Salmon Poisoning" Trematode, Nanophyetus salmincola, on the Swimming Ability of Juvenile Salmonid Fishes

Coho salmon, Oncorhynchus kisutch, and steelhead trout, Salmo gairdneri, 57 to 60 mm in total length, were exposed either once for 1 hr to 1,500 cercariae of Nanophyetus salmincola, or daily for 1 hr to 100 of these parasites for 15 days. The effect of the parasites on the swimming ability of fish exposed only once was determined immediately after exposure, or after 6, 12, 24, or 96 hr, or after 15 days; fish exposed daily were tested immediately after the last exposure. Groups of control and infected fish were tested together in a swimming tube. In some tests, the water velocity in the tube was increased gradually at 10-min intervals, and the velocity at which each fish became fatigued was recorded. In other tests, the water velocity was suddenly increased to a high, constant level, and the duration of swimming of each fish at that velocity was recorded. In both types of tests, there was a marked effect of the parasite on the swimming ability of fish exposed once and tested 0 to 96 hr later and on those exposed repeatedly and tested after their last exposure. The mean swimming speeds or times of groups of steelhead trout were reduced by 10 to 58%, as compared with those of controls, and corresponding per cent reduction values for coho salmon ranged from 4 to 95%. This impairment of the swimming ability of infected fish is attributed to tissue injury caused by migrating parasites. The swimming ability of fish that were exposed once and tested 15 days later, when the parasites were encysted, was little affected. However, it is likely that the effect would have been greater had the infections been heavier.