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

Abstract

Coho salmon, Oncorhynchus kisutch, and steelhead trout, Salmo gairdneri, 57 to 60 mll 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 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. The salmon poisoning trematode, Nanophyetus salmincola, can kill experimentally infected fish, and the same may be true for fish in nature (Millemann and Knapp, 1970). But we have no information on the sublethal effects of the parasite on fish, such as impairment of swimming ability and retardation of growth. This paper reports the results of a study, carried out from September 1968 to August 1970, on the effects of N. salniincola on the swimming ability of juvenile coho salmon, Oncorhlynchus kisutch, and summer-run steelhead trout (anadromous rainbow trout), Salito gairdneri. Effects of the parasite on the growth of these fish species will be reported in a subsequent paper. MATERIALS AND METHODS Experimental animals Coho or fertilized eggs were obtained from the Alsea River Salmon Hatchery of the Fish Commiission of Oregon, and steelhead Received for publication 16 February 1971. * This investigation was supported in part by Public Health Service Research Grant 5 RO1 AI06599 from the NIAID, and in part by the National Science Foundation Institutional Sea Grant GH 97, and constitutes part of doctoral thesis research done by the senior author at Oregon State University. Technical Paper No. 3026, Oregon Agricultural Experiment Station. trout from the Oregon State Game Commission's Oak Springs Fish Hatchery. The former hatchery is in the enzootic area of the trematode, but the used in the experiments were obtained as fry in late winter, when N. salmincola cercariae are not present in the hatchery water supply, or were hatched and reared in parasite-free well water. The hatchery at which the steelhead trout had been reared was outside the enzootic area, but the fish were hatched there from eggs obtained from adult fish taken in the Siletz River (Lincoln County, Oregon), which is within the enzootic area. Thus, all experimental fish were initially uninfected but originated from stocks that have had long evolutionary association with the parasite. The fish were maintained in outdoor cement tanks provided with flowing dechlorinated tap water and were fed commercial fish pellets several times a week. When the fish were used in experiments, their total lengths ranged from 57 to 60 mm and their weights from 1.4 to 1.7 g, but the fish used in any one experiment never differed from each other in weight by more than 0.2 g. Oxyjtrema silicula snails were collected from the Big Elk River and Beaver Creek in Lincoln County, Oregon. Those found to be infected with N. salmincola were maintained thereafter in the laboratory in plastic trays provided with flowing chlorine-free tap water at 15 to 16 C. Clam shells in the trays served as a calciium source. Snails were fed either lettuce or alder leaves. When cercariae were needed for an experiment, approximately 25 snails were placed in 1.0 liter of dechlorinated, aerated tap water at room temperature (near 21 C) and left there overnight, because exposure to the elevated temperature for 8