COMPARATIVE SUSCEPTIBILITY OF NATIVE SCOTTISH AND NORWEGIAN STOCKS OF ATLANTIC SALMON, SALMO-SALAR L, TO GYRODACTYLUS-SALRAIS MALMBERG - LABORATORY EXPERIMENTS

Salmon parr of each of three hatchery-reared stocks from the Rivers Shin and Conon in northeast Scotland, and from the River Lierelva in southeast Norway, were infected by exposing them concurrently to wild Norwegian salmon parr naturally infected with the ectoparasitic monogenean Gyrodactylus salaris. After exposure the fish were transferred to six holding tanks, two tanks to each stock. Each tank held 50 fish in a pooled group and another 12 fish individually isolated in small cages. At weekly intervals fish in each tank were anaesthetised and parasite numbers counted. No natural resistance was observed in the three stocks of salmon tested and all three were susceptible to G. salaris reproduction. However, 3-5 weeks post-infection a heterogeneity in the course of infection was observed in all three stocks, with highly susceptible, moderately susceptible, and apparently responding individuals in each tank. The results demonstrated an increasing ability to tolerate G. salaris with increasing size on the part of the salmon parr. In the pooled fish, parasite populations increased throughout the experiment, whereas in the isolated fish they tended to decrease 30-36 days post-infection. The experiments ended 50 days post-infection, mainly because of fish mortality. Possible explanations for the observed pattern of parasite population growth are discussed, together with the significance of these results for salmon populations in both countries.