A REVIEW OF THE INTERSPECIFIC AND INTRASPECIFIC VARIABILITY IN SALMONID HOSTS TO LABORATORY INFECTIONS WITH GYRODACTYLUS-SALARIS MALMBERG

Host specificity tests in the laboratory have demonstrated that Gyrodactylus salaris Malmberg can attach to and reproduce on species of Salmo, Salvelinus, Thymallus and Oncorhynchus. Inter- and intraspecific variations in host specificity and transmission capacity of G. salaris are described briefly. All Norwegian salmon individuals were initially susceptible and under 10% were later able to mount a response against the infection. Generally, heterogeneity in susceptibility/resistance to G. salaris seems less prominent in the Norwegian salmon stocks tested, than the Baltic salmon. Laboratory experiments indicate no correlation between temperature and host specificity, but positive correlations between temperature and the innate capacity for increase and the transmission rate. The results presented support the view that heavy salmon epidemics in G. salaris-infected rivers in Norway may occur because of this parasite. However, the importance of temperature on G. salaris reproduction and transmission suggests seasonal differences in the development and outcome of the colonization of infected rivers in Norway, along e.g. the north-south gradient.