Salinity tolerance of Gyrodactylus salaris (Platyhelminthes, Monogenea): laboratory studies

The salinity tolerance of the freshwater monogenean Gyrodactylus safaris, infecting Atlantic salmon (Salmo salar) parr, was studied experimentally. Following direct transfer of infected fish from fresh water to 5.0 parts per thousand salinity, parasite population growth increased at the same rate as in fresh water and was positively correlated with temperature (1.4, 6.0, and 12.0 degrees C). In 7.5 parts per thousand salinity the populations declined and became extinct after a maximum of 56 days, without any significant difference between 6.0 and 12.0 degrees C. However, some infrapopulations demonstrated short periods of growth. At higher salinities (10.0, 15.0, 20.0, and 33.0 parts per thousand) the survival time decreased, and there was a negative correlation between survival time and temperature(1.4, 6.0, and 12.0 degrees C). When transferred directly to sea water (33.0 parts per thousand) the parasites became opaque and ceased moving after a few minutes. There was no difference in parasite survival time between direct and gradual transfer from fresh water to 7.5 and 10.0 parts per thousand, except for one infrapopulation which demonstrated population growth from day 22 after some fluctuations following gradual transfer to 7.5 parts per thousand. The present findings support the hypothesis of brackish water dispersal of G. salaris with infected fish migrating between rivers in fjord systems.