Synergistic Effects of Hybridization and an Invasive Pathogen on Extinction Risk in a Salmonid Fish

The monogenean parasite Gyrodactylus salaris is a recent introduction to many Norwegian watercourses, where it has led to strong reductions in the population size of Atlantic salmon (Salmo salar). Concurrently, interspecific hybrids between Atlantic salmon and brown trout (S. trutta), typically found at low proportions, seem to be extremely prevalent in rivers where salmon are infected with G. salaris. Interspecific hybrids compete for space and food in the rivers as juveniles but leave few, if any, offspring. One reason for their high prevalence in rivers with G. salaris may be the low encounter rate of conspecific mates in salmon populations having been strongly reduced by the parasite. Interspecific hybrids show intermediate tolerance to G. salaris, whereas the parent species are either susceptible or resistant. This begs the question whether infestation by G. salaris and interspecific hybridization can lead the affected populations into an extinction vortex. We evaluate this idea using field and experimental data, and modelling of host-parasite relationships.