VANN: Hybridisation between Atlantic salmon and brown trout in Gyrodactylus-infected rivers: consequences for population viability

Extremely high proportions of hybrids between Atlantic salmon and brown trout have recently been detected in rivers Vefsna and Driva. Both rivers have been infected by the parasite Gyrodactylus salaris since the 1970s, causing a dramatic decline in juveni le salmon densities. Reasons for the remarkable hybridisation rates are probably the dramatically reduced wild salmon stocks and a high frequency of non-native salmon. Increased hybridisation may cause vulnerable populations to become even less viable, an d this needs to be modelled in a stock-recruitment context for both Atlantic salmon and brown trout. On top of this ecological effect, high numbers of hybrids may increase the likelihood of rare events such as back-crosses to one or other parental species and introgression of genes from one species to the other. We have already identified triploid backcrosses in both rivers, and detailed genetic analyses are needed to investigate whether diploid backcrosses are formed. Ongoing investigations in both rive rs supply material for studying host-parasite dynamics and some laboratory support for identifying first-generation hybrids. This current proposal suggests to increase the molecular investigations to be able to answer questions related to 1) the occurrenc e of hybridisation beyond the first generation, and 2) the population of origin of the parental fish engaged in hybridisation. Results from the molecular studies will be used to model population viability in the absence/presence of introgression, using me thods developed by a current RCN project.