Atlantic Salmon and Gyrodactylus Parasites in the Geological Theater in North-Western Europe

In seventies, many Norwegian Atlantic salmon rivers were fatally attacked by an unexpected novel parasite, later confirmed to be Gyrodactylus salaris which was described in 1957 in a Swedish hatchery. The parasite was seldom observed and never reported as pathogenic in the Baltic basin. Thus, it was suggested that the Baltic salmon was resistant and Atlantic stocks susceptible. The phylogeography of the Atlantic salmon by allozyme markers suggested strong division (1 Myr) between Eastern and Western Atlantic salmon, but the position of the Baltic populations was unsolved. Because the continental ice covered the whole Northern Europe, it was natural to expect that the salmon arrived to the Baltic basin from Atlantic, less than 10 000 BP. This hypothesis failed when tested by updated markers. The Baltic salmon has a much longer history of isolation. The Baltic basin  was repopulated from the freshwater lakes east and south of the ice cap, and there is no gene flow from the Atlantic. Also, a molecular analysis proved that G. salaris deserved more than  just one name tag. The parasites are viviparous hermaphrodites, but propagate mostly parthenogenetically. There are several sister lineages, which are host-specific on salmon, grayling, or farmed rainbow trout. The molecular analysis of the parasite populations in the Baltic basin demonstrated that the salmon specific G. salaris strain is probably a hybrid between two separate strains on grayling (T. thymallus). The hybridization and host switch possibly occurred  during the Eemian interglacial 132 000 BP, when the White Sea and Baltic basins were shortly connected. Consequently, the landlocked salmon populations have had more than 100 000 years to adapt to the parasite. The Baltic salmon is winning the game: the parasite prevalence in the great Russian lakes is about 1%. When the Baltic parasites have been transported to Norway, or to the White Sea, they have been able to kill up to 98% of the salmon juveniles.