Genetic introgression of anadromous Atlantic salmon threatens persistence of a nonanadromous riverine population

The upper River Namsen harbours landlocked Atlantic salmon Salmo salar with a riverine life history, isolated 9500 years ago. This isolation is reflected in large genetic distances ( F ST = 0.37) to anadromous salmon. During 1950–1976, thousands of fry of anadromous salmon were stocked annually in landlocked habitat, and from 1976, fish ladders allowed anadromous spawners access to a mixing zone in the lower landlocked stretch. We demonstrate limited hybridization between anadromous and landlocked salmon caused by fry releases (0.4%) and extensive hybridization following fish ladders to the mixing zone. There, anadromous salmon dominate with 76% in 1998 and 96% in 2017, whereas landlocked salmon decreased from 20% in 1998 to 0% in 2017, the remaining being hybrids. Hybrids were also identified among smolt-like juveniles in 1978. This suggests that hybrid individuals may outmigrate as anadromous salmon while landlocked salmon are resident. The outmigration of hybrids may lead to the recovery of landlocked salmon above the waterfalls, provided that anadromous salmon are not given access to the mixing zone.