Interspecific hybridization and introgression are associated with stock transfers in salmonids

Domestic stocks may endanger wild populations by escapes or deliberate releases, not only through competition for resources but also through gene pool contamination if they reproduce in the wild. One of the risks is increased interspecific hybridization by interspecific matings. Stocking of domesticated genomes into wild ecosystems leads to a heightened frequency of interspecific hybridization in sympatric south European Atlantic salmon Salmo salar and brown trout Salmo trutta. Introgressive hybridization involving brown trout males of domestic origin was demonstrated. Following cessation of domestic stock releases in eight south European rivers, hybridization decreased significantly. These results emphasize the risks associated with stock transfers and escapes of domestic individuals in wild ecosystems and illustrate the potential role of domestic stocks when they contribute to secondary contact between species.