Incidence of hybridization between Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in Ireland

The substantial growth of the farmed salmon industry in Europe since the 19 70s has highlighted concerns regarding the genetic impact of escaped farmed salmon on wild salmonid stocks. High incidences of salmon x trout hybrids have been recorded in rivers situated near intensive salmon farming in Norw ay and Scotland, which may be indicative of a breakdown in reproductive iso lation between salmon, Salmo salar L., and brown trout, Salmo trutta L, In the present study, salmonid fry and 0+ parr were collected from rivers in w estern Ireland. Allozyme and minisatellite DNA analysis were carried out on fry to determine the frequency of F1 hybrids from 10 rivers located within 38 km of salmon farms and three rivers at least 80 km from salmon farms. A total of 49 hybrids were recorded from 4135 salmonid fry (frequency = 1.2% ). Mitochondrial DNA analysis showed that all hybrids arose from Atlantic s almon female x brown trout male crosses. Hybrid parr were recorded from one of the low-risk rivers (1.0%), but were present in seven out of the 10 cat chments located within 38 km of salmon farms, with frequencies ranging from 0.7% to 3.1%. The results of the present survey, which represents the firs t extensive record of the levels of salmon-trout hybridization in Ireland, are discussed in relation to the continued growth of salmon farming in this country.