An unusually high frequency of Atlantic salmon x brown trout hybrids in the Loch Lomond catchment, west-central Scotland

ABSTRACT A genetic study to examine population structuring of fish identified by anglers on the basis of external morphology, as Atlantic salmon, Salmo salar, returning as sea-migrants to the Loch Lomond catchment, recorded a rate of hybridisation with brown trout, Salmo trutta, of 10.4%. This is much higher than previously reported for adult fish elsewhere and considerably higher than amongst juveniles sampled from across the catchment (0.7%). The sea-migrant hybrids recorded here were F1-hybrids derived from matings comprising one brown trout female and at least three different salmon females. Mitochondrial DNA haplotypes suggest that these are associated with spawning in different parts of the Loch Lomond catchment. The cause(s) of the high incidence among adults is uncertain. However, these findings challenge the general view, derived from existing literature, that lifetime fitness in Atlantic salmon x brown trout hybrids is low. Suggesting that, at least occasionally, hybrid survival to sexual maturity may be similar to that of individuals in the parental populations. The potential impact of high hybrid survivorship on parental species dynamics is discussed