Electrophoretic examination of the population structure of brown trout, Salmo trutta L., from the Lough Neagh catchment, Northern Ireland

Samples of brown trout, Salmo trutia L., from 34 locations throughout the Lough Neagh system in north‐east Ireland were electrophoretically examined for genetic variation at 28 enzyme loci. Patterns of allelic variation at 12 polymorphic loci indicated the existence of genetic differentiation within as well as among several river systems, suggesting the existence of multiple brown trout populations. Significant gene frequency differences were detected over distances as little as 3 km, demonstrating the propensity of this species for microgeographic genetic differentiation. This was confirmed by a hierarchical analysis of genetic variance, some 35% of among‐sample variance being distributed within tributaries. Within Lough Neagh itself significant genetic differentiation was detected between two morphotypes (dollaghan and salmon‐trout) and for one of these (dollaghan) among samples from different years and from different areas of the lough. This suggests the existence of genetically differentiated subpopulations of originating from separate river catchments.