Genetic composition of facultatively anadromous brown trout <i>Salmo trutta</i> L. in a freshwater‐lake system

Brown trout (Salmo trutta) has adapted to a wide range of habitats and displays a variety of life-history strategies. Being facultatively anadromous, populations without migration barriers to the sea consist of both resident and anadromous individuals in different proportions. This affects genetic drift and the level of gene flow between populations, and therefore shapes the metapopulation structure. Here, utilizing genetic variation at 95 SNP markers, we describe the genetic structure and effective size of a brown trout population inhabiting the Fremstad catchment that supports both anadromous and freshwater resident life histories. Compared to other sea trout populations in the region, the Fremstad population is genetically divergent with a unique genetic signal. A weak and significant genetic structure was observed in the Fremstad catchment, and anadromous individuals were not assigned to a specific lake or stream. The effective population size of brown trout in the Fremstad catchment is large compared to the average annual entry of sea trout into the catchment; hence, a large proportion of non-anadromous spawners likely contribute to the population. Approximately 2% of sea trout entering the catchment were strays from other populations. Because of a large proportion of non-anadromous spawners, the proportion of strays into the overall spawning population is likely to be lower than observed among the sea trout, and contribute to the genetic divergence of the Fremstad population from other populations. This study adds to our understanding of the evolutionary drivers that affect the genetic structure of anadromous brown trout populations and demonstrates how small spawning streams can be evolutionarily important in supporting a large and genetically unique population.