Spatial and temporal genetic variation in small and declining populations of Atlantic salmon

Atlantic salmon is protected under national and European legislation but most populations are declining. The research investigated population size, habitat quality, population structure and stability over time using Atlantic salmon parr in the River Frome (a chalk stream in Dorset, England) surveyed at 15 sites, in summer and autumn over three consecutive years. The hypotheses were: * Abundance of juvenile Atlantic salmon in a chalk stream is related to habitat quality. * There is significant genetic differentiation between Atlantic salmon within one river, despite small geographic distance between sites and no barriers to migration. * Temporal stability of spatial population structure of Atlantic salmon occurs within a river. Habitat quality, assessed using HABSCORE, varied between sites and over time. Habitat quality was not a predictor of juvenile density but presence of adult brown trout reduced juvenile numbers. DNA for molecular analysis was extracted from fin-clips (removed non-lethally) and genetic variation and distribution of genetic variation was assessed using 5 microsatellites. Relatedness tests indicated that some 0+ parr within a site were closely related. It was inferred that 1+ parr had moved from their natal site, as they could not be assigned to a site of origin. Moderate heterozygosity was detected at each sample site. Low, but significant, genetic differentiation over all sites was detected in summer and autumn for three consecutive years. Isolation by distance was detected at two out of the six sample times, despite small distances between sites. This was strong evidence for structuring within the population. Using a new statistical test, no temporal stability of spatial population structure was detected in the River Frome, but significant temporal stability was detected using published data for Atlantic salmon in a Canadian river