Spatial and temporal genetic differentiation and effective population size of brown trout (Salmo trutta, L.) in small Danish rivers

The spatial and temporal genetic structure of brown trout populations from three small tributaries of Lake Hald, Denmark, was studied using analysis of variation at eight microsatellite loci. From two of the populations temporal samples were available, separated by up to 13 years (3.7 generations). Significant genetic differentiation was observed among all samples, however, hierarchical analysis of molecular variance (AMOVA) showed that differentiation among populations accounted for a non-significant amount of the genetic differentiation, whereas differentiation among temporal samples within populations was highly significant (0.0244, P < 0.001). Estimates of effective population size (N-e) using a maximum-likelihood based implementation of the temporal method, yielded small values (N-e ranging from 33 to 79). When a model was applied that allows for migration among populations, N-e estimates were even lower (24-54), and migration rates were suggested to be high (0.13-0.36). All samples displayed a clear signal of a recent bottleneck, probably stemming from a period of unfavourable conditions due to organic pollution in the 1970-1980's. By comparison to other estimates of N-e in brown trout, Lake Hald trout represent a system of small populations linked by extensive gene flow, whereas other populations in larger rivers exhibit much higher N-e values and experience lower levels of immigration. We suggest that management considerations for systems like Lake Hald brown trout should focus both on a regional scale and at the level of individual populations, as the future persistence of populations depends both on maintaining individual populations and ensuring sufficient migration links among these populations.