Evolutionary Dynamics of Brown Trout In Small Streams

Properly managing living resources requires understanding how the resource may respond to environmental variability. Several evolutionary processes interact and together determine the adaptability of a population. In small populations random stochastic forces may be especially important. These forces may be both genetic and demographic. Here, we present a detailed study of the evolutionary dynamics of a stream-living brown trout Salmo trutta population using long-term mark-recapture data combined with parentage assignment based on extensive microsatellite genotyping. The stream is small and we sampled intensively over 1.5 km over c. 10 years. In general individual trout are stationary, rarely moving beyond c. 50 m. This induces a weak genetic isolation-by-distance genetic structure. Both sexes are highly polygamous, and have large reproductive skew (large variance in individual reproductive success). Based on these observations and detailed demographic data, we estimated effective population size (N_e) using a wide range of models. Overall, the estimates were all low, being approximately at 100 individuals. As expected, the observed variance in reproductive success reduced N_e relative to the number of spawners. Reproductive success increases with size for both sexes. There was significant among-year and within-stream variation in individual growth rate, but there was no directional or stabilizing selection for size-at-age or growth rate. Overall, heritability for length-at-age varied between 0.16-0.31, with reasonably narrow confidence bands. Further, total phenotypic variance increased with age. Parental size (for both sexes) positively influenced length-at-age for juvenile trout – either through direct parental effects or through genotype-environment correlations. Length-at-age is a complex trait, comprising the results of a number of physiological, behavioural and ecological processes. Our data show that fitness-related traits such as length-at-age can retain high levels of additive genetic variance even when effective population size is low and total phenotypic variance is high. This indicates that the adaptability of such populations may be high.