Genetic monitoring reveals temporal stability over 30 years in a small, lake-resident brown trout population

Abstract

Knowledge of the degree of temporal stability of population genetic structure and composition is important for understanding microevolutionary processes and addressing issues of human impact of natural populations. We know little about how representative single samples in time are to reflect population genetic constitution, and we explore the temporal genetic variability patterns over a 30-year period of annual sampling of a lake-resident brown trout (Salmo trutta) population, covering 37 consecutive cohorts and five generations. Levels of variation remain largely stable over this period, with no indication of substructuring within the lake. We detect genetic drift, however, and the genetically effective population size (N(e)) was assessed from allele-frequency shifts between consecutive cohorts using an unbiased estimator that accounts for the effect of overlapping generation. The overall mean N(e) is estimated as 74. We find indications that N(e) varies over time, but there is no obvious temporal trend. We also estimated N(e) using a one-sample approach based on linkage disequilibrium (LD) that does not account for the effect of overlapping generations. Combining one-sample estimates for all years gives an N(e) estimate of 76. This similarity between estimates may be coincidental or reflecting a general robustness of the LD approach to violations of the discrete generations assumption. In contrast to the observed genetic stability, body size and catch per effort have increased over the study period. Estimates of annual effective number of breeders (N(b)) correlated with catch per effort, suggesting that genetic monitoring can be used for detecting fluctuations in abundance.

Figure 1

Location of the study area in central Sweden () and schematic representation of Lake Blanktjärnen and connected water systems. Small arrows indicate the direction of waterflow and numbers represent elevations (m).

Figure 2

Estimates of effective size (temporal method, corrected for age structuring) for the Lake Blanktjärnen population for all pairs of consecutive cohorts (○) and the corresponding mean N_e obtained from moving averages of F′_s over five () and ten () consecutive cohorts (cf. Table 2). Cohort on the X-axis represents the first cohort in a pair used for each N_e estimate. The dotted line represents the total estimate for the entire period (N̂_e=74; 95% confidence interval 50–141). Note the broken Y-axis and that large N_e estimates are given as numbers (∞=infinity).

Figure 3

N_e estimates (LD approach, not accounting for age structuring) for single collection years (1980–2010) for the Lake Blanktjärnen population. The dotted line represents the total estimate for the entire period (N̂_e=76). Note the broken Y-axis and that large N_e point estimates are given as numbers (∞=infinity).