Genetically-Derived Estimates of Contemporary Natural Straying Rates and Historical Gene Flow Among Lake Michigan Lake Sturgeon Populations

Natural straying rates are difficult to quantify over large spatial scales using direct observations, particularly for long-lived fish species characterized by delayed sexual maturity and long inter-spawning intervals. Using microsatellite genotypes and likelihood-based statistical methods, we quantified rates of immigration and emigration for six genetically differentiated (mean F_ST = 0.041) Lake Michigan lake sturgeon populations based on adults (n = 437) captured in tributaries during the spawning season.  Estimated rates of straying were high (mean = 0.105), asymmetrical, and highly variable across populations. We found no significant association between total length (a surrogate measure of age) of individuals that strayed and those that did not (p = 0.099). Linear distance between streams was more predictive of straying rates and F_ST than least-cost distances based on lake-scape features (bathymetry and lake current patterns). Historical rates of gene flow estimated using coalescent analysis indicated that a fully parameterized model with variable evolutionarily effective population sizes (θ; range 0.684 – 0.989) and variable, nonsymmetrical migration rates best explained the genetic data. Comparatively high estimates of relative historical gene flow from several numerically depressed populations suggest those populations were once larger contributors to basin-wide gene flow than indicated based on estimates of contemporary straying.