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

Natural rates of straying are difficult to quantify over large spatial scales using direct observations, particularly for long-lived fish species characterized by delayed sexual maturity and long interspawning intervals. Using multilocus microsatellite genotypes and likelihood-based statistical methods, we quantified rates of immigration and emigration for six genetically differentiated (mean F-ST = 0.041) lake sturgeon Acipenser fulvescens populations in Lake Michigan 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 the total length (a surrogate measure of age) of individuals that strayed and those that did not. Linear distance between streams was more predictive of straying rates and F-ST than least-cost distances estimated based on lakescape features (bathymetry and lake current patterns). Historical rates of gene flow estimated using coalescent analysis indicated a fully parameterized model with variable evolutionarily effective population sizes (theta; range, 0.684-0.989), and variable and nonsymmetrical migration rates best explained the genetic data. Comparatively high estimates of relative historical gene flow from several numerically depressed populations suggest that these populations were once larger contributors to basinwide gene flow than indicated by estimates of contemporary straying rates. High rates of interpopulation straying contrast with high F-ST, suggesting that straying rates are poor indicators of successful reproduction following dispersal.