Temporal Dynamics of Genetic Variation within the Escanaba Lake Walleye Population: Implications for Managing the Genetic Resources of Naturally Recruiting Walleye Populations

Abstract

Maintaining genetic variation within Wisconsin's walleye (Sander vitreus) populations is necessary to ensure their long-term productivity and viability. A fundamental concern when managing the genetic resources of small, insular populations, typical of most of our inland walleye fisheries, is the loss of genetic variation as the result of random genetic drift. An inverse relationship exists between the rate at which a walleye population will lose genetic variation due to genetic drift and the population's effective size (Ne). Numerous aspects of walleye reproductive biology and ecology are suspected to cause Ne to be less then the population's census size (Nc), thereby, increasing the magnitude of random genetic change that would be expected to occur over time. Increased understanding of the relationship between inter- and intra-generation mechanisms of genetic change will improve our ability to conserve the genetic resources of managed walleye populations. In order to identify aspects of walleye biology and management that can affect the genetic integrity of naturally reproducing walleye populations, I used microsatellite DNA isolated from archived scale samples to examine the temporal dynamics of genetic variation within the Escanaba Lake walleye population. Between 1952 and 2002, the genetic composition of the walleye population in Escanaba Lake changed significantly. The magnitude of the observed changes was consistent with a long-term Ne (165.3) that would be considered quite restricted. Estimates of Ne increased between 1962 (34.1) and 2002 (826.3), suggesting the influence of genetic drift decreased over time, which was consistent with the allele frequency homogeneity observed between 1982 and 2002. This conclusion is further supported by the high effective number of breeding individuals (harmonic mean= 223.6), the low degree of relatedness, and lack of significant genetic divergence observed among the six sequential walleye cohorts sampled between 1997 and 2002. These findings are consistent with a response by Escanaba Lake walleye to changes in the population's breeding structure caused by high exploitation ( x = 29% ). Exploitation can result in a downward shift in size structure leading to an increase in walleye growth rates and a decrease in age at maturity, thereby, reducing the lifetime variance in reproductive success, a critical factor influencing Ne. Another important factor that has affected the genetic integrity of the Escanaba Lake walleye population was the apparent introgression between stocked and resident walleye because of supplemental stockings that occurred between 1954 and 1961. A strong indication that introgression had occurred was the 30% increase in allelic diversity (Wilcoxon signed-rank, P = 0.02) between 1952 and 1982 resulting from an apparent influx of 18 new alleles and the significant genetic divergence between pre- and post-stocking sample. These findings suggest the supplementation efforts had a significant long-term influence on the genetic composition of the Escanaba Lake walleye population despite the apparent minimal demographic impacts suggested by other researchers. Knowledge and understanding of the key factors influencing the temporal dynamics of genetic variation in the Escanaba Lake walleye population provides a foundation for developing research and management initiatives aimed at conserving the genetic integrity of Wisconsin's walleye populations.