Genetic Structure of Rock Bass and Johnny Darters: Implications for Gamefish Management in Wisconsin

Abstract

Understanding the patterns of spatial genetic structure within and among populations has become a critical component of fisheries management practices. The hierarchical genetic structure of aquatic organisms is highly influenced by watershed boundaries or other key geological features. Previous genetic research on walleye (Sander vitreus) and muskellunge (Esox masquinongy) across their native range in Wisconsin showed sampled populations in the Upper Chippewa River watershed were more similar to fish from the neighboring Upper Wisconsin River watershed than they were to fish in the lower reaches of the Chippewa River watershed. The discordance is likely the result of glacial processes or widespread, cross-boundary stocking. The underlying cause is important in implementing stock-based management of coolwater gamefish in Wisconsin. The objective of this study was to determine if the previously observed genetic structures for walleye and muskellunge populations in Wisconsin are likely the result of zoogeographic processes or anthropogenic events. Three primary sub-objectives were: (1) to determine if rock bass exhibit genetic structure among populations in northern Wisconsin, (2) to determine if johnny darters exhibit genetic structure among populations in northern Wisconsin, and (3) to determine if the genetic structures of rock bass and johnny darter are consistent with the previously identified genetic structures of walleye and muskellunge. The genetic structures were evaluated by sampling 22 rock bass populations and 16 johnny darter populations in the five major watersheds in northern Wisconsin. Genetic diversity was assessed with 10 microsatellite markers for rock bass and 14 for johnny darters. A modified genetic stock identification (GSI) method using a Bayesian hierarchical process for determining genetic structure across a landscape combined with analysis of molecular variance (AMOVA) was employed to delineate genetic structure. Analysis of genetic structures of rock bass and johnny darters revealed 14 and 16 unique genetic units, respectively. Because of limitations in managing a large number of genetic units, putative management groups were identified using AMOVA and a minimum ratio of among group variance (Va) to within-group variance (Vb) of one. This approach identified nine genetic units for rock bass and eight for johnny darters. The rock bass and johnny darter genetic structures were inconsistent with the previously identified walleye and muskellunge genetic structures, but were consistent with watershed boundaries. The current genetic structures of walleye and muskellunge are most likely the result of past cross-watershed boundary stockings rather than natural biogeography. Two management alternatives should be considered in the future management of walleye and muskellunge in northern Wisconsin. The first option is to manage based on the contemporary genetics and risk altering communities downstream. The second alternative is to manage based on contemporary watershed boundaries and risk altering populations that have already been altered through past anthropogenic events.