Predation by walleye Stizostedion vitreum and sauger S.

Fish stocking, often from multiple source populations, is a common management practice frequently conducted without the means or effort to determine the reproductive contributions of stocked fish. Historically, the Minnesota Department of Natural Resources (MNDNR) has stocked four strains of muskellunge Esox masquinongy, but the contribution of these strains to current populations was unknown. Two strains came from Minnesota lakes, Shoepack Lake and Leech Lake, and the other strains came from Wisconsin and Iowa hatcheries and were of uncertain origin. The MNDNR discontinued stocking the Shoepack strain in the 1980s when that strain displayed poor growth in stocked waters. Managers were concerned that ancestry from this strain might be limiting the genetic potential for muskellunge to attain trophy size in stocked populations. Using 13 microsatellite DNA markers, we determined the ancestry of muskellunge in 10 supplemented native populations and 10 introduced populations. The ancestry from each of the four stocked strains of muskellunge was detected in some populations, but the level of ancestry was unrelated to the amount of stocking of a strain. Ancestry from native populations persisted in six of the supplemented populations despite years of stocking. The potential effects of Shoepack strain ancestry on fish size were limited in most lakes because of its low persistence. All stocked strains reproduced in at least some of the lakes, but some lakes had no evidence of reproduction by any stocked strain. Our results will help MNDNR manage genetic diversity among muskellunge populations and direct efforts toward appropriate actions to improve size structure. This study reinforces how genetic data are often useful for evaluating ancestry in stocked fish populations, whereas stocking histories may be poor indicators of current genetic composition.

Muskellunge Esox masquinongy are large piscivores stocked into numerous lakes in North America, but the potential effects of these stockings remains poorly understood. To investigate potential fish community changes associated with Muskellunge stocking, we contrasted catch per unit effort (CPUE) for seven fish species commonly found in Minnesota, along with mean weight for two predator species, before and after Muskellunge introduction in 36 lakes with that of paired reference lakes. Because of the known importance of Cisco Coregonus artedi as prey for Muskellunge, we also examined these data by separating the lakes into those with and without Cisco. Across all 36 lakes, we observed several significant differences between stocked and reference lakes. Yellow Perch Perca flavescent CPUE in gill nets were significantly higher after Muskellunge were introduced, as was mean weight for Northern Pike Esox lucius. Gill‐net CPUE for Northern Pike and White Sucker Catostomus commersonii were both lower following Muskellunge introduction. When subsets of lakes with and without Cisco were considered, results for the 21 lakes without Cisco were similar to the results of the entire set of lakes. In the 15 lakes with Cisco, the only significant result was lower Northern Pike CPUE after Muskellunge stocking. Our results are evidence that the introduction and management of Muskellunge in these lakes has not adversely affected game fish populations.

The effects of stocked fish on native populations of the same species are poorly understood. During the 1960s-1980s, the Minnesota Department of Natural Resources (MNDNR) stocked muskellunge Esox masquinongy derived from Shoepack Lake in the Hudson Bay drainage of northern Minnesota into Moose Lake in the Upper Mississippi River drainage. In the mid-1980s, the MNDNR recognized that stocked Shoepack Lake-strain fish (hereafter, Shoepack strain) were not attaining large sizes, and sources were switched statewide; subsequently, muskellunge from a Wisconsin hatchery were stocked once into Moose Lake. We used 14 microsatellite DNA markers to estimate the genetic contribution by each source population to muskellunge sampled in four different years from 1981 to 2004, and we evaluated the effects of Shoepackstrain ancestry on fish size. Two samples in the 1980s had mostly fish with a single ancestry; thus, these fish were probably Shoepack-strain fish stocked a few years earlier or native fish. By 2004, when most fish would have been naturally produced in the lake, contributions from all three source populations were detected, including admixed individuals from crosses among all ancestral sources. The estimated ancestry by source was 52% native Moose Lake ancestry, 29% Wisconsin strain, and 19% Shoepack strain, with 24% of the individuals having pure Moose Lake ancestry. Fish without Shoepack-strain ancestry had a greater median length than did those with Shoepack-strain ancestry and had a greater proportion of legal-sized individuals (total length ! 1,016 mm) in the population. In contrast, Moose Lake and Wisconsin-strain descendents were common among all sizes of fish. Management options include doing nothing, accepting that extensive introgression has already occurred, or manipulating the population by either stocking to dilute Shoepackstrain ancestry or genetic screening to purge the population of Shoepack-strain ancestry.