The Genetic Legacy of Stocking Muskellunge in a Northern Minnesota Lake

Abstract

AbstractThe 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 Shoepack‐strain 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 Shoepack‐strain ancestry or genetic screening to purge the population of Shoepack‐strain ancestry.