Identifying Potential Sources of Natural Recruitment for Muskellunge (Esox masquinongy) in Green Bay, Lake Michigan

Abstract

Historically, Green Bay, Lake Michigan, supported a self-sustaining muskellunge Esox masquinongy population. However, like many muskellunge populations throughout the Great Lakes, they were extirpated from Green Bay by the mid-1930s. Re-establishing a self-sustaining population within Green Bay is a goal of several active management and restoration efforts, including the Lower Green Bay and Fox River Area of Concern (AOC) Remedial Action Plan and the Wisconsin Department of Natural Resources Great Lakes Spotted Musky management plan. Annual stockings within lower Green Bay and the Sturgeon Bay area have resulted in the creation of a world-renowned trophy fishery. However, limited evidence of natural reproduction suggests that natural recruitment of muskellunge is low and the population remains reliant on stocking. Several recent research studies have focused on understanding the influence of habitat characteristics associated with muskellunge egg deposition and successful hatching in Green Bay tributaries. The results of these studies suggest that major tributaries to Green Bay may provide limited suitable spawning habitat for muskellunge and potentially low hatching success. However, both anecdotal evidence and telemetry data suggest that nearshore (≤ 1000 m from shore) locations in lower Green Bay proper, including the Sturgeon Bay area, may represent a potential source of natural recruitment. Due to uncertainties surrounding muskellunge natural recruitment in nearshore, non-tributary habitats within Green Bay, my objectives were to determine if: 1) successful hatching occurs in nearshore locations in Green Bay proper, including locations in the Sturgeon Bay area; 2) presence of eggs or age-0 muskellunge at a location is related to a suite of habitat characteristics including dissolved oxygen concentration, water temperature, substrate composition, substrate diversity, coarse woody habitat presence, depth, bottom slope, aquatic vegetation relative abundance, distance from shore, and shoreline alteration; and 3) muskellunge spawning in the Sturgeon Bay area contribute to the overall Green Bay population. During 2024 and 2025, a total of 312 individual sites in lower Green Bay and the Sturgeon Bay area were sampled for eggs using a combination of a diaphragm pump, D-frame dip nets, and egg mats. A total of 281 individual sites were sampled for larval muskellunge using a combination of conical ichthyoplankton trawls and D-frame dip nets. Fifty-seven sites were sampled for juvenile muskellunge using a seine. In addition, habitat characteristics were recorded at each site. To assess patterns in habitat use, I characterized and compared distributions of habitat variables between sites where eggs and/or age-0 muskellunge were and were not present. For this, I utilized box-and whisker plots and Mann-Whitney U tests (α = 0.05) for habitat variables with continuous data (dissolved oxygen concentration, water temperature, substrate composition, substrate diversity, depth, bottom slope, and distance to shore). For categorical habitat variables (shoreline alteration, aquatic vegetation relative abundance, and coarse woody habitat presence), I created contingency tables showing the frequencies of each category and used chi-square tests to determine if these frequency distributions differed between sites where eggs and/or age-0 muskellunge were and were not present. Using these distributions and the results of these comparisons I attempted to develop a series of general guidelines to characterize where muskellunge eggs and age-0 muskellunge were found relative to where they were not to help guide future sampling and habitat restoration efforts. Muskellunge eggs were collected at 14 sites, with four sites in lower Green Bay, nine sites in Little Sturgeon Bay, and one site in Sawyer Harbor. Egg sites were characterized by shallow (< 1 m), near shore (≤ 10 m) habitats with relatively high substrate diversity. Nine age-0 muskellunge were captured, with two captured in Sawyer Harbor and seven captured in Little Sturgeon Bay. Age-0 muskellunge were associated with shallow (≤ 1.25 m), near shore (< 8 m) habitats that included coarse woody habitat, aquatic vegetation, and organic matter within the substrate. To assess contribution of adult muskellunge that spawn in the Sturgeon Bay area to the overall Green Bay population, 20 individuals across Little Sturgeon Bay, Sawyer Harbor, and Sturgeon Bay proper were implanted with acoustic transmitters during the annual spawning window. Three individuals were tagged in 2024 and 17 were tagged in 2025. Data collection is ongoing and will span the ten-year battery life of the transmitters. Preliminary detection data from the three individuals tagged in 2024 suggests that muskellunge spawning in the Sturgeon Bay area leave the area and move into southern Green Bay after spawning. Two of three individuals were subsequently detected within or near the Lower Green Bay and Fox River AOC, with one individual detected on a receiver located near the northern boundary of the Lower Green Bay and Fox River AOC and another individual detected by a receiver located along Renard Island. Muskellunge hatching is occurring in the Sturgeon Bay area, particularly in Little Sturgeon Bay and Sawyer Harbor. Little Sturgeon Bay yielded both the highest number of sites with eggs and the highest number of age-0 muskellunge. This evidence indicates that these locations support both viable spawning and nursery habitat. In light of these observations, managers could adjust current stocking strategies and stock additional muskellunge within Little Sturgeon Bay and Sawyer Harbor to promote natural recruitment under the premise that stocked fish will eventually return to these locations to spawn. This approach could support progress toward establishing a self-sustaining muskellunge population within Green Bay.