Movements and Spawning Habitat of Muskellunge Esox Masquinongy in Green Bay, Lake Michigan

Abstract

Muskellunge (Esox masquinongy) have been extirpated or have declined significantly in many areas of the Great Lakes, including Green Bay. Restoring a self-sustaining population of Muskellunge to Green Bay is a goal of several management efforts. Currently, annual stocking supports a world-renowned trophy fishery for anglers seeking to catch large Muskellunge. However, there has been limited evidence of natural reproduction as few juvenile Muskellunge of wild origin have been collected. Lack of juvenile Muskellunge in Green Bay suggests natural recruitment is limited, possibly due to habitat limitations. Previous work has identified potential Muskellunge spawning locations in Green Bay and provided initial descriptions of Muskellunge spawning habitat, but it is not known whether Muskellunge successfully hatch at these locations or what habitat characteristics result in successful hatching. Additionally, it is not known if Muskellunge in Green Bay exhibit reproductive homing and return to the same locations to spawn in consecutive years, or if Muskellunge return to their stocking location to spawn. Due to the uncertainties surrounding Muskellunge reproduction in Green Bay, my objectives were to determine if: 1) Muskellunge spawning locations and occurrence of successful hatching were related to habitat variables, 2) proportions of Muskellunge spawning in or outside of tributaries to Lower Green Bay were different, 3) most Muskellunge (>75%) returned to stocking locations to spawn, and 4) adult Muskellunge display high site fidelity (>75%), returning to the same locations to spawn in consecutive years. Results from my research allowed me to quantify the availability of suitable spawning habitat in the Fox and Menominee rivers, and to qualitatively characterize general movement patterns of Muskellunge in Green Bay and its tributaries. My study was the culmination of a 4-year effort focusing on Muskellunge spawning behavior and success in southern Green Bay and its tributaries. During 2017-2019, adult Muskellunge (N = 60) were captured and surgically implanted with radio and acoustic transmitters. Muskellunge were actively tracked during the open water season to identify putative spawning sites using a combination of aerial and boat-based telemetry. Muskellunge were also monitored passively using acoustic receivers at fixed locations. Egg sampling was conducted at putative spawning site using an airlift pump, followed by larval sampling using conical ichthyoplankton nets, D-frame nets, and quatrefoil light traps. Habitat variables were recorded at each putative spawning site and at a large number of random sites that were also sampled for eggs. Multiple logistic regression was used to model the habitat variables significantly influencing the probability of Muskellunge egg presence at a given location. Data from acoustic receivers and active tracking efforts were used to determine seasonal movement patterns and tributary use of Muskellunge. Chi-square tests were used to compare proportions of Muskellunge spawning in tributary vs. bay habitats, as well as analyze reproductive homing to stocking locations and spawning site fidelity. Side-scan sonar in combination with my logistic regression model was used to quantify the amount of suitable Muskellunge spawning habitat in the Fox and Menominee rivers. In total, 278 sites were sampled for eggs, and as a result 58 egg sites were identified. Additionally, 436 individual sampling events resulted in the capture of two larval Muskellunge. Because only two larval Muskellunge were captured, I was not able to quantify habitat characteristics related to successful hatching. Therefore, my final logistic model only includes habitat variables related to egg deposition. My final logistic regression model indicates bottom slope, depth, dissolved oxygen, distance from shore, percent of gravel substrate, and percent of organic matter as a substrate best predict Muskellunge egg presence at a given location. Of these habitat variables, bottom slope had the greatest influence on presence of Muskellunge eggs. The likelihood Muskellunge eggs were present at a given site decreased by 50% when slope steepness increased by 0.5 m/m. A 1-m increase in depth resulted in a 49% decrease in probability Muskellunge eggs were present. A one unit increase in the dissolved oxygen concentration (1 mg/L) at a given site increased the probability Muskellunge eggs were present 1.4 times. Additionally, for every 1 m increase in distance to shore the probability of Muskellunge egg presence decreased by 14%. Finally, a 10% increase of gravel or organic matter in the substrate increased the probability of Muskellunge egg presence by 27% and 19%, respectively. Currently, based on surface area, 1.3% of the Fox River and 8.3% of the Menominee River are suitable Muskellunge spawning habitat (suitable for egg deposition). Proportions of Muskellunge spawning in or outside of tributaries were not significantly different in 2018, 2019, or 2020 based on chi-square analysis. Of the Muskellunge spawning in tributaries to Green Bay during my study, the majority spawned in the Fox River. Telemetry data suggest Muskellunge are spawning in the Fox, Menominee, Suamico, and Peshtigo rivers, and potentially in Duck Creek. Eggs were collected to confirm spawning in the Fox, Menominee, Suamico, and Peshtigo rivers and in multiple locations throughout Green Bay proper. In all but one case, Muskellunge with known stocking locations (N = 6) returned to their stocking location to spawn. Finally, Muskellunge in Green Bay displayed moderate levels of spawning site fidelity (45-70%), and the upper proportion of this range was not significantly different than 75%. Muskellunge in Green Bay generally moved south towards the mouth of the Fox River as water temperatures cooled in fall. A handful of Muskellunge overwintered in the Fox River in 2018, 2019, and 2020. Peak spring spawning activity was observed at water temperatures comparatively higher to reported Muskellunge spawning activity in most locations. Entry into tributaries for spawning each spring was staggered over a long period of time prior to peak spawn, whereas exit out of tributaries after presumed spawning was comparatively more truncated. In the Fox River, average residency times for each year were 27, 27, and 21 days respectively, with males generally residing longer than females. My results demonstrate that habitat suitable for Muskellunge egg deposition may be lacking in the Fox and Menominee rivers. Additionally, the lack of Muskellunge larvae captured during my study suggests that successful hatching may not be occurring in Lower Green Bay and the Fox River or is occurring at relatively low levels. A lack of naturally produced recruits indicates the Green Bay Muskellunge population is not self-sustaining, and stocking is still required to support the current fishery. My results also demonstrate that choice of stocking locations may play an import role as a management tool, as Muskellunge are likely to return to stocking locations to spawn and generally display moderate site fidelity. Stocking Muskellunge in tributaries at shallow areas with minimal human alteration of the shoreline may result in increased egg deposition and subsequent natural reproduction. Furthermore, stocking Muskellunge in Green Bay proper at sites closely resembling wetland or coastal marshes, such as in Deadhorse Bay, may increase Muskellunge spawning in these locations and lead to successful hatching. Due to the proportion of Muskellunge in Green Bay not spawning in tributary habitats, known spawning areas in Green Bay proper should be considered for enhancement or protection efforts in addition to those in tributaries. Management objectives and goals related to the Green Bay Muskellunge population should emphasize actions that result in increased knowledge of factors regulating successful hatching, enhancement and protection of suitable spawning habitat, and identifying natural reproduction.