Food Web Interactions Among Walleye, Lake Whitefish, and Yellow Perch in Green Bay, Lake Michigan
College of Natural Resources, University of Wisconsin-Stevens Point
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Green Bay supports important fisheries for walleye Sander vitreus, lake whitefish Coregonus clupeaformis, and yellow perch Perca flavescens. While walleye and lake whitefish populations have expanded in Green Bay, yellow perch numbers remain at historically low levels. As one of the primary piscivores in the Green Bay food web, walleyes could influence lake whitefish and yellow perch abundance, and yellow perch are important prey for walleye within many ecosystems. Specifically, walleye and yellow perch populations in southern Green Bay have exhibited contrasting trends in abundance since 1986. Walleye predation has been hypothesized to represent a potential recruitment bottleneck for yellow perch in Green Bay, which has been documented in other areas throughout the Great Lakes. These three species likely interact in a variety of ways, including predation and diet overlap, but these interactions are poorly understood because contemporary information on diets is lacking. This information is needed to guide management decisions, because changes in population status of one species will likely affect fisheries for the other two species. The specific objectives of my research were to determine if: 1) lake whitefish and yellow perch represent important prey for walleyes in Green Bay; 2) diets of these three species vary spatially and temporally; 3) diet overlap among species is evident; and 4) the extent of walleye predation is sufficiently high to influence recruitment potential of lake whitefish and yellow perch in the portion of Green Bay south of Chambers Island (Zone 1). Fish were collected each month from May through October in two zones of Green Bay during 2018 and 2019 using monofilament graded mesh gill nets. Additional fish were collected by natural resource agencies, commercial fishers, and recreational anglers. A total of 4,423 stomachs were dissected and diet items were removed and wet weighed to generate diet compositions using diet information from 687 walleye, 533 lake whitefish, and 696 yellow perch with nonempty stomachs. For each species, diet composition was described using mean proportion by wet weight for each year, zone, and month combination. Analysis of similarities (ANOSIM) was used to test for spatial and temporal differences in diet composition, Pianka’s (1974) index of niche overlap was used to calculate the extent of diet overlap, and a combination of bioenergetics and statistical catch-at-age (SCAA) modeling was used to estimate total walleye consumption of lake whitefish and yellow perch from May 1 through October 31 during 2018. Diet compositions suggest that lake whitefish and yellow perch comprise between 5-6% of walleye diets overall, though temporal and spatial variation in diets was evident. Lake whitefish were seasonally important prey consumed only during May and June in Zone 1, comprising 36% of June walleye diets. Also, this study was the first to document the importance of lake whitefish to walleye diets within the Great Lakes. In both zones, yellow perch contributed to walleye diets at a broader temporal scale than lake whitefish, but perch never comprised more than 15% of walleye diets in any month throughout Green Bay. Walleye diet compositions were significantly different between zones, potentially reflecting differences in prey fish availability. Additionally, significant temporal variation was identified in Zone 1, with distinct seasonal transitions in the consumption of prey species. Walleye diets were generally similar between 2018 and 2019, though fish collections during 2019 occurred at a reduced scale. Furthermore, variability in diet compositions was less evident for lake whitefish and yellow perch. However, a significant difference was detected between zones which was attributed to the greater contribution of round gobies Neogobius melanostomus in lake whitefish and yellow perch diets identified in Zone 2. Similarly, a significant temporal change in yellow perch diets was detected, largely attributed to increased consumption of round gobies during the fall throughout Green Bay. Diet overlap was strongest between lake whitefish and yellow perch, while weak to moderate overlap was observed between walleye and both lake whitefish and yellow perch. The highest degree of diet overlap between walleye and yellow perch occurred during October in Zone 1, corresponding with increased contribution of gizzard shad Dorosoma cepedianum to the diets of both species, and September in Zone 2, which was influenced most by predation on round goby and yellow perch. Moreover, diet overlap was highly influenced by the invertebrate prey group used in my diet analyses, as all invertebrate taxa were pooled. More specific identification of invertebrate taxa in diets would likely increase the resolution and reliability of my diet overlap indices, but this would be difficult because invertebrates were rarely intact in many diets, especially for lake whitefish. In conjunction with observed growth and monthly diet data, a range of abundance estimates (i.e., 95% confidence limits) from SCAA models were used to predict the influence of walleye predation on lake whitefish and yellow perch recruitment potential. Based on the range of walleye consumption estimates and lake whitefish recruitment estimates, walleye consumption appeared unlikely to influence recruitment potential of lake whitefish in Zone 1 of Green Bay. However, given the high abundance of walleye and low abundance of yellow perch estimated in Zone 1, yellow perch recruitment potential is likely affected by walleye predation. These results corroborate other studies that have identified walleye predation as a significant recruitment bottleneck for yellow perch populations throughout the Great Lakes. This study also suggests that previous concerns related to walleyes negatively impacting lake whitefish may be unrealistic. Additionally, alewife Alosa pseudoharengus, gizzard shad, and invasive round gobies may serve as predation buffers for lake whitefish and yellow perch and predation by my three focal species may help in regulating round goby abundance in Green Bay. My results provide fisheries managers and stakeholders with important information regarding the complex interactions among walleyes, lake whitefish, and yellow perch in Green Bay, so that future management decisions can be made with a better understanding of the potential implications for all three fisheries and prey resources.