Walleye (Sander vitreus; Figure 1) are one of the most sought-after fish among recreational anglers in the Upper Midwest. Prized for their mild flavor and flaky texture, they are a staple of Friday night fish fries in Wisconsin. Walleye are also culturally significant to the Native American tribes in the region such as the Ojibwe and Menominee.
Walleye are a cool water species found in large rivers and lakes throughout North America. Their name is derived from the reflective tissue in their eyes called the tapetum lucidum, which allows the fish to see in low light conditions. Walleye can typically be found during the day in deeper water or areas with abundant cover such as boulders, logs, and thick vegetation. Around dawn and dusk, they make their way into shallow water to feed, primarily on fish. During the spring, walleye spawn over rocks and gravel in areas with a strong current that will aerate the eggs and prevent fine sediments from settling.
A recent study led by researchers from the Wisconsin Department of Natural Resources and the University of Wisconsin-Madison’s Center for Limnology found that walleye populations have been in decline throughout northern Wisconsin over the past several decades (Figure 2). After analyzing the data collected from population surveys in 473 lakes between 1990 and 2012, it was found that walleye production (the increase in biomass over time) and recruitment (the number of young fish that survive the larval stage and become juveniles) had decreased significantly during that time period.
The study found that walleye populations generally had higher production in lakes where no stocking took place and the population naturally reproduced. In “combination” lakes where stocking and natural reproduction occurs, production declined by about 47%. Walleye production was found to have decreased by 63% in lakes that were stocked with fingerlings but did not have a naturally reproducing population, though the habitat in these lakes may be marginal for the species.
The walleye decline may be influenced by warming water temperatures, overfishing, habitat loss due to increased development along lake shorelines and interactions with largemouth bass. Largemouth bass (Micropterus salmoides; Figure 3) are a warmwater gamefish that are popular among anglers due to their aggressive behavior and fighting ability. They are a highly predatory species that will feed on anything that fits into their mouths, including fish, frogs, crayfish, and insects. While walleye numbers are declining, largemouth bass have become more abundant in many of these lakes.
In another study, Craig Kelling and Dr. Dan Isermann from the USGS-Wisconsin Cooperative Fishery Research Unit at the University of Wisconsin-Stevens Point led an effort to examine the stomach contents of walleye and largemouth bass in four lakes in northern Wisconsin to investigate possible predation and competition between the two species. From late spring until early fall, the fish were captured by electrofishing along the shoreline after dusk. Electrofishing is a common technique used in freshwater fisheries surveys where an electrical current is passed through the water to temporarily stun fish so they can be easily caught with a net. Once captured, the stomach contents were collected by gastric lavage, where water is pumped into the stomach and the contents are expelled through the mouth. After recovery, the fish were released back into the lake.
Back at the laboratory, the fish and invertebrates collected from the bass and walleye stomachs were identified. Because these species quickly digest their prey, a molecular technique called DNA barcoding was used to identify any partially digested remains that could not be identified visually. With this method, DNA collected from the tissues are sequenced and compared to a reference database for identification. Using this technique, the amount of unidentifiable fish decreased to less than 1% of the total diet.
The scientists found that largemouth bass and walleye had similar diets. They both largely favored yellow perch (Perca flavescens; Figure 4), bluegill (Lepomis macrochirus) and pumpkinseed (Lepomis gibbosus) in August and September. The high overlap suggests that these two species may be competing for food resources, but to confirm true competition, researchers will additionally need to sample the availability of these prey species in each lake. Between the four lakes, only one instance of largemouth bass predation on juvenile walleye was recorded. It seems unlikely that bass are influencing walleye recruitment.
In addition to Wisconsin, walleye production has also decreased in lakes throughout Minnesota, Michigan, and Ontario. The increase in populations of largemouth bass do not appear to be the only driver of the walleye decline, it is likely caused by a complex variety of factors. Future management practices should also consider the effects of changing environmental conditions to restore walleye in northern lakes.
Kelling, C.J., D.A. Isermann, B.L. Sloss, and K.N. Turnquist. 2016. Diet overlap and predation between largemouth bass and walleye in Wisconsin lakes using DNA barcoding to improve taxonomic resolution. North American Journal of Fisheries Management 36(3): 621-629.
Rypel, A.L., D. Goto, G.G. Sass, and M.J. Vander Zanden. 2018. Eroding productivity of walleye populations in northern Wisconsin lakes. Canadian Journal of Fisheries and Aquatic Sciences 75(12): 2291-2301.