Cooperative Fish and Wildlife Research Units Program: Wisconsin Fishery
Education, Research and Technical Assistance for Managing Our Natural Resources

Walleye recruitment has declined in many northern Wisconsin lakes that historically supported natural recruitment. Previous research has shown a recruitment bottleneck is occurring for age-0 walleye at or before mid-July. While declines in walleye recruitment are likely related to climate-induced changes in lake environments, the specific causes of this bottleneck remain unknown. As walleye recruitment has declined, adult largemouth bass abundance increased in many lakes. However, a previous study demonstrated that largemouth bass rarely preyed upon walleye in these lakes, although diet overlap for small bluegill and yellow perch was observed. Possibly, increased abundance of adult largemouth bass also indicates that abundance of all centrarchids has increased, but standard sampling gears used by the Wisconsin Department of Natural Resources (WDNR) do not effectively sample fish < 100 mm total length (TL), yet these fish likely comprise a significant portion of the fish biomass in many lakes. These small fish are more likely to interact with larval walleye through predation or competition for zooplankton resources. Yellow perch are also an important component of most northern Wisconsin fish communities, yet little information on perch population trends is available because targeted sampling for this species is not conducted. Specifically, data needed for indexing trends in yellow perch recruitment are not available. The objectives of this project are to determine if:
1) sampling precision, efficiency, and length frequency distributions of centrarchids and yellow perch varies among multiple sampling gears that might be used to target fish < 100 mm TL;
2) a qualitative index based on visual assessment during electrofishing accurately reflects overall abundance of centrarchids < 100 mm TL;
3) catches of age-0 fish in selected sampling gears are highly correlated with catch of age-1 fish the following year (i.e., age-0 catch provides a useful indicator of age-1 year-class strength);
4) current relative abundance estimates for centrarchids and juvenile yellow perch are related to walleye recruitment history (defined as declining or sustained recruitment);
5) previous estimates of centrarchid relative abundance in select Wisconsin lakes are associated with walleye recruitment history.
Sampling for objectives 1-4 will occur during August 2019 and 2020 on 11 of the 13 lakes that were used to identify the timing of the walleye recruitment bottleneck and will include lakes exhibiting declining walleye recruitment and lakes that have sustained walleye recruitment. Sampling methods that will be assessed include mini-fyke nets, clover leaf traps, standard boat electrofishing, and electrofishing from a boat using a using a hand-held probe similar to a backpack electrofishing unit. For boat electrofishing we will use standard catch-per-effort as an index of relative abundance of small centrarchids, but we also assess the validity of a qualitative approach that will be based on observations of small centrarchids by netters (i.e., the confetti index). Micromesh gill nets will also be used to assess juvenile yellow perch abundance in both years. To assess objective 3, we will correlate catch of age-0 fish in 2019 and 2020 with catch of corresponding catch of age-1 fish in 2020 and 2021; sampling in 2021 will be completed by Wisconsin Cooperative Fishery Research Unit staff.
Using our sampling data and data collected by the WDNR we will develop a composite index of centrarchid abundance for each lake and will determine if lakes with different walleye recruitment histories differ in terms of various metrics of centrarchid abundance (age-0, age-1, juvenile, adult, composite) and juvenile yellow perch abundance. We will also use mini-fyke net and spring electrofishing data collected from a sample of Wisconsin lakes in the late 1990s and early 2000s to assess whether centrarchid abundance at that time was related to subsequent walleye recruitment history.