Indexing Age‐0 Walleye Abundance in Northern Wisconsin Lakes before Fall

Abstract

Recruitment of age‐0 Walleye Sander vitreus is often indexed using fall electrofishing surveys. However, collecting fish before fall may provide timely information regarding stocking decisions and factors influencing recruitment. We evaluated sampling methods for age‐0 Walleye in northern Wisconsin lakes that could be used to assess recruitment in spring and summer. Initial assessments on two lakes indicated surface tows of ichthyoplankton nets at night during May to early June and 0.64‐cm‐micromesh gill nets set in July provided the highest catches of age‐0 Walleye among the methods we evaluated. Additional sampling on 13 lakes over 2 years indicated catch per effort (CPE) of age‐0 Walleye using these two methods did not correlate with age‐0 CPE in fall electrofishing. However, presence or absence of age‐0 Walleye in micromesh gill nets was 92% accurate in predicting whether age‐0 CPE in fall electrofishing was ≥15 fish/h, the threshold above which eventual recruitment to the fishery is expected to occur. Micromesh gill netting may provide a useful tool for allocating fingerling Walleye that are stocked in fall, a resource that is often limited due to space and forage constraints associated with propagation. Additionally, our sampling protocol may help to identify timing of potential recruitment bottlenecks occurring in some lakes.

Recruitment Bottlenecks for Age‐0 Walleye in Northern Wisconsin Lakes

“…To sample larval Walleye, a 1,000-μm mesh conical ichthyoplankton net was towed at the surface for 5 min at nearshore and offshore locations on each lake (Isermann and Willis 2008). All towing was conducted at night (Boehm et al 2020). Lakes were divided into three to six zones of approximately similar surface area, with two tows conducted in each zone on each sampling date.…”

“…Multiple 46-m-long × 1.2-m-tall gill nets (0.64-cm mesh, 0.10-mm-diameter monofilament twine; referred to as "micromesh gill nets" herein) were used to collect age-0 (i.e., postlarval) Walleye in mid-July of 2016 and 2017. Micromesh gill nets were set perpendicular to shore in depths <5 m. Between four and eight nets were set on a single night at randomly selected locations, but we ensured that at least some nets were set on relatively open sandy flats, where higher catch rates were expected (Boehm et al 2020), as our sampling objective was to determine presence or absence of age-0 Walleye. The number of sets varied with lake surface area.…”

Special Section Overview: Effects of Ecosystem Change on North American Percid Populations

“…The exact mechanisms for observed changes in percid populations are often unknown and may be difficult to identify. To illustrate, the final presentation of the symposium chronicled nearly a decade of research in northern Wisconsin that was conducted to identify mechanisms causing Walleye recruitment declines (Richter 2015;Kelling et al 2016;Boehm et al 2020;Gostiaux et al 2022, this special section). This research was informed by broad-scale analyses of statewide or regionwide data that documented relationships between Walleye population metrics and many abiotic and biotic variables (Hansen et al 2015a;Honsey et al 2020;Brandt et al 2022), but the direct cause-and-effect aspects of these relationships were not always clear.…”

Relative Effectiveness of D‐Frame Dip Nets, Quatrefoil Light Traps, and Towed Ichthyoplankton Nets for Larval Muskellunge

“…Although fry stockings are used less frequently (Hanson et al 1986;Margenau 1999), D-frame dipnetting could be used as a tool to assess short-term survival of stocked fry depending on size at stocking and growth rates. Furthermore, the ability to sample fish before fall could provide timely information regarding stocking decisions for fingerling or yearling Muskellunge (e.g., Anderson et al 1998;Boehm et al 2020). Although further investigation would be required to determine relationships between larval Muskellunge abundance in Dframe dip nets and abundance at later life stages (i.e., whether larval abundance in dip nets could provide an early index of year-class strength), identifying a sampling gear to reliably capture larval Muskellunge in complex habitats, as we have done here, represents an important first step in the process.…”

“…Moreover, reproductive failure of Muskellunge has been documented in many systems that previously supported self-sustaining populations (Zorn et al 1998;Rust et al 2002;Kapuscinski et al 2007). This failure has been attributed to a variety of factors, many of which may operate during the larval stage (Dombeck et al 1986;Inskip 1986;Kapuscinski et al 2007), which reinforces the importance of effectively sampling larvae, as understanding recruitment bottlenecks is important for developing management solutions and rehabilitation efforts (Anderson et al 1998;Boehm et al 2020).…”