Identifying Recruitment Bottlenecks for Age-0 Walleye Sander vitreus in Northern Wisconsin Lakes
Boehm, Hadley I. A.
University of Wisconsin-Stevens Point, College of Natural Resources
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Walleye Sander vitreus recruitment (measured as age-0 catch-per-effort in fall electrofishing) has declined in many northern Wisconsin lakes and the reasons for these declines are not known. Recruitment declines are a significant management concern for the Wisconsin Department of Natural Resources, as many of these walleye populations previously supported popular recreational fisheries. Understanding mechanisms and timing associated with walleye recruitment bottlenecks during the first year of life is important in developing management solutions, as changes to harvest regulations or stocking strategies may be warranted depending on when and where bottlenecks occur. Therefore, the objectives of my research were to: 1) develop sampling protocols for collecting larval and age-0 post-larval walleyes; 2) identify timing of recruitment bottlenecks for age-0 walleyes in two northern Wisconsin walleye lakes with a declining recruitment history (D-NR) relative to trends observed in two lakes with sustained recruitment histories (S-NR); and 3) evaluate differences in abiotic and biotic variables between lakes with the two different recruitment histories. In 2014 and 2015, I sampled Kawaguesaga and Sawyer lakes (D-NR) and Escanaba and Big Arbor Vitae lakes (S-NR). Adult walleyes were captured in spring using nighttime electrofishing, egg mats were used to verify spawning, and towed ichthyoplankton nets, quatrefoil light traps, beach seines, micro-mesh gillnets, and electrofishing were used to capture age-0 walleyes throughout their first year of life. Water quality data, zooplankton samples, and panfish (potential predator of larvae) diet information were also collected. I compared biotic and abiotic metrics between recruitment histories using repeated-measures analysis of variance. My results indicated temporal trends in relative abundance of walleyes during their first year of life can be monitored using a combination of ichthyoplankton nets towed at night during mid to late May (i.e., 1-3 weeks after peak walleye spawning), 0.64-cm mesh gill nets set in mid to late July, and fall electrofishing. Age-0 walleyes were not captured in D-NR lakes after the larval stage, while age-0 walleyes were captured at multiple life stages during both years in S-NR lakes. These results suggest a recruitment bottleneck for age-0 walleyes occurred at or before the larval stage in D-NR lakes. Panfish did not appear to be major predators of larval walleyes on any lake in either year. Temperature and dissolved oxygen metrics were similar for all lakes. Although D-NR lakes were slightly clearer (greater Secchi depth), the observed differences were not statistically significant. Mean total length (TL) of adult walleyes was significantly greater (P < 0.01, f = 213.11, df = 1) on D-NR lakes than S-NR lakes, which suggests low recruitment. Mean coefficient of variation in May daily water temperature, average density and TL of the most common zooplankton taxa, and walleye egg density did not differ significantly between S-NR and D-NR lakes. Continued research should focus on longer term collection of abiotic and biotic metrics on these lakes, and include addition of more study lakes to determine if trends in age-0 walleye abundance persist and are similar in other D-NR and S-NR lakes. This expanded sampling might also provide more information on the factors responsible for potential recruitment bottlenecks. Future studies might also include experimental stocking of walleye fry to determine if this strategy can be used to circumvent recruitment bottlenecks that occur at or before the larval stage.