Yellow Perch Recruitment and Zooplankton Availability in Northern Wisconsin Lakes with Different Walleye Recruitment Histories

Abstract

Some northern Wisconsin lakes that previously supported natural Walleye Sander vitreus recruitment have exhibited recruitment declines over the last few decades. Previous research conducted on thirteen lakes in northern Wisconsin suggested that a recruitment bottleneck was occurring at or before mid-July in lakes with declining Walleye recruitment. Recent research suggested that Walleye and Yellow Perch Perca flavescens recruitment are influenced by similar environmental factors, but current status and historical trends of Yellow Perch recruitment in Wisconsin are unknown. To better understand factors influencing both Walleye and Yellow Perch recruitment, the objectives of my study were to determine if: 1) differential trends in age-0 Yellow Perch abundance occurred between lakes with different Walleye recruitment histories (i.e., declining, sustained); 2) catch per effort (CPE) of larval Yellow Perch in ichthyoplankton tows or electrofishing CPE of age-0 perch in June-July predicted perch year-class strength as indexed by age-0 CPE in fall electrofishing; and 3) zooplankton densities and spatial and temporal trends in these densities differed among lakes with different Walleye recruitment histories. In 2021, I sampled three lakes with declining Walleye recruitment (D-NR) and three lakes with sustained Walleye recruitment (S-NR) to evaluate age-0 Yellow Perch CPE and zooplankton densities. In 2022, I expanded the study to include four additional lakes for a total of five D-NR and five S-NR lakes. I used ichthyoplankton nets in May to mid-June and hand-held anode electrofishing in late-June to October to collect age-0 Yellow Perch. In 2021, zooplankton samples were collected via vertical and horizontal tows concurrent with larval fish sampling. Additionally, vertical tows were continued through the last week of August on a subset of four lakes to assess temporal trends. In 2022, zooplankton sampling was limited to nighttime vertical tows concurrent with larval fish sampling. I used t-tests, repeated-measures analysis of variance, and mixed effects models to compare age-0 Yellow Perch CPE between D-NR and S-NR lakes at several points throughout their first year of life. I also regressed loge transformed age-0 Yellow Perch CPE against electrofishing sampling periods to compare mortality rates between lakes with different Walleye recruitment histories. Spearman’s rank correlations were used to determine if CPE of larval Yellow Perch and CPE of age-0 Yellow Perch in June-July were correlated with CPE of age-0 perch in October. Additionally, error matrices were used to evaluate whether strong or weak larval perch CPE and CPEs of age-0 perch in June-July accurately predicted strong or weak CPE of age-0 Yellow Perch. I used t-tests, generalized linear models, and repeated-measures ANOVA to compare zooplankton densities and total lengths between Walleye recruitment histories. Larval Yellow Perch CPE was similar between lakes with different Walleye recruitment histories within years, and similar between years for the six lakes sampled during both 2021 and 2022. Age-0 Yellow Perch electrofishing CPE was significantly higher in S-NR lakes than D-NR lakes in June-July 2021, but there were no significant differences in any other electrofishing sampling period. Age-0 Yellow Perch mortality rate estimates did not differ between recruitment histories in either year. Larval and age-0 June-July Yellow Perch CPEs were significantly positively correlated with age-0 October Yellow perch CPE (larval rs = 0.60, P = 0.01; post-larval rs = 0.73, P < 0.01). Both larval CPE and age-0 Yellow Perch CPE in June-July were 69% accurate in correctly predicting relative year-class strength of age-0 Yellow Perch in October. Mean zooplankton densities and total lengths of Daphnia spp., cyclopoid copepods, and calanoid copepods collected in nighttime vertical tows during May-June were similar in D-NR and S-NR lakes during both years. There were no significant interactions between Walleye recruitment history and depth for both day and night when comparing densities and total lengths for Daphnia spp. and both orders of copepods, but there were significant differences in density of Daphnia spp. during the day among depths, loge density of cyclopoid copepods between recruitment histories during the day, and calanoid copepod total length during the day among depths. For the four lakes where vertical zooplankton tow samples were conducted through the last week of August, there were no significant interactions between Walleye recruitment history and sampling period when explaining variation in loge transformed densities for Daphnia spp. and both orders of copepods and for total lengths of both orders of copepods, but there were significant differences in loge density of Daphnia spp. and cyclopoid total length between several sampling periods. There was a significant interaction between recruitment history and sampling period for Daphnia spp. total length, but there were no significant differences between lakes with different Walleye recruitment histories during an individual sampling period. My results suggest that lakes with declining Walleye recruitment are capable of producing age-0 Yellow Perch year classes that are similar to year classes observed in S-NR lakes. Although age-0 Yellow Perch CPE was only significantly different between sampling periods in June-July 2021, observed catch rates were higher in S-NR lakes in subsequent periods but also more variable. This trend did not continue in 2022, as some S-NR lakes sampled in 2021 showed decreased Yellow Perch recruitment in 2022 and some D-NR lakes had relatively high age-0 Yellow Perch CPE. Similarities in age-0 Yellow Perch CPE between lake types may be due to similar environmental conditions across northern Wisconsin. More research is necessary to better describe Yellow Perch recruitment trends, as perch recruitment is highly variable. While factors contributing to declines in Walleye recruitment remain unclear, this study provides additional evidence that a lack of zooplankton in larval Walleye diets is likely not due to low zooplankton densities, and that larval Yellow Perch may be a more preferred prey. Collectively, field-based research on Walleye recruitment declines in northern Wisconsin indicate that identifying the specific mechanisms resulting in Walleye recruitment declines in northern Wisconsin lakes may be difficult given the inherent variation in these systems. Additionally, identifying these mechanisms may not provide for direct management actions that can be implemented to increase Walleye recruitment. Consequently, efforts to identify lakes where management actions like changes to harvest regulations or stocking may be effective in maintaining Walleye fisheries within a Resist-Accept-Direct framework seems prudent