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dc.contributor.authorGostiaux, Jason C.
dc.date.accessioned2020-05-21T21:02:35Z
dc.date.available2020-05-21T21:02:35Z
dc.date.issued2018-05
dc.identifier.urihttp://digital.library.wisc.edu/1793/80144
dc.descriptionExecutive Summary: Many northern Wisconsin lakes that historically supported naturally-recruiting walleye Sander vitreus populations have shown declines in recruitment over the last 10 15 years. Previous research conducted on four northern Wisconsin lakes suggested a recruitment bottleneck was occurring before mid-July in lakes with declining walleye natural recruitment. Effective management of walleye populations involves understanding these recruitment bottlenecks, as potential management solutions may vary in relation to when and why this recruitment failure is occurring. To further assess these recruitment bottlenecks, I expanded on the previous assessment to determine if: 1) timing of a recruitment bottleneck for age-0 walleyes was consistent among lakes with declining recruitment; 2) abiotic and biotic metrics differed between lakes with declining (D-NR) and sustained (S-NR) walleye recruitment, with a specific focus on the abundance of edible zooplankton and 3) catch-per-effort (CPE) of larval and post-larval walleyes can be used to predict the presence, absence, and relative strength of walleye year-classes indexed by standard fall electrofishing conducted by the Wisconsin Department of Natural Resources and the Great Lakes Indian Fish and Wildlife Commission. In 2016 and 2017, I sampled six D-NR lakes and seven S-NR lakes distributed across northern Wisconsin. I used ichthyoplankton nets in spring, micro-mesh gill nets in summer, and nighttime electrofishing in fall to collect age-0 walleye at multiple stages during their first year of life. In addition, adult walleye were collected in spring to verify that adult abundance was sufficient to support natural recruitment. Limnological data and zooplankton samples were also collected during spring. I used repeated-measures analysis of variance or t-tests to compare a suite of metrics describing lake characteristics (e.g., water temperature and Secchi depth), adult walleye populations (e.g., relative abundance and mean total length), interspecific competition (e.g., larval yellow perch Perca flavescens relative abundance), and zooplankton (e.g., relative abundance, edible size relative abundance and mean total length) between lakes with different recruitment histories. Age-0 walleye were collected during larval and post-larval stages in most S-NR lakes. Larval walleye were collected in four of six D-NR lakes, but lack of age-0 walleye in five of the D-NR lakes after the larval stage supported the conclusion that a recruitment bottleneck was occurring before mid-July. One D-NR lake (Bony Lake) supported a limited level of natural recruitment because age-0 walleye were collected during fall electrofishing in both years. In addition, age-0 walleye were never collected at any stage in one S-NR lake (Windfall Lake), suggesting a lack of natural recruitment. Statistical analyses indicated that only the mean total length of Daphnia spp. was significantly different between recruitment histories, but this difference was largely a result of the relatively large size of Daphnia spp. in Escanaba Lake (an S-NR lake). Removal of Escanaba Lake from the analysis resulted in no significant difference in mean total length of Daphnia spp. between recruitment histories. Significant interactions between recruitment history and year were also detected when comparing relative abundance of Daphnia spp. and relative abundance of edible Daphnia spp. between lakes with different recruitment histories, but pairwise comparisons indicated that these metrics did not differ between recruitment histories in either year of sampling. Relative abundance of larval walleye during spring and relative abundance of post-larval walleye during summer were not significantly correlated with relative abundance of age-0 walleye in fall electrofishing. Across all lakes, if larval walleye were collected, there was only a 40% probability that age-0 walleye would be collected in fall electrofishing at a rate above the threshold for eventual recruitment to the fishery (CPE ≥ 15 walleye/h). However, when I examined only S-NR lakes, there was a 75% probability that age-0 electrofishing CPE would be ≥ 15 walleye/h if larval walleye were present in spring ichthyoplankton tows. If post-larval walleye were encountered in July gill nets, there was an 80% probability that age-0 walleye would be collected in age-0 electrofishing at a rate ≥ 15 walleye/h; this probability was the same when examining only S-NR lakes. My results indicate that larval towing and mid-summer micro-mesh gill nets could provide useful tools for allocating walleye fingerlings for stocking, as lakes where age-0 walleyes were not captured in one of these gears could be prioritized for stocking over lakes where age-0 walleyes were collected. The presence of larval walleye in some D-NR lakes followed by lack of age-0 walleye in micro-mesh gill nets and fall electrofishing surveys suggests a recruitment bottleneck is occurring at some point before mid-July and this could be before, during, or immediately after the larval stage. However, the possible causes of this bottleneck remain unclear because I detected only one difference in abiotic and biotic metrics between recruitment histories. Possibly, difference in average size of Daphnia spp., which may reflect differences in species composition, could influence larval walleye survival, but this difference was largely a reflection of the relatively large Daphnia spp. present in Escanaba Lake. Moreover, edible zooplankton were available in similar abundances between lakes with different recruitment histories. Difficulties with understand the mechanisms regulating these bottlenecks makes it challenging to prescribe management actions that might alleviate walleye recruitment problems. Possibly, increasing the abundance of larval walleye by maintaining higher adult walleye densities or through fry stocking may circumvent this bottleneck. However, higher larval abundance may not result in eventual recruitment. For instance, larval walleye abundance in Sawyer Lake (D-NR) during 2017, exceeded abundance in all but one S-NR lake, yet no age-0 walleyes were collected from Sawyer Lake in micro mesh gill nets during July or in fall electrofishing. Currently, stocking fingerling walleye represents one method that might maintain walleye fisheries in these lakes. Ongoing WDNR evaluations of fingerling stocking and changes in walleye harvest regulations will help to determine if these management actions can be used to maintain these fisheries and possibly re-establish natural recruitment in some lakes.en_US
dc.description.sponsorshipWisconsin Department of Natural Resourcesen_US
dc.language.isoen_USen_US
dc.publisherUniversity of Wisconsin-Stevens Point, College of Natural Resourcesen_US
dc.titleExpanded Assessment of Recruitment Bottlenecks for age-0 Walleye Sander Vitreus in Northern Wisconsinen_US
dc.typeThesisen_US


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