Spawning Habitat Assessment and Relationships to Walleye Recruitment in Northern Wisconsin Lakes

Abstract

Walleye Sander vitreus are one of the most sought after sport fish in Wisconsin and support important recreational and tribal fisheries within the state. A 2011 national survey conducted by the U.S. Fish and Wildlife Service reported that 45% of all Wisconsin anglers were targeting Walleye and Sauger (S. canadense, a closely related congeneric), contributing substantially to an annual $1.6 billion fishing economy in the state. Due to their social and economic importance, Walleye are intensively managed in Wisconsin. Previous research aimed at understanding Walleye recruitment variability has largely ignored the potential effects of spawning habitat availability and quality. Therefore, the primary objective for this study was to determine if spawning habitat characteristics varied among Walleye populations in northern Wisconsin lakes exhibiting high and low recruitment. A critical precursor of this primary objective was the need to develop a practical, flexible, cost-effective technique to describe and quantify Walleye spawning habitat. Relatively low-cost side-scan sonar offered a promising alternative to transect-based methods for collecting near-shore habitat information. Therefore, my secondary objective was to determine if side-scan sonar could be used to accurately and efficiently classify substrate composition in the nearshore littoral zone of north temperate lakes compared to a traditional transect-quadrat based method. Sixteen lakes in northern Wisconsin were sampled. To address the secondary objective, substrate composition estimates from the first 4 m of the littoral zone were collected and compared using side-scan sonar methods and a traditional transect-based quadrat sampling method. To address the primary objective, habitat data collected using the traditional transect method were used to calculate several key spawning habitat metrics including, mean water depths (m), proportion of coarse substrates (i.e., gravel, cobble, and rubble), proportion of suitable spawning habitat (i.e., coarse substrates embedded ≤ 1.5), and the proportional area with ≥ 50% egg deposition probability and then those metrics were compared between lakes with high and low Walleye recruitment. The side-scan sonar method was determined to be a practical, accurate, and efficient technique to assess substrate composition in the nearshore littoral-zone of north temperate lakes. Using this method, lake-wide substrate composition estimates equivalent to a transect-quadrat based method were obtained with a 60% reduction in effort. Ultimately, no differences in the selected spawning habitat characteristics were detected between lakes with high and low Walleye recruitment. Given the complex interactions that can influence Walleye recruitment, finding no differences in available spawning habitat characteristics between high and low recruitment lakes was not surprising. Despite the lack of meaningful differences in available spawning habitat between lakes with high and low Walleye recruitment, my findings have important implications for Walleye management. My findings demonstrate that the highest quantity and quality of Walleye spawning habitat in inland Wisconsin lakes is likely within 2 m of shore. My findings highlight the importance of developing shoreline habitat protection plans to limit erosion and shoreline development. Furthermore, my results call attention to the potential impacts of water level fluctuations that could reduce or prevent access to large areas of quality spawning habitat, which may ultimately result in poor Walleye recruitment.