Walleye (Sander Vitreus) Spawning Habitat Selection and Dynamics in a North-Temperate Wisconsin Lake
Raabe, Joshua K.
University of Wisconsin-Stevens Point, College of Natural Resources
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Critical to the conservation and management of sustainable populations of walleye, Sander vitreus, throughout North America is the need to understand what comprises and compromises high quality spawning habitat. In particular, questions remain regarding the actual selection of spawning habitat by walleye, how physical characteristics and functional dynamics (e.g., thermal conditions, wave energy, water levels) impact habitat quality, and the quantity of spawning habitat necessary to sustain healthy walleye populations. Therefore, the spawning habits and habitat of naturally reproducing walleye in Big Crooked Lake, a north temperate Wisconsin lake, were evaluated in 2004 and 2005, with specific objectives to: 1) describe the physical characteristics of natural walleye spawning reefs in order to develop detailed blueprints of successful natural walleye spawning habitat and to develop walleye spawning habitat resource selection functions, 2) evaluate thermal conditions on walleye spawning reefs and the relative affect they have on reproductive success across two years, and 3) assess the potential impact of wind activity, wave energy, and water level on walleye egg movement and survival. Walleye spawned primarily close to shore (outer boundary x distance = 2.72 m), in shallow water (outer boundary x depth = 0.29 m) and over gravel substrate ( x coverage = 64.3%) with low embeddedness ( x = 1.30) at all sites. Only 39% of available rock shoreline (14% of total shoreline) and no offshore reefs were utilized by walleye. The best resource selection function (overall correct classification = 97.6%) predicted that the relative probability of egg deposition increased with gravel and cobble substrates and decreased with distance from the shoreline, higher substrate embeddedness, and with increasing sand and fine organic material. Evaluating habitat quality (i.e., egg survival), regression models found percent cobble substrate to be the only variable positively related to the percent survival of deposited eggs in egg collection chambers. Two- and three-dimensional spawning reef blueprints were developed to visually depict the successful natural walleye spawning habitat studied. Evaluating thermal conditions, despite gradual and stable rises in water temperature and high densities of eggs in 2004, percent survival was lower than in 2005, potentially due to lower thermal lethal limits (<6°C) exceeded the first six days of the spawning period. In 2005, water temperature fluctuated, prolonging the development and hatching of walleye embryos but had minimal direct impact on egg survival. Significant differences (p < 0.05) in mean daily water temperature throughout the spawning period were observed between reefs and sometimes within reefs. Close to shore, shallow water responded more to fluctuations in air temperatures than further, deeper water at all reefs. Wind and wave velocities showed a positive, significant (p < 0.001) correlation. Periods of wave energy were sufficient to initiate movement of walleye eggs over 28% of the incubation period and also to move various substrate size classes, potentially impacting egg survival through transportation or exposure, abrasion, and burial. Egg movement was primarily closer towards shore and along shorelines of the littoral zone having sand substrates. Larger substrates and steeper riparian shoreline areas at reefs retained eggs better than flatter sand regions. Water level fluctuations were minimal (+1.20 cm) and most likely did not impact the amount of available habitat or strand and desiccate eggs. This study increases our quantitative understanding of walleye spawning areas and can assist biologists in understanding future processes affecting habitat quality and in developing protection strategies of natural reefs and potentially aid in the design of artificial reefs.