The walleye, Stizostedion vitreum (Mitchill), population and sport fishery of the Big Eau Pleine, a fluctuating, central-Wisconsin reservoir
Joy, Earl Thomas
University of Wisconsin-Stevens Point, College of Natural Resources
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A 17-month study of the walleye, Stizostedion vitreum (Mitchill), population and sport fishery of the 2,832-ha, fluctuating Big Eau Pleine Reservoir in central Wisconsin indicated that total annual mortality rate (a = 0.64), annual expectation of natural death (v = 0.37), growth to age V (180, 299, 393, 458, and 498 mm total length at successive annuli),and water harvest of walleye (0.162 fish/angler-hour, 0.6 kg/ha) were high; exploitation rate (u = 0.27) was average; and the spring population estimate (age II and older walleye: 20,091 fish, or 7.1 fish/ha) and open-water walleye harvest (0.059 fish/angler-hour,0.7 kg/ha.) were low in comparison with findings reported from other waters. The expression 1t = 596 ( 1 - e -0.3658(t - 0.0468) ) described growth in length of walleye, and length-weight relationships were y = 3.1309x - 5.3896 for walleye and y = 3.0613% - 5.4179 for northern pike, where y = log_10 weight in grams and x = log_10 total length in millimeters. Fish (chiefly yellow perch, Perca flavescens; Johnny darter, Etheostoma nigrum; and bullheads, Ictalurus melas and I. natalis) make up the bulk of the diet of walleye of all sizes, but insects and zooplankton were also important food for walleye less than 200 mm long. Fishing pressure was most intense in the winter months. A large portion of the walleye harvest consisted of age I+ fish, which had completed their second growing season. A minimum length limit regulation was considered inadvisable because of the history of winterkills in the reservoir and on the basis of calculations of equilibrium yield per unit recruitment, which indicated increases in the weight harvested of only 0.4 and 3.3% with simulated length limits of 330 and 381 mm respectively and a 1.1% decrease with a limit of 432 mm. Walleye probably spawned in 1974 along shoreline riprap in the reservoir, at dates and water temperatures common for walleye spawning in other Wisconsin waters. Walleye dispersed throughout the reservoir after spawning and appeared to move less extensively for the remainder of the year. Tag returns indicated that some walleye, perhaps 10% annually, escape the reservoir through the dam. The 1968 and 1974 year classes were large possibly because they were augmented by the stocking of 5 million fry (1766 fry/ha) in those years. An apparent increase in mean length of young-of-the-year walleye from fall to the following spring suggested that winter mortality may have been greater among smaller fish.