Variation among Stocks of Walleye (Stizostedion vitreum vitreum): Management Implications

Abstract

Identification of races, strains, or subpopulations of walleye (Stizostedion vitreum vitreum) by examining differences in the morphology or biochemistry has met with relatively limited success. Although there is some genotypic evidence for stock discreteness, most evidence points to differences (age, growth, fecundity, maturity) which are believed to be phenotypic expressions induced by the environment. Thus, in the absence of clear genetic evidence, the biological differences are the reliable means of identifying and managing walleye stocks.Walleye populations are spatially distinct, varying greatly in their rate of growth, rate of maturity, and longevity throughout their distributional range. Any genotypic expression of these population characteristics would probably be masked by environmental influences. There is no concrete evidence that differences in growth, age to maturity, fecundity, and longevity among walleyes are inheritable. The possibility of southern walleye populations being obligative riverine spawners suggests genetic differences, but this needs to be substantiated.Some of the physiological variables used to delineate the responses of stocks to their environment can be useful as management tools to measure their responses to various disturbances, for example, exploitation and habitat modification. Measurement of specific fecundity indicates the relative ability of species to replace themselves. Measurements of mean age of the catch and age of onset of sexual maturity provide information used in protecting the brood stock and rate of maturity indices are more recent examples of these management initiatives.The trend in time responses (mean age, age to maturity, catch per unit effort, etc.) to exploitation of walleye stocks in the Great Lakes and larger inland lakes illustrate the appropriateness of monitoring these variables. Age to maturity, mean age of the catch, and fecundity will also be useful variables in determining the compensatory reserve to population reduction among walleye stocks inhabiting various energy and nutrient regimes. Using examples of heavily exploited walleye populations, we have made an initial attempt to show the degree of compensation in different energy regimes. A crisis curve is given to warn when various walleye stocks are in danger of collapse due to commercial overexploitation.Key words: walleye, phenotypic and genotypic difference, stock, distribution, rate of maturity, fecundity, walleye management