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

Abstract

NEPSZY. 1981. Variation among stmks of walleye (Stizostedkorz vitreum virrelarat): management implications. Can. J. Fish. Aquat. Sci. 38: 1814-1831.Identification of races, strains, or subpopulations of walleye (StizostecSPorz \'itreurn vitrcwn) 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 bc 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 rivcrinc spawners suggests genetic differences. but this needs to be substantiated.Some of the physiological variables used to delineate thc 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 fcc~~ndity 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 (incan 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 varic~us walleye stocks are in danger of collapse due to commercial overexploitation. . 198 1. Variation among stocks of walleye (Stizostedion vitrertraz vitreum): management implications. Can. J. Fish. Aquat. Sci. 38: 1814-1831.L'identification de races. lignCes ou sous-populations de dorks jaunes par examcn de diffkrences morphologiques ou biochimiques a connu un succes relativenaent limit6. Bicn 'This paper fornms part of the

Microsatellite and mitochondrial DNA markers show no evidence of population structure in walleye (Sander vitreus) in Lake Winnipeg

“…The number of strays or migrants per generation required to break down genetic structure can be very small (Waples, 1998); a rate of gene flow as little as 1% can be sufficient to homogenize the gene pool (Carvalho and Hauser, 1994;Waters et al, 2002). Furthermore, although the migratory behavior of walleye has been extensively examined using tagging studies in multiple lake systems (Eschmeyer, 1950;Eschmeyer and Crowe, 1955;Forney, 1963;Foust and Hynes, 2007;Ward et al, 1989), these tagging studies provide no direct evidence that the homing behavior is to an individual's natal site-although it is widely assumed that this is the case (Spangler et al, 1977;Colby and Nepszy, 1981). Olson et al (1978) suggested that homing (or rather site fidelity) in walleye is not a natally-imprinted behavior, but rather learned as an adult.…”

“…Multiple genetic stocks can co-exist in a single water body if their reproduction is separated either temporally or spatially. Natal homing or spawning site philopatry, a behavior whereby individuals exhibit a tendency to reproduce at the site where they were born, has been extensively documented in walleye by tagging studies in multiple lake systems (Eschmeyer, 1950;Eschmeyer and Crowe, 1955;Forney, 1963;Foust and Hynes, 2007;Ward et al, 1989), but appears to be facultative in some systems (Olson and Scidmore, 1962;Colby and Nepszy, 1981). When present, philopatry results in a spatial separation of spawning aggregations across the lake, providing potential for genetic variation to develop over several generations, and for potentially discrete genetic stocks to form within the lake.…”

Temporal trends of mercury concentrations in Wisconsin walleye (Sander vitreus), 1982–2005

“…We included lake latitude, season in which each fish was collected, and fish length (cm) in our models to account for possible changes in the spatial distribution of lakes sampled, the seasonal timing of walleye sampling, and the size of walleye targeted for sampling. Lake latitude is also correlated with important gradients in pH and alkalinity, lake productivity, and fish growth (Colby and Nepszy 1981;Lillie and Mason 1983;Lathrop et al 1989;Quist et al 2003;Nate 2004). Fish length is known to be an important determinant of walleye Hg concentrations and must be accounted for in comparing lakes or estimating trends (Wiener et al 1990;Lathrop et al 1991).…”

Degree-day accumulation influences annual variability in growth of age-0 walleye

“…Walleye typically undergo a series of ontogenetic diet shifts in their first growing season, moving from zooplankton, to macroinvertebrates and finally to fish (Beck et al, 1998;Fox, 1989;Fox and Flowers, 1990;Fox et al, 1989;Johnston and Mathias, 1994a;Mathias and Li, 1982). The complexity of age-0 walleye food habits during the first growing season allows for many different scenarios in which food could be limiting (Colby and Nepszy, 1981).…”