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
972. Effects of eutrophication on salmonid communities in oligotrophic lakes. J' Fish' Res. Bd. Canada 29:975-983.Oligotrophic lakes respond to progressive eutrophication by a sequence of predictable events. Increased nutrient loads and subsequent increased plant production result in alterations in the abiotic environment, including changes in the color and transparency of the water, increased turbidity, oxygen depletion in the hypolimnion, and increased chemical stratification. The physico-chemical changes precipitate biotic changes among the phytoplankton, littoral algae, zooplankton, and benthos. The salmonid community may respond initially with an increased body growth rate in various taxa and a higher incidence of parasitism, but later inhibition of natural reproduction occurs, and finally, the taxa are replaced by others that can survive in the changed environment.A relation between natural nutrient loading (expressed in terms of a morphoedaphic index) and yield (both quantitative and qualitative) is proposed as an aid to determining the natural successional status of a lake. Knowing the natural baseline of a particular lake the fisheries managers can judge the nature and size of responses due to cultural nutrient loading and then alter the rate of cultural nutrient loading to modify the ecological effects, or they can use biological engineering to capitalize on the present conditions'Among the most important effects of eutrophication is the increased vulnerability of sedentary discrete stocks to changes in other stresses such as fishing. Corsv, P. J., G. R. SraNcr,nn, D. A. Hunuv, .qNo A. M. McCoMers. 1972. Effects of eutrophication on salmonid communities in oligotrophic lakes. J. Fish. Res. Bd. Canada 29:975-983.
Several species interrelationships influenced by actural or preceived disturbances were described which managers should consider when manipulating fish populations and communities. For example, factors controlling homeostasis of adult northern pike (Esox lucius) and white suckers (Catostomus commersoni) in northern lakes may still operate despite walleye reductions, suggesting less niche overlap than we previously expected. In more northern centrarchid type communities, percid abundance and condition depend on how well northern pike and other predators control both white sucker and bluegill (Lepomis macrochirus) abundance. Evidence from walleye (Stizostedion vitreum vitreum) fry and fingerling plantings suggest that when intense interactions between species determine their abundance, these interactions occur during the very early life stages. Climatic changes may also be influential in determining abundance of several fish species common in north-temperate lakes. Rehabilitation of preferred species by removal of less desirable fishes can be successful in some ecosystems, but we warn that an undesirable compensatory response may also occur.
Embryonic development of lake herring (Coregonus artedii) was observed in the laboratory at 13 constant temperatures from 0.0 to 12.1 C and in Pickerel Lake (Washtenaw County, Michigan) at natural temperature regimes. Rate of development during incubation was based on progression of the embryos through 20 identifiable stages.An equation was derived to predict development stage at constant temperatures, on the general assumption that development stage [Formula: see text] is a function of time (days, D) and temperature (T). The equation should also be useful in interpreting estimates from future regressions that include other environmental variables that affect egg development.A second regression model, derived primarily for fluctuating temperatures, related development rate for stage [Formula: see text], expressed as the reciprocal of time, to temperature (x). The generalized equation for a development stage is:[Formula: see text]In general, time required for embryos to reach each stage of development in Pickerel Lake agreed closely with the time predicted from this equation, derived from our laboratory observations. Hatching time was predicted within 1 day in 1969 and within 2 days in 1970.We used the equations derived with the second model to predict the effect of the super-imposition of temperature increases of 1 and 2 C on the measured temperatures in Pickerel Lake. Conceivably, hatching dates could be affected sufficiently to jeopardize the first feeding of lake herring through loss of harmony between hatching date and seasonal food availability.
Young-of-the-year walleyes Stizostedion vitreum are more abundant in even-numbered years than in odd-numbered years in Savanne Lake, Ontario. Differences among years were related to emergences of the burrowing mayfly Hexagenia limbata. Adult walleyes from evennumbered year classes are more abundant than those from odd-numbered year classes. We hypothesize that pulse production of H. limbata in even-numbered years positively affects walleye recruitment by enhancing egg production by adult walleyes and buffering young of the year against predation and cannibalism.
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