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Relative impacts of bottom-up (producer controlled) and top-down (consumer controlled) forces on the biomass and size structure of five major components of freshwater pelagic systems (piscivores, planktivores, zooplankton, phytoplankton, and total phosphorus availability) were estimated. Predictions that emerge are (1) maximum biomass at each trophic level is controlled from below (bottom-up) by nutrient availability, (2) this bottom-up regulation is strongest at the bottom of the food web (i.e. phosphorus → phytoplankton) and weakens by a factor of 2 with each succeeding step up the food web, (3) as energy moves up a food web, the predictability of bottom-up interactions decreases, (4) near the top of the food web, top-down (predator mediated) interactions are strong and have low coefficients of variation, but weaken with every step down the food web, (5) variability around the bottom-up regressions can always be explained by top-down forces, and (6) interplay between top-down and bottom-up effects changes with the trophic status of lakes. In eutrophic lakes, top-down effects are strong for piscivore → zooplankton, weaker for planktivore → zooplankton, and have little impact for zooplankton → phytoplankton. For oligotrophic lakes, the model predicts that top-down effects are not strongly buffered, so that zooplankton → phytoplankton interactions are significant.

The possibility for fishery-induced evolution of life history traits is an important but unresolved issue for exploited fish populations. Because fisheries tend to select and remove the largest individuals, there is the evolutionary potential for lasting effects on fish production and productivity. Size selection represents an indirect mechanism of selection against rapid growth rate, because individual fish may be large because of rapid growth or because of slow growth but old age. The possibility for direct selection on growth rate, whereby fast-growing genotypes are more vulnerable to fishing irrespective of their size, is unexplored. In this scenario, faster-growing genotypes may be more vulnerable to fishing because of greater appetite and correspondingly greater feeding-related activity rates and boldness that could increase encounter with fishing gear and vulnerability to it. In a realistic whole-lake experiment, we show that fast-growing fish genotypes are harvested at three times the rate of the slow-growing genotypes within two replicate lake populations. Overall, 50% of fast-growing individuals were harvested compared with 30% of slow-growing individuals, independent of body size. Greater harvest of fast-growing genotypes was attributable to their greater behavioral vulnerability, being more active and bold. Given that growth is heritable in fishes, we speculate that evolution of slower growth rates attributable to behavioral vulnerability may be widespread in harvested fish populations. Our results indicate that commonly used minimum size-limits will not prevent overexploitation of fast-growing genotypes and individuals because of sizeindependent growth-rate selection by fishing.behavior ͉ fisheries ͉ selection ͉ temperament I t is well known that fisheries tend to select for larger and older fish individuals because of preference and/or regulations imposing minimum size limits for harvest. The result of sustained and heavy size-selective harvesting over time has been the removal of larger and/or later-maturing individuals from populations, leaving behind populations consisting of small, earlymaturing individuals, with low fecundity (1-3). Because growth rate affects fish size at age and size at maturity, a size-selective fishery may indirectly remove faster-growing individuals from a population. Studies suggest that this effect may represent contemporary evolution, leaving behind genotypes that are slowergrowing and early-maturing; this then can lead to reductions in harvestable biomass and population fecundity that in turn hinders population recovery from harvest (2-5). The possibility for evolutionary responses should not be surprising given high heritability of growth rate and other life history parameters in fish and the intensity of size-selective fish harvest reviewed in refs. 2, 6, and 7. However, we are aware of only two studies providing strong evidence of fisheries-induced evolution of growth and/or other life history traits (4, 5).A fishery may select upon growth rate through both indirec...

The feeding activity of warm-and coolwater fishes can be severely restricted during the long period of cold temperatures characteristic of winter in temperate zone lakes and rivers. The effect of such restriction is greater for smaller fish. Weight-specific basal metabolism increases as size decreases; however, there is no corresponding increase in energy storage capacity. Thus, smaller fish tend to be less tolerant of starvation conditions because they exhaust their energy stores sooner. Such size dependence of starvation endurance has often been observed in laboratory experiments. In wild populations commonly subject to winter starvation, population viability hinges on the ability of young of year to complete a minimum amount of growth during their first year of life. From south to north, this ability is increasingly restricted as the growing season shortens and the starvation period lengthens. We show that this constraint is sufficient to explain the present locations of the northern distributional limit for yellow perch Percaflavescens in central and western North America, the northern distributional limit for Eurasian perch P. fluviatilis in Eurasia, and the northern distributional limit for smallmouth bass Micropterus dolomieui in central North America. We also forecast how shifts in North American climate may relax this constraint and permit both yellow perch and smallmouth bass to thrive well to the north of their present distributions.

In laboratory experiments, fed and starved young-of-the-year yellow perch (Perca flavescens) lost weight when kept under simulated overwinter temperature and photoperiod regimes, small fish losing a greater proportion of their mass than larger ones. Perch in fed and starved treatments suffered 1 and 46% mortality, respectively, mortality being higher among the smaller individuals. Winter duration was an important determinant of both total mortality and the intensity of size-selective mortality. Size-selective mortality also occurred in yellow perch from the same stock kept overwinter in an in situ lake enclosure with natural food. Fall and spring population estimates for two cohorts of young-of-the-year yellow perch from Lake St. George also indicated the occurence of size-selective overwinter mortality. We present a quantitative technique for identification of size-dependent mortality and size-dependent growth from sequential length frequency distributions. This technique allowed identification of overwinter size-selective mortality for five natural cohorts sampled in the field. Results from a stochastic simulation model, incorporating observed variability in both first year growth and winter duration suggest that overwinter starvation mortality can cause substantial variability in year-class strength that is independent of adult stock size.