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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.

We develop a model for somatic growth in fishes that explicitly allows for the energy demand imposed by reproduction. We show that the von Bertalanffy (VB) equation provides a good description of somatic growth after maturity, but not before. We show that the parameters of the VB equation are simple functions of age at maturity and reproductive investment. We use this model to show how the energy demands for both growth and reproduction trade off to determine optimal life-history traits. Assuming that both age at maturity and reproductive investment adapt to variations in adult mortality to maximize lifetime offspring production, our model predicts that: (i) the optimal age of maturity is inversely related to adult mortality rate; (ii) the optimal reproductive effort is approximately equal to adult mortality rate. These predictions are consistent with observed variations in the life-history traits of a large sample of iteroparous freshwater fishes.

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Thermal preference and performance provide the physiological frame within which fish species seek strategies to cope with the challenges raised by the low temperatures and low levels of oxygen and food that characterize winter. There are two common coping strategies: active utilization of winter conditions or simple toleration of winter conditions. The former is typical of winter specialist species with low preferred temperatures, and the latter is typical of species with higher preferred temperatures. Reproductive strategies are embodied in the phenology of spawning: the approach of winter conditions cues reproductive activity in many coldwater fish species, while the departure of winter conditions cues reproduction in many cool and warmwater fish species. This cuing system promotes temporal partitioning of the food resources available to young-of-year fish and thus supports high diversity in freshwater fish communities. If the zoogeographic distribution of a species covers a broad range of winter conditions, local populations may exhibit differences in their winter survival strategies that reflect adaptation to local conditions. Extreme winter specialists are found in shallow eutrophic lakes where long periods of ice cover cause winter oxygen levels to drop to levels that are lethal to many fish. The fish communities of these lakes are simple and composed of species that exhibit specialized adaptations for extended tolerance of very low temperatures and oxygen levels. Zoogeographic boundaries for some species may be positioned at points on the landscape where the severity of winter overwhelms the species' repertoire of winter survival strategies. Freshwater fish communities are vulnerable to many of the shifts in environmental conditions expected with climate change. Temperate and northern communities are particularly vulnerable since the repertoires of physiological and behavioural strategies that characterize many of their members have been shaped by the adverse environmental conditions (e.g. cool short summers, long cold winters) that climate change is expected to mitigate. The responses of these strategies to the rapid relaxation of the adversities that shaped them will play a significant role in the overall responses of these fish populations and their communities to climate change.

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Predicted increases in water temperature in response to climate change will have large implications for aquatic ecosystems, such as altering thermal habitat and potential range expansion of fish species. Warmwater fish species, such as smallmouth bass, Micropterus dolomieu, may have access to additional favourable thermal habitat under increased surface-water temperatures, thereby shifting the northern limit of the distribution of the species further north in Canada and potentially negatively impacting native fish communities. We assembled a database of summer surface-water temperatures for over 13 000 lakes across Canada. The database consists of lakes with a variety of physical, chemical and biological properties. We used general linear models to develop a nationwide maximum lake surface-water temperature model. The model was extended to predict surface-water temperatures suitable to smallmouth bass and under climatechange scenarios. Air temperature, latitude, longitude and sampling time were good predictors of present-day maximum surface-water temperature. We predicted lake surface-water temperatures for July 2100 using three climate-change scenarios. Water temperatures were predicted to increase by as much as 18 1C by 2100, with the greatest increase in northern Canada. Lakes with maximum surface-water temperatures suitable for smallmouth bass populations were spatially identified. Under several climate-change scenarios, we were able to identify lakes that will contain suitable thermal habitat and, therefore, are vulnerable to invasion by smallmouth bass in 2100. This included lakes in the Arctic that were predicted to have suitable thermal habitat by 2100.

Life history characteristics of 54 Ontario lake trout (Salvelinus namaycush) populations vary with differences in lake area (range 25-450 000 ha) and total dissolved solids (TDS) (range 15-180 mg·L-1). Populations from large lakes exhibit greater maximum sizes, greater ages and sizes at first maturity, lower natural mortality rates, and lower sustainable yields. Populations from high-TDS lakes exhibit higher growth rates in early life, lower ages at first maturity, larger sizes at first maturity, and higher natural mortality rates. Angler catchability increases significantly at low population densities. With these relationships included in an age-structured population model, we found that the fishing mortality rate at maximum equilibrium yield ranges from 0.12·year-1 for a 100-ha, low-TDS lake to 0.37·year-1 for a 10 000-ha, high-TDS lake; the fishing effort level at maximum equilibrium yield ranges from 6.6 angler-h·ha-1· year-1 for a 100-ha, low-TDS lake to 4.0 angler-h·ha-1·year-1 for a 10 000-ha, high-TDS lake. Populations from small, low-TDS lakes are more sensitive to overexploitation than populations from large, high-TDS lakes. Easily measured, environmental correlates of life history characters may be common among fish species and are useful in developing exploitation guidelines for populations that are not well studied.