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Characteristics of temperature-dependent metabolic adaptation, as well as their implications for climate-dependent energy budgets, biogeography and fitness are reviewed and analysed for populations of Atlantic cod Gadus morhua in relation to findings in other fish species from northern and southern hemispheres, especially various species of eelpout (Zoarcidae). The analysis builds on the recently posited concept of oxygen-and capacity-dependent thermal tolerance in aquatic ectotherms. Mechanistic physiological studies are used to explain both functional differences between populations and current observations (such as the northward movement of cod, or the changes in seasonal abundance of eelpout due to global warming). Available data support the hypothesis that natural selection favours individuals that maximize growth and energy efficiency at the expense of ranges of thermal tolerance. The levels of …