Temperature affects rates of chemical reactions and can have large effects. Aerobic metabolism changes of 2 to 3 fold (Q₁₀ = 2 to 3) for every 10° C change in temperature.

The effects of temperature described above only apply within normal functional temperature range of an enzyme. Higher/lower temperatures denature tertiary or quaternary structures and eventually kill.

Fish can acclimate to temperature to at least some degree at the enzymatic level. For example, in the muscles fish from different latitudes have various isoforms of contractile proteins which facilitate development of similar maximum muscle stress of 200-300 kN/m² at normal environmental temperatures for Antarctic, temperate and tropical species. However, times required to activate and relax muscle increase among species as Antarctic>temperate>tropical. So that maximum beat frequencies for propulsors and muscle power output are lower for fish resident in colder habitats.

A polygon can be constructed showing how some physiological measure of performance varies with acclimation temperature. The polygon is delineated by ultimate lethal temperatures, those at which a fish just begins to be harmed, at which life expectancy is reduced. The polygon encloses the tolerance zone for the temperature for the species.

The tolerance polygon is surrounded by the resistance zone, in which temperature death is resisted for some period of time, depending on the magnitude of the temperature difference from the ultimate lethal level.

Temperature response polygons can be constructed for other activities. These always lie inside the polygon for ultimate lethal levels. More complex activities, such as reproduction are typically restricted to the smallest range of temperatures.

Standard - minimum level for a post-absorptive animal at rest under defined test conditions, such as temperature, toxicants etc.

Active - maximum sustainable rate, usually measured in an increasing velocity swimming trial, and referred to the test time at which fish swim at each speed; e.g. 45 to 60 minutes.

Metabolic Scope = active - standard metabolic rates. Indicates energy range that can be used for various activities.

Routine (or activity) metabolism - energy expended by animals during voluntary (routine) activities. Typically routine metabolic rate is 2-3 times Q_S.

Metabolic scope and maximum swimming speed increase with temperature, although there is often an optimum temperature above which they decline. This optimum temperature is typically close to the preferred temperature chosen by fish in a temperature gradient.

Fishes from different latitudes tend to converge on similar standard rates for metabolism.

The body temperature achieved by an organism depends on the environment, the ability to conserve heat, and the ability to generate heat. these factors determine body temperature. Approaches are classified on the basis of the importance of various heat sources in determining body temperature.

Maintenance ration - zero growth. Minimal food requirements plus all the costs associated with obtaining and processing the ration needed to provide a surplus equal to standard metabolic rate.

Maximum ration - the maximum that can be consumed over a reasonable period. Plaice - probably limited by heat increment within metabolic scope.

Appetite - amount eaten. Sturgeon, appetite can be large, but large food intake cannot be sustained.

Maximum growth rate - tangent to curve, defining an optimum ration lower than maximum ration.

Scope for growth - maximum ration - maintenance ration.

Principle of Conservation of Energy; all energy entering and leaving a system is accounted for. Metabolic requirements are met first. All animal responses to the environment are ultimately met at the metabolic level. Environmental effects will appear as changes in metabolism and an inverse relationship will therefore apply to growth.