Some Principles in the Thermal Requirements of Fishes

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Abstract
After pointing out that aquatic gill-respiring vertebrates can not escape from environmental temperature and that tissue temperature and environmental temperature are constantly equilibrated at the gill surfaces, the author restates the fundamental requirement: an external temperature best suited to the internal tissues. There is a wide thermal tolerance to which tissues are adapted. Lethal limits exist and vary between species. A table of lethal temperatures and thermal tolerances for 23 species of fish with 7 references is given. In general, cold-water fishes have a lower level of thermal tolerance than those of warmer waters. The degree of tolerance and the activity range is variously extended among species through reversible acclimation. This process is more rapid for a rising temperature than for a falling one. For maintaining life, a slow rate of decrease is of more importance than a slow rate of increase. Temperature may be a lethal factor since it constantly conditions fishes through acclimation while governing the scope of the metabolic rate. Performance is best in the region of the preferred temperature. A sensitivity to minute gradients of temperature may act as a directive factor. The author concludes that the role of temperature is a multiple one.