Metabolic and acid-base changes during selection of warmer water by cold-acclimated fish

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Abstract
Largemouth bass (Micropterus salmoides) acclimated to 3.degree. C were placed in a thermal gradient. The bass selected the final thermal preferendum of 28.degree. C in .apprx. 18 h. The movement into warmer water was initially rapid but became progressively slower. Other bass were acclimated to 8.degree. C, cannulated in the dorsal aorta and placed in a temperature-controlled chamber. O2 uptake, blood pH and total CO2 were measured as the chamber temperature was increased to 28.degree. C following a time course similar to that followed by bass in the gradient. O2 uptake was always elevated above the resting level, and this elevation increased as higher temperatures were encountered. Just prior to placement in the chamber the pH of the arterial blood was 7.98 .+-. 0.05 (mean .+-. SE). As the temperature was increased neither the pH nor the total CO2 content of the blood exhibited major changes. At temperatures between 26-28.degree. C, the pH (8.01 .+-. 0.03) was .apprx. 0.3 pH units above predicted normal values. During the return to the final thermal preferendum fish experience overall metabolic rates and extracellular acid-base levels that deviate progressively farther from normal resting levels. Neither factor appears likely to be the major determinant of the behavioral response.