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

Largemouth bass (Micropterus salmoides) acclimated to 3 degrees C were placed in a thermal gradient. The bass selected the final thermal preferendum of 28 degrees C in about 18 h. The movement into warmer water was initially rapid but became progressively slower. 2) Other bass were acclimated to 8 degrees C, cannulated in the dorsal aorta, and placed in a temperature-controlled chamber. Oxygen uptake, blood pH, and total CO2 were measured as the chamber temperature was increased to 28 degrees C following a time course similar to that followed by bass in the gradient. 3) Oxygen uptake was always elevated above the resting level, and this elevation increased as higher temperatures were encountered. 4) 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 and 28 degrees C, the pH (8.01 +/- 0.03) was about 0.3 pH units above predicted normal values. 5) 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.