Interdependence of Acute Temperature Preference and Respiration in the Plains Minnow

Although the temperature preferences of fishes in air-saturated water have been studied intensively, very little is known of fishesˈ preferred temperatures in waters that are oxygen-deficient. Plains minnows Hybognathus placitus acclimated near 21 C selected 30 C in a horizontal temperature gradient when dissolved oxygen was maintained between 5 and 9 mg˙liter−1. But when dissolved oxygen was lower, the acutely preferred temperature declined-from near 30 C at 4.8 mg˙liter−1 to 17 C at 2 mg˙liter−1. Subsequent experiments on routine metabolism of plains minnows at various constant temperatures between 16.5 and 29 C gave results consistent with the following hypothesis: In oxygen-deficient water, the plains minnow prefers the highest temperature (less than 30 C) at which it can operate within the zone of respiratory independence. In nature, a reduction of acutely preferred temperature would reduce oxygen demand and might lead the fish into more highly oxygenated water. Received October 22, 1982 Accepted May 14, 1984