Low-temperature dormancy in fish.

The importance of low ambient temperature in the physiology of winter dormancy was studied in the brown bullhead (Ictalurus nebulosus) and the largemouth bass (Micropterus salmoides). The bullheads frequently entered a sleep-like state at low temperatures; the likelihood of being aroused from this state was inversely proportional to the ambient temperature. Spontaneous activity for both species was relatively constant from 17 to 7 degrees C; at lower temperatures activity decreased. The selected temperature was lowered in both species as a consequence of acclimation to 3 degrees C; if given the opportunity, fish of both species moved to temperatures above 25 degrees C within 1 day in spite of the consequent acid-base and metabolic imbalances. In bass, food intake was very low for acclimation temperatures of 8 degrees C and below; at higher temperatures the relationship between food intake and acclimation temperature required 4 wk to stabilize. Quiescent brown bullheads exhibited discontinuous breathing. Alteration of brain temperature with implanted thermodes indicated that the main locus of control of this breathing pattern is in the medulla; lesser influences emanate from the anterior hypothalamus and the midbrain. Metabolism was measured at a series of acclimation temperatures between 3 and 17 degrees C for both species. No evidence of a discontinuous function (metabolic shutdown) was seen for either species.