Morphological and physiological correlates with swimming performance in juvenile largemouth bass

Winter- and summer-acclimatized largemouth bass (Micropterus salmoides) were collected from hatchery ponds in eastern Colorado during late winter and midsummer, then challenged with two prolonged swimming performances (step test and constant-velocity endurance). Variation in the step test performances was significantly correlated with variation in the endurance performances in the winter-acclimatized but not in the summer-acclimatized fish. Fourteen physiological and morphological traits were measured on each fish, and correlations among these traits and swimming performance were tested. None of the traits measured were correlated with performance variation in both the winter- and summer-acclimatized fish. The only significant correlate with swimming performance in the summer-acclimatized fish was white muscle lactate dehydrogenase activity (n = 19). Six of the seven factors correlating with winter swimming performance (n = 18-19) could be divided into two categories: traits associated with fasting (condition factor and liver enzymatic activity) and those associated with oxygen delivery (heart mass, heart and red muscle cytochrome oxidase activity). The results of this study suggest that morphological and physiological correlates of swimming performance in juvenile largemouth bass are profoundly influenced by seasonal variation.