The effect of exercise training on the metabolic interaction between feeding and locomotion in the juvenile southern catfish (Silurus meridionalis Chen).

The southern catfish exhibits the largest decrease in critical swimming speeds (U(crit)) during digestion among the fish species that have been investigated. To test whether the maximum metabolic capacity of the southern catfish was improved after exercise training to alleviate the competitive interaction between digestion and swimming, we measured postprandial metabolic responses, U(crit) and oxygen consumption rates (MO(2)) during swimming in both fasting and digesting fish. Twenty-one days of training (50 min swimming at 60% U(crit) followed by 10 min chasing) did not produce significant differences in resting MO(2) (MO(2rest)) or postprandial peak MO(2) (MO(2peak)). However, it did result in a significant decrease in energy expenditure during digestion. Feeding caused a significant decrease in U(crit) and an increase in active MO(2) (MO(2active)), whereas training caused a significant increase in U(crit) but no significant change in MO(2active). Neither digestion nor training had a significant effect on metabolic scope (MO(2active)-MO(2rest)). Training had no interactive effect on postprandial changes in any measured variable, so we conclude that training did not alleviate the competitive interaction between digestion and swimming. Our results suggest that: (1) the metabolic capacity of nontrained fish cannot support the metabolic demands of both digestion and locomotion simultaneously, and swimming metabolism, therefore, is sacrificed to sustain digestion when feeding and locomotion are combined (digestion-prioritization mode); (2) the metabolic capacity and metabolic mode of competition did not change after training, but trained fish did exhibit improved swimming performance, possibly due to their increased rate of O(2) extraction.