SUMMARYTo test the hypothesis that digestion has a more notable physiological effect on ambush foragers than on active foragers, we investigated the behavioural, digestive and metabolic characteristics, as well as the postprandial locomotory capacity, of four species of juvenile fish distributed along the Yangtze River, China, with distinct foraging strategies. The ambush foraging southern catfish (Silurus meridionlis) had the fewest movements per minute (MPM), lowest per cent time spent moving (PTM), slowest critical swimming speed (U crit ), lowest maintenance metabolism (V O2rest ) and lowest maximum locomotory metabolism (V O2max ). However, the southern catfish had the highest feeding level and maximum feeding metabolism (V O2peak ) and the greatest decrease in U crit after consumption of a large meal. Thus, this fish is highly adapted to its ambush behavioural strategy and sedentary life style. In the herbivorous grass carp (Ctenopharyngodon idellus), a low digestive capacity led to little change in postprandial locomotory performance, which benefits its frequent grazing behaviour. In this species, the greater amount of energy spent on routine activity and avoiding predators versus U crit might be related to its herbivorous life style and high predation risk. The active foraging crucian carp (Carassius auratus) adopts a unique high energy cost strategy that allows for high capacity in both routine activity and digestion, and the great flexibility of its cardio-respiratory capacity (increased V O2max after feeding) guarantees a small decrease in U crit even after maximum feeding. Finally, the sluggish foraging darkbarbel catfish (Pelteobagrus vachelli) has low digestive and locomotory capacity, but its energy-efficient venomous defence strategy may be related to its abundance. These results show that the digestive, behavioural and metabolic strategies differ among these fish species. The locomotory capacity in the sedentary fishes decreased profoundly after feeding, whereas it decreased little or not at all in the active fishes. The maintenance of high locomotory capacity after eating in the active fishes is probably related to a large metabolic capacity, a lower digestive capacity or an improvement in cardio-respiratory capacity after feeding.

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Effects of feeding on pre-exercise VO(2) and excess post-exercise oxygen consumption (EPOC) after exhaustive exercise were investigated in sedentary southern catfish, active herbivorous grass carp, omnivorous crucian carp, and sluggish omnivorous darkbarbel catfish to test whether feeding had different effects on EPOC and to compare EPOC in fishes with different ecological habits. For fasting fish, the pre-exercise and peak post-exercise VO(2) were higher and recovery rates were faster in crucian carp and grass carp compared to those of darkbarbel catfish and southern catfish. EPOC magnitudes of grass carp and southern catfish were significantly larger than those of crucian carp and darkbarbel catfish. Feeding had no significant effect on peak post-exercise VO(2), recovery rate, and EPOC magnitude in grass carp. Both the pre-exercise and peak post-exercise VO(2) increased with meal size, while the EPOC magnitude and duration decreased significantly in the larger meal size groups of crucian carp and southern catfish. In darkbarbel catfish, both the pre-exercise and peak post-exercise VO(2) increased with meal size, but the VO(2) increment elicited by exercise was larger in feeding groups compared with the fasting group. These results suggest that (1) the characteristics of the post-exercise VO(2) profile, such as peak post-exercise VO(2) and recovery rate, were closely related to the activity of fishes, whereas the EPOC magnitude was not and (2) the effects of feeding on EPOC were more closely related to the postprandial increase in VO(2).

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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.