The effects of fasting on the metabolic interaction between digestion and locomotion in juvenile southern catfish (Silurus meridionalis)

To investigate the effect of fasting on maintenance metabolism, feeding metabolism and aerobic swimming performance as well as their metabolic interactions in juvenile southern catfish, we measured the following: (1) the postprandial oxygen consumption (M˙O2) response (16% body mass meal size) after 0 (control), 1, 2 and 4 weeks of fasting and (2) the swimming performance of non-digesting and digesting fish after either 0, 1, 2 or 4 weeks of fasting. The fasting groups displayed with lower resting M˙O2 (M˙O2rest), lower peak postprandial M˙O2 (M˙O2peak), larger energy expenditures and longer digestive processes than those of the control groups. The critical swimming speed (Ucrit), the active M˙O2 (M˙O2active) and the metabolic scope (M˙O2active -M˙O2rest, MS) of both non-digesting and digesting fish all decreased progressively after 1, 2 and 4 weeks of fasting, with those of non-digesting fish decreased more acutely than digesting fish (P < 0.05). Digesting fish displayed with a 14%, 23%, 27% and 71% significantly higher M˙O2active than that of non-digesting fish in the 0-, 1-, 2- and 4-week fasting groups. Digestion only caused a significantly lower Ucrit and MS in both the 0- and 1-week fasting groups (P < 0.05). The M˙O2 increased greatly with the swimming speed, and digestion caused a higher M˙O2 when compared to that of the fasting fish in any groups. The M˙O2 of fish in the 4-week fasting group was significantly lower than that of other groups when the swimming speed was the same. In conclusion, both digestive and locomotive functions were down-regulated during fasting. In the 0- and 1-week fasting groups, the decreased MS for swimming during digestion caused a lower Ucrit (i.e., a digestion priority model). However, because the M˙O2active of digesting fish decreased much more slowly than that of fasting fish, the MS of the 2- and 4-week fasting groups did not change during digestion, and the fish could handle both physiological activities independently (i.e., an additive model).