The effects of dissolved oxygen level on the metabolic interaction between digestion and locomotion in juvenile southern catfish (Silurus meridionalis Chen)

To investigate the effect of dissolved oxygen level ([O2]) on maintenance metabolism, feeding metabolism, aerobic swimming performance and their metabolic interaction in juvenile southern catfish (Silurus meridionalis Chen), we measured the following: (1) the resting oxygen consumption rate (MO2rest) over a range of water [O2] and from this we calculated the critical oxygen tension (Pcrit) of fasting fish; (2) the postprandial MO2 response (10% body mass meal size) at water [O2] of 1, 2, 4 and 8 mgO2 L− 1; and (3) the swimming performance of fasting and digesting fish at water [O2] of 1, 2, 4 and 8 mgO2 L− 1 at 25 °C. The MO2rest remained constant over a broad range of water [O2] but then dropped markedly upon reaching the Pcrit (16.4% saturation). Hypoxic groups presented lower peak postprandial MO2 (MO2peak) (1 mgO2 L− 1 group), larger energy expenditure and longer digestive process (both 1 and 2 mgO2 L− 1) than those of normoxic groups. Both critical swimming speed (Ucrit) and the active metabolic rate (MO2active) of fasting fish remained unchanged over a decrease in water [O2] from 8 to 4 mgO2 L− 1 and then decreased significantly with further decreases in water [O2]. These parameters in fed fish showed a pronounced decrease as water [O2] decreased from 8 to 1 mgO2 L− 1. Feeding caused a significantly lower Ucrit in the 2 mgO2 L− 1 water [O2] group, a significantly higher MO2active in both the 2 and 8 mg L− 1 water [O2] groups and a significantly higher metabolic scope (MO2active − MO2rest) in both the 2 and 4 mgO2 L− 1 water [O2] groups compared to fasting fish. The MO2 increased greatly with swimming speed in the higher water [O2] groups, whereas it leveled off as swimming speeds approached the Ucrit in the lower water [O2] groups. Within all water [O2] groups, feeding caused a higher MO2 compared to fasting fish when fish swam at the same speeds, except in the 1 mgO2 L− 1 group. This finding showed that the critical water [O2] for maintenance of metabolism, digestion, swimming and postprandial swimming increased sequentially due to the increasing O2 demand, which challenges respiratory capacity. With the administration of the medium-sized meal that was used in this study, the metabolic mode of juvenile southern catfish changed from an additive mode during normoxia to a digestion-priority mode under moderate hypoxia and to locomotion-priority mode under hypoxia.