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

Abstract

To investigate the effect of dissolved oxygen level ([O(2)]) 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 (MO(2rest)) over a range of water [O(2)] and from this we calculated the critical oxygen tension (P(crit)) of fasting fish; (2) the postprandial MO(2) response (10% body mass meal size) at water [O(2)] of 1, 2, 4 and 8mgO(2)L(-1); and (3) the swimming performance of fasting and digesting fish at water [O(2)] of 1, 2, 4 and 8mgO(2)L(-1) at 25 degrees C. The MO(2rest) remained constant over a broad range of water [O(2)] but then dropped markedly upon reaching the P(crit) (16.4% saturation). Hypoxic groups presented lower peak postprandial MO(2) (MO(2peak)) (1mgO(2)L(-1) group), larger energy expenditure and longer digestive process (both 1 and 2mgO(2)L(-1)) than those of normoxic groups. Both critical swimming speed (U(crit)) and the active metabolic rate (MO(2active)) of fasting fish remained unchanged over a decrease in water [O(2)] from 8 to 4mgO(2)L(-1) and then decreased significantly with further decreases in water [O(2)]. These parameters in fed fish showed a pronounced decrease as water [O(2)] decreased from 8 to 1mgO(2)L(-1). Feeding caused a significantly lower U(crit) in the 2mgO(2)L(-1) water [O(2)] group, a significantly higher MO(2active) in both the 2 and 8mgL(-1) water [O(2)] groups and a significantly higher metabolic scope (MO(2active)-MO(2rest)) in both the 2 and 4mgO(2)L(-1) water [O(2)] groups compared to fasting fish. The MO(2) increased greatly with swimming speed in the higher water [O(2)] groups, whereas it leveled off as swimming speeds approached the U(crit) in the lower water [O(2)] groups. Within all water [O(2)] groups, feeding caused a higher MO(2) compared to fasting fish when fish swam at the same speeds, except in the 1mgO(2)L(-1) group. This finding showed that the critical water [O(2)] for maintenance of metabolism, digestion, swimming and postprandial swimming increased sequentially due to the increasing O(2) 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.