The effects of diel-cycling hypoxia acclimation on the hypoxia tolerance, swimming capacity and growth performance of southern catfish (Silurus meridionalis).

To investigate the effects of diel-cycling hypoxia acclimation on the hypoxia tolerance, swimming and growth performance of juvenile southern catfish, we initially measured the critical oxygen tension (P(crit)), oxygen thresholds of aquatic surface respiration (ASR) and loss of equilibrium (LOE) of diel-cycling hypoxia-acclimated (15 d, 7:00-21:00, dissolved oxygen level (DO) = 7.0 ± 0.2 mg L(-1); 21:00-7:00, DO = 3.0 ± 0.2 mg L(-1)) and non-acclimated (15 d, DO = 7.0 ± 0.2 mg L(-1)) southern catfish at 25 °C. We then measured the critical swimming speed (U(crit)) and metabolic rate (MR) of hypoxia-acclimated and non-acclimated fish (under both hypoxic and normoxic conditions). The feeding rate (FR), feeding efficiency (FE) and specific growth rate (SGR) of fish in hypoxia-acclimated and non-acclimated groups were also measured. The P(crit), ASR and LOE of hypoxia-acclimated fish were significantly lower than those of non-acclimated fish. Hypoxia acclimation resulted in a significantly higher U(crit) when the individuals swam in hypoxia. The U(crit), maximum metabolic rate (MMR) and metabolic scope (MS) of both the hypoxia-acclimated and non-acclimated fish all decreased with the decrease of DO. However, the U(crit), MMR and MS decreased by 31, 43 and 54%, respectively, in non-acclimated fish, whereas these values decreased by 15, 28 and 29%, respectively, in hypoxia-acclimated fish, which suggests that hypoxia-acclimated fish were less sensitive to the DO decrease. The FR, FE and SGR all decreased by 21, 20 and 45%, respectively, in the hypoxia-acclimated group compared to the non-acclimated group. This result suggests that diel-cycling hypoxia acclimation improved the hypoxia tolerance and aerobic swimming performance of southern catfish, whereas impaired the growth performance. The high hypoxia tolerance and physiological plasticity to hypoxia-acclimated southern catfish may be related to its lower maintenance energy expenditure, sit-and-wait lifestyle and bottom-dwelling living environment condition (usually facing oxygen fluctuation). The growth performance of so-called 'visceral type' fish species, such as southern catfish, are more sensitive to hypoxia compared to other fish species because of their high peak post-prandial metabolic rate, which may be restrained by the limited aerobic metabolic scope in hypoxia.