Individual variation in metabolism and swimming performance in juvenile black carp ( Mylopharyngodon piceus) and the effects of hypoxia

We investigated inter-individual variation in metabolism, swimming performance and the relationship between metabolism and swimming performance under normoxic and hypoxic oxygen conditions ([O-2]) in juvenile black carp (Mylopharyngodon piceus). We measured the standard metabolic rate (SMR), critical swimming speed (U-crit), active metabolic rate (AMR) and aerobic scope (AS) of 40 fish (body weight, 9.7-11.0g; body length, 8.4-8.8cm) at 20 degrees C under normoxic (100% air saturation) and hypoxic (30% air saturation) conditions. Hypoxia resulted in a significant decrease in all the investigated parameters (p<0.05). The SMR, U-crit and AMR exhibited consistent individual differences (repeatability) (p0.022), whereas the AS had no consistency across different water [O-2] conditions (p=0.088). The SMR was positively correlated with the U-crit, AMR and AS (p0.002) under normoxic conditions. The SMR was also positively correlated with the U-crit and AMR under hypoxic conditions (p0.003) but there was no correlation between the SMR and AS (p=0.141). The slope of the correlation between the SMR and U-crit was shallower under hypoxic conditions than under normoxic conditions (F-1,F-76=13.844, p<0.001), which indicated that the swimming performance decreased more profoundly under hypoxic conditions in individuals with a high SMR.