Hypoxia tolerance is unrelated to swimming metabolism of wild, juvenile striped bass (Morone saxatilis)

Juvenile striped bass residing in Chesapeake Bay are likely to encounter hypoxia that could affect their metabolism and performance. The ecological success of this economically valuable species may depend on their ability to tolerate hypoxia and perform fitness-dependent activities in hypoxic waters. We tested whether there is a link between hypoxia tolerance (HT) and oxygen consumption rate (M-O2) of juvenile striped bass measured while swimming in normoxic and hypoxic water, and to identify the interindividual variation and repeatability of these measurements. HT (loss of equilibrium) of fish (N=18) was measured twice collectively, 11 weeks apart, between which M-O2 was measured individually for each fish while swimming in low flow (10.2 cm s(-1)) and high flow (similar to 67% of critical swimming speed, U-crit) under normoxia and hypoxia. Both HT and M-O2 varied substantially among individuals. HT increased across 11 weeks while the rank order of individual HT was significantly repeatable. Similarly, M-O2 increased in fish swimming at high flow in a repeatable fashion, but only within a given level of oxygenation. M-O2 was significantly lower when fish were swimming against high flow under hypoxia. There were no clear relationships between HT and M-O2 while fish were swimming under any conditions. Only the magnitude of increase in HT over 11 weeks and an individual's M-O2 under low flow were correlated. The results suggest that responses to the interacting stressors of hypoxia and exercise vary among individuals, and that HT and change in HT are not simple functions of aerobic metabolic rate.