A methodological evaluation of the determination of critical oxygen threshold in an estuarine teleost

One measure of hypoxia tolerance is the critical oxygen threshold, P_crit, which is the point where standard metabolism can no longer be maintained through aerobic processes. Traditionally, P_crit was determined using closed respirometry, whereby the fish's respiration naturally lowered O₂ More recently, intermittent flow techniques have been adopted, where N₂ is used to displace O₂, which ostensibly reduces end-product build-up. This study used a paired design on the marine teleost, red drum. P_crit is comparable between closed (4.6±0.2 kPa; mean±s.e.m.) and intermittent flow (4.4±0.2 kPa; mean±s.e.m.) respirometry. pCO₂, ammonia and pH changes within the chamber were measured prior to the onset of P_crit and at the end of a typical P_crit trial and revealed changes in water chemistry in both closed and intermittent flow. P_crit values were similar in both methods of hypoxia induction regardless of subsequent water chemistry changes that occurred in both methods.