Excess posthypoxic oxygen consumption in rainbow trout (Oncorhynchus mykiss): recovery in normoxia and hypoxia

Under certain conditions, a number of fish species may perform brief excursions into severe hypoxia and return to water with a higher oxygen content. The term severe hypoxia describes oxygen conditions that are below the critical oxygen saturation (S crit ), defined here as the oxygen threshold at which the standard metabolic rate becomes dependent upon the ambient oxygen content. Using rainbow trout ( Oncorhynchus mykiss (Walbaum, 1792), this study quantified the excess posthypoxic oxygen consumption (EPHOC) occurring after exposure to oxygen availability below S crit . Tests showed that S crit was 13.5% air saturation (O 2sat ). Fish were exposed to 10% O 2sat for 0.97 h, and the EPHOC was quantified in normoxia (≥95% O 2sat ) and hypoxia (30% O 2sat ) to test the hypothesis that reduced oxygen availability would decrease the peak metabolic rate (MO 2peak ) and prolong the duration of the metabolic recovery. Results showed that MO 2peak during the recovery was reduced from 253 to 127 mg O 2 ·kg –1 ·h –1 in hypoxia compared with normoxia. Metabolic recovery lasted 5.2 h in normoxia and 9.8 h in hypoxia. The EPHOC, however, did not differ between the two treatments. Impeded metabolic recovery in hypoxia may have implications for fish recovering from exposure to oxygen availability below S crit .