Atlantic cod Gadus morhua experienced oxygen deficit (
D
O
2
) when exposed to oxygen levels below their critical level (c. 73% of pcrit) and subsequent excess post‐hypoxic oxygen consumption (CEPHO) upon return to normoxic conditions, indicative of an oxygen debt. The mean ± s.e. CEPHO:
D
O
2
was 6·9 ± 1·5, suggesting that resorting to anaerobic energy production in severe hypoxia is energetically expensive.
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