Aerobic respiration, in which oxygen is used as the terminal electron acceptor, is crucial to all water-breathing fish.  When fish are deprived of oxygen, they require other ways to produce ATP.  Thus, a switch from aerobic metabolism to anaerobic metabolism occurs at the onset of hypoxia.  Glycolysis and substrate-level phosphorylation are used as alternative pathways for ATP production.  However, these pathways are much less efficient than aerobic metabolism.  For example, when using the same substrate, the total yield of ATP in anaerobic metabolism is 15 times lower than in aerobic metabolism.  This level of ATP production is not sufficient to maintain a high metabolic rate, therefore, the only survival strategy for fish is to alter their metabolic demands.