The behavioural thermoregulation of rainbow trout Oncorhynchus mykiss was investigated in a shuttlebox at normoxia and at three levels of hypoxia: 13.3, 9.3 and 5.3 kPa. The final preferred temperatures at normoxia, and at 13.3, 9.3 and 5.3 kPa, were 16.1, 14.9, 15.0 and 12.7°C, respectively. A decreased rate of metabolism and an increased blood oxygen-affinity are among the physiological advantages of selecting a lower temperature during hypoxia. It is suggested that catecholamines may play a part in controlling temperature selection during hypoxia. In natural environments, this behaviour may result in habitat shifts of fish living in heterothermal environments with changing oxygen tensions.

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