J. exp. Biol. 157, 75-86 (1991) 75 Printed in Great Britain © The Company of Biologists Limited 1991 THE INFLUENCE OF HYPOXIA ON THE PREFERRED TEMPERATURE OF RAINBOW TROUT ONCORHYNCHUS MYKISS BY H. SCHURMANN 1 , J. F. STEFFENSEN 2 * AND J. P. LOMHOLT 1 1 Department of Zoophysiology, University of Aarhus, DK-8000 Aarhus and 2 Marine Biological Laboratory, University of Copenhagen, DK-3000 Helsing0r, Denmark Accepted 17 December 1990 Summary 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. Introduction Fish placed in a temperature gradient will initially select temperatures that depend on the acclimation temperature. The selected temperature during the first 2 h in the gradient is termed the acute preferred temperature. If left in the gradient for a longer period, usually 24 h (Reynolds and Casterlin, 1980), the fish will select a species-specific preferred temperature. This temperature is termed the final preferendum and is defined by Fry (1947) as 'the temperature at which the individuals will ultimately congregate regardless of their thermal experience before being placed in the gradient, and at which acclimation temperature and acute preferred temperatures are equal'. Usually the selected temperature consists of a range of temperatures bounded by an upper and lower avoidance tempera- ture, so the final preferendum is actually a statistical measure of central tendency in the temperature distribution (Reynolds and Casterlin, 1979). Recently, the acute preferred temperature for fish (Bryan et al. 1984) has been shown to decrease as a result of exposure to hypoxia. Similar results were obtained in reptiles (Hicks and Wood, 1985), salamanders and crayfish (Dupre and Wood, *To whom reprint requests should be addressed. Key words: hypoxia, rainbow trout, preferred temperature, Oncorhynchus mykiss.