Lethal oxygen levels at different temperatures and the preferred temperature during hypoxia of the Atlantic cod, Gadus morhua L.

The behavioural thermoregulation of Atlantic cod Gadus morhua L. was investigated in a shuttlebox at normoxia and at three levels of hypoxia: 30, 20 and 15% oxygen saturation. The preferred temperatures at normoxia, 30, 20 and 15% oxygen saturation were 13·9, 13·1. 10·0 and 8·8° C, respectively. A decrease in metabolism and an increased blood oxygen affinity are among the physiological advantages of selecting a lower temperature during hypoxia. Furthermore the chances of surviving low oxygen saturations are better at low temperatures. In natural environments, this behaviour may result in habitat shifts of fish living in heterothermal environments with changing oxygen saturations, especially in coastal areas with eutrophication, as for example the Baltic Sea.