The response of Atlantic cod, Gadus morhua, to progressive hypoxia: fish swimming speed and physiological stress

Atlantic cod, Gadus morhua, were exposed to a progressive stepwise decline in water oxygen pressure ð19:9; 13:2; 10:5; 8:4; 6:2 and 4:3 kPa PO2Þ: Fish swimming speed and indicators of primary and secondary stress (e.g. blood cortisol and lactate) were measured to assess whether a severe shift in physiological homeostasis (i.e. stress) preceded any change in behaviour or vice versa. Swimming speed increased by 18% when PO2 was reduced rapidly from 19.9 kPa to 13.2 kPa and was interpreted as an initial avoidance response. However, swimming speed was reduced by 21% at a moderate level of steady PO2 (8.4 kPa) and continued to drop by 41% under progressively deep hypoxia (4.3 kPa). Elevations in plasma cortisol and blood lactate indicated major physiological stress but only at 4.3 kPa, which corresponds to the critical oxygen tension of this species. We propose that the drop in speed during hypoxia aids to offset major stress and is adaptive for the survival of cod in extensive areas of low oxygen. Abbreviations PO2 : Partial pressure of oxygen