PHYSIOLOGY AND BEHAVIOR OF FREE-SWIMMING ATLANTIC COD (GADUS-MORHUA) FACING FLUCTUATING SALINITY AND OXYGENATION CONDITIONS

1. Atlantic cod (Gadus morhua L.) acclimated to a temperature of 5 deg rees C and 30 parts per thousand salinity were equipped with ultrasoni c transmitters which allowed continuous monitoring of their heart rate and their position in the water column, Fish were placed in a 125 m(3 ) tower tank which permitted various environmentally relevant modifica tions of the salinity and oxygenation conditions. Cod physiological an d behavioural responses were followed in parallel to the environmental manipulations, Some of the experimental conditions studied in the tow er tank were also reproduced in a swimming respirometer where fish oxy gen consumption and heart rate were monitored at various levels of act ivity. 2. Lowering salinity from 30 to 26 parts per thousand did not c hange resting oxygen consumption, but increased active oxygen consumpt ion. 3. Lowering salinity from 30 to 26 parts per thousand increased h eart rate over the whole range of swimming speeds except at maximum sp eed. 4. Lowering oxygen tension to 9 kPa decreased oxygen consumption over the whole range of swimming speeds and decreased resting heart ra te. 5. Low salinity did not significantly affect the relationship betw een heart rate and oxygen consumption. 6. Low oxygen levels decreased the oxygen transported per heart beat. 7. In the tower tank, bursts of activity were associated with tachycardias. 8. In uniform conditions, fish swam more deeply during the day than at night. 9. After an explo ratory period of approximately 6 h, fish chose to remain in a low-sali nity upper layer of the tank, Thereafter, high salinities were avoided , Fish tended to select low salinities if a choice was provided, 10. F ish generally avoided zones of low oxygen (<9 kPa) but continued volun tarily to enter regions with values as low as 3.0 kPa for short excurs ions or if food was offered.