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

Atlantic cod (Gadus morhua L.), acclimated to 5 degrees C, were equipp ed with ultrasonic transmitters which allowed the continuous monitorin g of their vertical movements and heart rate, Fish were then placed in a 125 m(3) tower tank in which the various thermal conditions they en counter in their natural environment were reproduced, Physiological an d behavioural responses of cod were followed in parallel to the induce d environmental changes, The experimental conditions studied in the to wer tank were also reproduced in a swimming respirometer, where oxygen consumption and heart rate could be monitored within the activity ran ge of a free-swimming animal. In a homogeneous water column, a rise in temperature induced marked increases in fish swimming activity, heart rate and heart beat-to-beat variability, In a thermally stratified en vironment, voluntary activity also increased when the thermal structur e of the water column was altered, though no temperature-dependent cha nges in heart rate were observed, In this case, fish avoided the new t emperature conditions, exhibiting distinct thermoregulatory behaviour, Stratification of the water column also prompted daily cyclic changes in fish distribution, animals tending to be in deeper and colder wate r layers during the day and in shallower and warmer layers at night. R espirometry experiments revealed that the thermoregulatory behaviour o bserved in free-ranging fish was probably driven by the energetic expe dient of maintaining the physiological status quo - i.e. avoiding bioe nergically costly reacclimation processes, Indeed, acute temperature i ncreases or decreases of 2.5 degrees C led to marked differences in ox ygen consumption, with metabolic rate changes of 15 and 30%, respectiv ely, The persistent linear relationship between heart rate and oxygen consumption allowed us to estimate, from the heart rate recorded in fr ee-swimming fish, the entire range of metabolic responses that cod und erwent voluntarily while experiencing a thermally stratified water col umn, The most profound metabolic effect, however, was observed with fe eding, when oxygen consumption increased by as much as 80%, resulting in an estimated 90% reduction in their subsequent scope for activity.