SUMMARY The possible repercussions of osmoregulatory processes on some indicators of classical and oxidative stress were examined during gradual acclimation of sturgeons ( Acipenser naccarii ) to full seawater (35% salinity) and after a period of 20∼days at this salinity. Erythrocyte constants and levels of cortisol, protein and glucose in the plasma were determined. In addition, plasma osmolality and muscle-hydration values, as well as liver and heart protein, were determined. Catalase, glutathione peroxidase and superoxide dismutase activities and lipidperoxidation levels were measured in blood (plasma and red blood cells) and tissue (liver and heart). A number of physiological responses, such as disturbance in body fluid, activation of osmoregulatory mechanisms, augmented antioxidant defences in blood and alteration of energy metabolites, were detected with increasing environmental salinity. After 20 days at 35% salinity, plasma osmolality, erythrocyte constants and muscle water content all returned to values usual for low environmental salinity, indicating that osmoregulatory processes have achieved their objective. However, cortisol values, antioxidant enzyme activities in the blood (plasma and red blood cells), lipid peroxidation in plasma, and hepatic proteins did not return to initial values, showing that osmoregulatory processes cause major physiological changes in the fish.

Atlantic cod ( Gadus morhua L.) acclimated to water salinities ranging from 28 ‰ to 7 ‰ were exposed to mild (8.0 kPa) or severe (4.0 kPa) hypoxic conditions for 6h. In each experiment, respiratory, acid-base, ionic, haematological and metabolic disturbances were analyzed. During mild hypoxia, a strong hyperventilatory response was observed, resulting in a respiratory alkalosis that persisted throughout the 6-h trial. Plasma Cl − and pyruvate levels were the only other variables to display significant changes: they both increased. In more severe hypoxic conditions, although the ventilatory response was the same, a weak metabolic acidosis was superimposed. The haematological response (increased haematocrit and decreased mean cellular haemoglobin content) suggested that catecholamines were released into the blood. Both Na + and Cl − concentrations increased significantly. Metabolic perturbations occurred: plasma lactate, pyruvate and glucose concentrations increased markedly. Though lactate concentrations in liver, heart and white muscle increased, the concentrations of pyruvate, glucose and glycogen did not change significantly. Water salinity affected the amplitude of the ionic responses during hypoxia: the amplitude decreased with decreasing salinity. Irrespective of water salinity, 23 of 29 fish survived the severe hypoxic conditions. This relatively good tolerance of low water oxygenation, as compared with other marine bottom-feeders, suggests that this species may face poorly oxygenated waters in the wild. Together with temperature and salinity, ater oxygen content may thus be an important variable to take into account in the study of the distribution and migration patterns of Atlantic cod.

Acid-base regulation in rainbow trout acclimated to about 3, 100 and 500 mmol l −1 Na + and Cl − , at constant water [HCO 3 − ], was assessed during 24h of exposure to 1% CO 2 and during recovery. The respiratory acidosis induced by a rise in plasma P CO CO2 to about 1.15kPa (8.5mmHg, 3mmol l −1 ), 1.33kPa (10mmHg, 100 mmol l −1 ) or 1.5 kPa (11.2 mmHg, 500 mmol l −1 ) was partially compensated for by accumulation of plasma HCO 3 − . The degree of pH compensation depended on the salinity of the environmental water, being about 61, 82 and 88% at 3, 100 and 300 mmol l −1 Na + and Cl − , respectively. [HCO 3 − ] in animals acclimated to 100 and 500 mmol l −1 rose to higher values than that in fish at 3 mmol l −1 . Plasma [Cl − ] decreased during hypercapnia as compared to control concentrations in all groups of fish. Plasma [Na + ] rose during the first 8 h of hypercapnia in fish acclimated to all three salinities, but recovered towards control values during the remainder of hypercapnia. The rise in plasma [HCO 3 − ] was significantly related to the fall in plasma [Cl − ], whereas the changes in plasma [Na + ] were unaffected by simultaneous changes in plasma [HCO 3 − ]. Time courses of changes in plasma [Na + ] and total ammonia concentration, [ T amm ], were similar but in opposite directions. The transepithelial potential (TEP) of blood relative to water was negative, close to zero and positive, averaging −21, −5.8 and +6.2 mV for fish acclimated to 3, 100 and 300 mmol l −1 Na + , respectively. After initiation of hypercapnia, which caused a quite heterogeneous response among groups, a clear trend towards depolarization was observed during the remainder of hypercapnia. These results confirm the role of active HCO 3 − /Cl − exchange processes for the compensation of extracellular pH during respiratory acidoses in fish.