FEED-INTAKE, GROWTH AND OSMOREGULATION IN ARCTIC CHARR, SALVELINUS-ALPINUS (L), FOLLOWING ABRUPT TRANSFER FROM FRESH-WATER TO MORE SALINE WATER

Groups of Arctic charr (150 g) were abruptly transferred from freshwater to water of salinities ranging from 10 to 35 ppt in April. Feed intake, growth and the ability of the fish to osmoregulate were then investigated on day 3 and day 30 following salt water transfer. Water temperature was kept at 8-degrees-C. No mortality occurred during the entire experimental period. Three days after transfer there were significant differences in feed intake, plasma osmolality and Cl- concentrations among fish exposed to the different salinity treatments. Feed intake was found to decrease and plasma electrolyte levels to increase with increasing salinity. At the termination of the experiment (30 days after transfer) no significant differences were found in feed intake, but plasma electrolyte levels were highest in fish held in water of the highest salinity. All plasma values were, however, within the normal range, indicating that osmo- and ionoregulatory homeostasis was achieved in all groups of fish. Increased salinity appeared to have an acute, but short lived appetite depressive effect in the experimental groups. Despite reduced feed intake in these groups on day 3, growth rates were high in all groups and there were no significant differences among salinity treatments. Thus, all groups of chaff appear to have adapted quickly to the new salinity conditions. The results indicate that Arctic charr can rapidly adapt to salinities up to 35 ppt in April, and that they may then have potentials for growth comparable to those of fish reared in freshwater. The results are discussed in relation to seasonal changes in seawater adaptability in salmonids.