ADAPTATION OF THE BROOK TROUT, SALVELINUS-FONTINALIS, TO DIRECT TRANSFER TO SEA-WATER IN SPRING AND SUMMER

Non-migratory brook trout, Salvelinus fontinalis, were introduced directly into sea water in May, June, July, and August to measure their ability to osmoregulate and their capacity to survive in this environment. Plasma osmotic pressure, and Na+ and Cl- ion concentrations were measured in fish kept in sea water for 1, 7, 14, 21, and 28 days and compared with brook trout in fresh water. Gill Na+, K+-ATPase levels of activity were also evaluated. The brook trout adapted very well to sea water in May and June and displayed survival rates of over 85%. In July and August, the fish showed problems in osmoregulation from the first day in sea water and their survival rates decreased substantially. The gill Na+, K+-ATPase maximal level of activity decreased in fish reared in fresh or sea water in July and August. In June enzyme activity began to increase earlier than at any other period and it coincided with the best osmoregulatory performances. It is concluded that to obtain good survival and growth performance this species should be introduced into sea water not later than June.