Behavior and plasma sodium regulation of chum salmon fry during transition into seawater

Abstract

Changes in seawater adaptability during the growth of chum salmon ( Oncorhynchus keta) fry were followed by examining plasma Na concentration after transfer to seawater. On direct transfer of fry weighing in excess of 1.8 g from fresh water to seawater, plasma Na concentration increased markedly after 3 h, reached a maximum after 6–12 h and was still higher than the seawater-acclimated level after 24 h. On the other hand, the smaller or younger fry weighing 0.4–1.4 g attained seawater-acclimated level within 24 h after transfer. When seawater-acclimated fish were transferred back into fresh water, plasma Na concentration of the larger fry decreased markedly (43%) after 6 h, and more than 50% of the fish died within a week after transfer. The Na concentration of the smaller fry decreased slightly (13%) during the first 3 days, and freshwater-acclimated level was restored 7 days after transfer. Thus, the osmoregulatory ability of the chum fry seems to decrease with an increase in body weight. Plasma Na concentration increased gradually during the course of seaward migration without showing any peak, and the change in the Na concentration correlated well with the increase in salinity. In the estuary, most of the fry were found in the mixed water of the surface, where the salinity was 10–14‰ (about 1 3 seawater). When fry weighing between 0.7 and 1.7 g were transferred from fresh water to 1 3 seawater (12‰), plasma Na concentration increased gradually and reached seawater-acclimated level after 12 h, and subsequent exposure to 100% seawater did not cause significant change in the plasma Na level. When these fry were maintained in 1 3 seawater for 6 h and then transferred to 100% seawater, the plasma Na concentration increased slightly but significantly during 6–12 h after transfer to seawater, and the seawater-acclimated level was attained after 24 h. As in the case of direct transfer from fresh water to seawater, the smaller fry tended to adjust their plasma Na levels more easily than the larger fry. Therefore, it seems to be of physiological significance for the chum fry to stay in dilute seawater in the estuary for more than 6 h for smooth adaptation to seawater.