Development and temporal decrease in seawater adaptability during early growth in chum salmon, Oncorhynchus keta

Abstract

Changes in seawater (SW) adaptability of chum salmon ( Oncorhynchus keta) were examined from just after hatching until juveniles weighed 10 g. The plasma sodium (Na +) concentrations of the alevins in fresh water (FW) were maintained at a level of 141–143 mEq/l. SW adaptability of alevins increased with their development over the first 5 weeks, and reached a maximal level at the stage of yolk absorption 7 weeks after hatching. Over the 24-h time-course in SW, the maximal level of plasma Na + concentration increased from 163 ± 1.5 mEq/l in juveniles weighing less than 1 g to 180 ± 2.2 mEq/l in juveniles weighing 3–4 g. The maximal level decreased gradually to 168 ± 2.5 for 6–7 g juveniles and it remained constant in juveniles weighing 7–10 g. Changes in SW adaptability of hatchery-reared chum juveniles weighing 1.1 – 2.3 g were also examined in relation to body size or time spent in FW with a 48-h time-course after abrupt SW transfer. The maximal level of plasma Na + concentration of 1.1 g juveniles in mid-May was significantly lower than that of the same group of juveniles weighing 1.4 g in mid-June and 2.3 g in mid-July. In three groups of juveniles weighing 1.1–1.6 g in mid-May, the smaller juveniles adjusted their plasma Na + levels more easily than the larger juveniles. These results indicate that the SW adaptability of chum alevins develops rapidly after hatching and reaches a climax at the stage of yolk absorption; the adaptability of juveniles slightly decreases with growth from 0.4 to 3–4 g body weight but then recovers.