Osmotic adaptation of Oncorhynchus kisutch Walbaum. III. Survival and growth of juvenile coho salmon transferred to sea water at various times of the year

Abstract

Juvenile salmon from two successive broods (1974 and 1975 spawnings) were transferred to sea water of various salinities at different periods of the year in 1976 to determine the tolerance to sea water in terms of mortality and growth. Fish of the 0 + age group, transferred directly to sea water (35‰) on 23 June (19.5 ± 3.3 g) and 13 July (27.1 ± 2.4 g) very rapidly showed an extremely high mortality. Progressive adaptation performed at the same dates indicated a higher tolerance in June compared to the July transfer, but still with significant mortalities. Direct transfers made between September and December with subyearlings showed a better adaptability in September–October than later. Yearling fish put in sea water on 3 March (68.3 ± 0.8 g) and 14 April (98.0 ± 2.4 g) had a very low 30-days mortality after contact with sea water of various salinities (25–30–35‰). However, growth assessment indicated better osmotic adaptation in April whereas the fish transferred in March showed a much lower average growth. The results suggest that the 30-day survival criterion alone is not sufficient to characterize a good osmotic adaptation to the marine environment. The simultaneous examination of growth and mortality data gives a more precise estimation of the tolerance to sea water. This study provided information on the seasonal variations of this tolerance to high salinities, and defined the minimum sizes required for a satisfactory adaptation. It is interesting to note that the periods of highest euryhalinity (April–June and September–October) correspond to the phases of highest gill Na +K + ATPase activity described in an earlier paper.