Seawater Adaptability in Autumn, and Freshwater Adaptability in Spring, in Juveniles of several Salmonid Species

Abstract

Salmon culture in coastal waters from autumn to early summer and transfer of cultured salmon to freshwater in spring is being attempted in Japan. From this view point four experiments were carried out in order to investigate seawater and freshawter adaptability in several salmonid species.
In Experiment 1, underyearlings of the rainbow trout, charr and chum salmon were directly transferred from freshwater to seawater in autumn. The rainbow trout died within three days, whereas charr and chum salmon exhibited higher tolerance to seawater (survival rate, 80-100%). In the latter two species, hematocrit value, serum sodium level, and moisture content of muscle greatly fluctuated during the first few days in seawater. Thereafter, these values reached those of seawater fish.
In Experiment 2, underyearlings of the rainbow trout and coho salmon weret fransferred from freshwater to various concentrations of seawater. Forty-eight hours following the transfer, an elevation of serum sodium levels was observed in the rainbow trout and coho salmon transferred to seawater higherthan 23‰, and 25‰, respectively. Dead fish were observed in seawater above 25‰in rainbow trout and 32‰ in coho salmon.
In Experiment 3, underyearlings of rainbow trout and amago salmon were transferred to seawater after acclimation in 50% seawater for 2-7 days. Long term acclimation improved seawater adaptability and survival rate.
In Experiment 4, yearlings of rainbow trout and amago salmon which reared in seawater were directly transferred to freshwater in spring. This transfer was carried out successfully without mortality and any serious decrease in serum sodium levels.