Salt and Water Balance in Rainbow Trout (Salmo Gairdneri) Rapidly Transferred from Fresh Water to Sea Water

ABSTRACT Physiological responses of rainbow trout (mean weight 13·3 g) to sudden changes in salinity were investigated. An initial period lasting about 8 h was characterized by increased drinking and an increase in plasma and body ions. Fish failed to survive more than 2 days in full strength sea water but in two-thirds sea water there were few mortalities and adaptation was complete in 7–10 days. During this period there were gradual physiological changes resulting in normal plasma ion concentrations but significantly increased body ionic content. The intracellular concentrations of muscle chloride showed the greatest increase.