Prolactin kinetics during freshwater adaptation of mature chum salmon, Oncorhynchus keta

When mature chum salmon (Oncorhynchus keta) caught in Otsuchi Bay (salinity 34 ppt), northern Honshu Island, Japan, were transferred directly to fresh water, they attained normal plasma ion levels within 24 hr. Plasma prolactin remained low in the fish kept in seawater. On transfer to fresh water, a significant increase in plasma prolactin concentration was seen only in females, but not in males. Metabolic clearance rate (MCR) and secretion rate of prolactin were calculated from its plasma levels after intra-arterial injection of chum salmon prolactin into chronically-cannulated fish. In both males and females, a significant increase in MCR was seen after transfer to fresh water, indicating that prolactin is involved in freshwater adaptation in both sexes. In females, the secretion rate increased significantly after 6 days in fresh water. No such change in the secretion rate was seen in males. It seems likely that prolactin secretion is affected by reproduction-related changes occurring in mature females in fresh water.