Chum salmon fry grow faster in seawater, exhibit greater activity of the GH/IGF axis, higher Na+, K+-ATPase activity, and greater gill chloride cell development

Chum salmon (Oncorhynchus keta) fry were reared in freshwater (FW), mimicking the hatchery enhancement program, and in seawater (SW) to determine the effect on growth. The fry groups in SW showed exponential growth, whereas those in FW grew in a linear manner in two experiments over 49 and 70 days. Fry adapted to SW for 49 days survived well after returning to FW for 7 days. Gill Na+, K+-ATPase activity in the SW group was higher than that of the FW group. The chloride cells in gill filaments increased in number and developed larger in size in SW fry. Pituitary growth hormone (GH) mRNA levels increased 14 days after transfer to SW. The mRNA levels of GH receptor (GHR) and insulin-like growth factor I (IGF-I) in the liver and gill increased in the fry reared in SW.