Effect of low salinity on the expression profile of Na⁺/K⁺-ATPase and the growth of juvenile chub mackerel (Scomber japonicus Houttuyn)

Chub mackerel (Scomber japonicus Houttuyn) is one of the most commercially important scombroid fish used as a food resource. Recently, there has been a demand for efficient rearing methods of this fish for a full-life cycle aquaculture. In the present study, we evaluated the physiological responses in the juvenile S. japonicus to different ambient salinities. A significantly higher gain of the body mass was observed in the juveniles reared in 24 g/L and 13 g/L seawater than in those reared in natural seawater (34 g/L) within 40 days of the experimental period without affecting mortality. A principal enzyme for osmoregulation, Na⁺/K⁺-ATPase, was expressed in the ionocytes located in the gill filaments of the juveniles. The number and the cell size of ionocytes and the enzymatic activity of Na⁺/K⁺-ATPase in the gills decreased within 10 days after the low-salinity challenge, which implies the reduction of the energy-consuming active ion secretion under the low-salinity environment. The physiological capacity for adaptation to low-salinity seawater in chub mackerel could be basic knowledge to carry out culturing of these fish in coastal sea pens where ambient salinity fluctuates. The improvement of the growth performance by rearing in low-salinity seawater will contribute to the efficient production of the seed juveniles for aquaculture.