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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. [ABSTRACT FROM AUTHOR]

In recent years, kisspeptin peptides, encoded by kiss genes have been used to manipulate reproductive processes in farmed animals, including fish. Our previous studies demonstrated that the chub mackerel brain expresses kiss1 and kiss2 and intramuscular injection of synthetic Kiss1 pentadecapeptide (Kiss1-15) but not Kiss2 dodecapeptide (Kiss2-12) accelerates spermatogenesis in prepubertal male chub mackerel ( Scomber japonicus). In the present study, we evaluated their effects in prepubertal female chub mackerel. The gonadosomatic index ( GSI) values of experimental fish did not show any significant changes. Condition factor ( CF) values increased significantly in Kiss1-15 treated fish, in comparison with control and Gn RH analogue (Gn RHa) injected fish. Histologically, only perinucleolar oocytes were found in all experimental fish. However, Kiss and Gn RHa treated fish showed a significant increase in the perinucleolar oocyte diameter, in comparison with the control fish. Gene expression analyses revealed decreased expression of gnrh1 in the telencephalon-preoptic region of the brain of Kiss2-12 and Gn RHa injected fish, in comparison with control fish. In contrast, Gn RHa injected fish exhibited higher levels of fshβ in the pituitary, with no changes in the levels of lhβ among different treatments. Levels of circulating sex steroids, testosterone, and estradiol-17β were significantly higher in Kiss1-15 injected fish, in comparison with control fish. These results indicate that synthetic kisspeptin peptides and Gn RHa can induce oocyte growth in prepubertal female chub mackerel. [ABSTRACT FROM AUTHOR]