Effect of oceanographic environments on sexual maturation, salinity tolerance, and vasotocin gene expression in homing chum salmon

Effects of the coastal oceanographic conditions on sexual maturation, salin ity tolerance, and expression of vasotocin (VT) gene in homing chum salmon, Oncorhynchus keta, were studied by four years of fieldwork and transfer ex periments. In fieldwork in 1992 and 1993, fish were sampled at three sites along their migratory pathway on the Sanriku coast, Japan. In transfer expe riments in 1994 and 1995, fish captured in the seawater (SW) environment we re transferred to SW or freshwater (FW) aquaria and sampled 1 to 4 days lat er. The distribution of cold and warm currents, which governs the oceanogra phic conditions of the Sanriku coast, were deduced from the mean sea surfac e temperature. Maturity of homing fish was estimated by gonadal states. Sal inity tolerance was estimated by plasma Na+ levels and mortality in SW envi ronment. Changes in VT gene expression were assessed by a quantitative dot blot analysis of the hypothalamic levels of VT mRNA. Homing fish were fully mature, and showed high plasma Na+ levels and high mortality in the SW env ironment in 1992 and 1994 when the warm current dominated. In the 1994 tran sfer experiment, VT mRNA levels markedly increased in the SW-retained males , whereas the levels were decreased by FW-transfer in both sexes. Homing fi sh were not fully matured in 1993 and 1995 when the branch of cold current reached the Sanriku coast. In the 1995 transfer experiment, VT mRNA levels did not change in both FW-retained and FW-transferred fish. In conclusion, the warmer oceanographic conditions affected the maturity and salinity tole rance in homing salmon, which in turn altered the osmotically-induced expre ssion pattern of VT gene.