Dependence of Hainan medaka, Oryzias curvinotus (Nichols & Pope, 1927), on salinity in the Tien Yen estuary of northern Vietnam

Abstract

Species of the genus Oryzias are models for studies of adaptation to osmotic environments because they demonstrate inter-species adaptability. Oryzias curvinotus , a fresh and brackish water species, is distributed from the coastal waters of southern China to northern Vietnam. The correlation of O. curvinotus ’ distribution with physical water conditions in the Tien Yen estuary of northern Vietnam was recorded during monthly collections for one year. For the current study, a total of 2916 individuals of O. curvinotus were caught, of which 942 were selected to be measured and have their developmental stage recorded: 41 larvae, 749 post-larvae and 152 juveniles or adults. A developmental description and an identification to the species level are also provided. Temperature, salinity and turbidity at collection sites ranged from ca. 17 to 34°C, from ca. 0 to 16 psu and from 0 to 524 NTU, respectively. A peak catch per unit effort was found in April and at the stations at which salinity was about 0.6 to 2.5 psu. In stations where salinity was greater than 17 psu or at 0 psu, few specimens were collected. Examination of distribution indicated a tendency of the species to be present along the estuary, with the center part as the main habitat of this species. The function of an estuarine environment for fishes, which inhabit fresh and brackish waters, is discussed.