Stock composition of green turtles Chelonia mydas foraging in the Ryukyu Archipelago differs with size class

The size composition of green turtles Chelonia mydas in their foraging grounds (FGs) in Japanese waters of the northwestern Pacific Ocean has suggested that turtles shift their FGs according to their size. To elucidate the size-related movements among FGs, we examined whether the origin of turtles differed depending on their size at 2 FGs in the Ryukyu Archipelago by performing mixed stock analysis using a 380 bp fragment of the mitochondrial DNA. At both sites, the Japanese stock primarily contributed to the 50–70 cm straight carapace length (SCL) size class, while the contribution of the southern stock from Micronesia and the Marshall Islands was higher in turtles < 50 and > 70 cm SCL. Since the above size-related differences in stock contribution were maintained for at least a decade, they were not attributable to the population trends in the Japanese stock. The higher contribution of the Japanese stock to the 50–70 cm SCL class suggests the possibility of juvenile natal homing by turtles from the Japanese stock, possibly from the main islands of Japan. The shift in the contribution of different stocks of origin to turtles > 70 cm SCL might have resulted from the movements of turtles to settle in their adult FGs. Additionally, the present results revealed that the Ryukyus are situated at the northern limit of the FGs for turtles from southern stocks. We hypothesize that this boundary may be created by a lack of adaptation of turtles from southern stocks to northern seasonal water temperatures.