Mass-stranding suggests natal area and migration of loggerhead turtle hatchlings in the Sea of Japan

Abstract

During the winter, from September 2012 to April 2013, 243 sea turtles were found along the Japanese coast of the Sea of Japan. Of these, 196 had been washed ashore, 44 were obtained as bycatch, and the remaining three came from an unknown source. The most notable finding was 119 post-hatchling loggerhead turtles, Caretta caretta, with carapace lengths of approximately 10 cm. Using mixed stock analysis, their natal regions were determined to be mainly areas in southern Japan, including the Okinawa Islands, Okinoerabu Island, and Yakushima Island. Furthermore, the post-hatchlings appear to have entered the Sea of Japan via the Tsushima Current from October to November 2012, which is when the post-hatchling bycatch increased, and 90 of the 107 post-hatchlings that washed ashore were found between December 2012 and January 2013. Small sea turtles, especially post-hatchlings, do not survive the low winter water temperatures in the Sea of Japan. However, two post-hatchlings were found in Mutsu Bay and eastern Hokkaido, indicating that post-hatchlings can exit the Sea of Japan to the Pacific Ocean via the Tsugaru Strait. Therefore, the dispersal of loggerhead post-hatchlings might not be entirely unsuccessful.