Overwintering and migration of sea turtles in Jeju Island of Korea: lessons from “SEAturtle” PICES special research project (2019-2023)

PICES special research project “SEAturtle” launched in 2019 to understand the ecology of sea turtles around Jeju Island in relation to environmental stressors. Though COVID 19 had interrupted the project, we had quite a successful outcome over the last 5 years. Until now (June 15, 2023), a total of 16 iridium transmitters were deployed on sea turtles (14 on green sea turtles and 2 on loggerhead sea turtles). Among them, we received the signals successfully from 15 sea turtles. We found that quite a proportion of green sea turtles released in Jeju Island (N = 4 out of 12, approx. 40%) overwintered nearby even in the cold sea where the temperature dropped to 15 °C. The diving duration increased to approx. 6 hrs with decreasing temperature. Most of migrating green sea turtles (N = 4) traveled toward southern Japan which suggests a strong link to the population in Japan. Our population genetics result on green sea turtles stranded suggests that a subunit of Jeju population also have an affinity to Japan population. On the other hand, one of our loggerhead sea turtles moved westward but the other moved southward from Jeju Island, suggesting that they may also have connectivity to both Japan and China. Our populations genetics and stable isotope analysis on the commensal barnacles support this. We also have actively worked on the threat of plastics on Jeju populations and found that derelict recreational fishing gears might cause more serious problems than commercial derelict fishing gears. Microplastics are other threats to them too. To conserve the population of sea turtles in Jeju Island, we need further extensive research and should keep up international cooperation.