Assessment of feeding grounds for loggerhead and green turtles in Korean water through stranding and bycatch data

Understanding occurrence patterns and the quantitative size of populations is essential to protect sea turtles. Here, we analyzed stranding and bycatch data to investigate the habitat use patterns of loggerhead ( Caretta caretta ) and green turtles ( Chelonia mydas ) in the seas around the Korean Peninsula, located at the northern range limit of the species in the northwestern Pacific. Depending on data from 116 loggerhead and 106 green turtles collected between 2017 and 2023, their regional and seasonal use patterns were analyzed using sea surface temperature, sea surface salinity, chlorophyll-a, and particulate organic carbon. Additionally, the climate simulation model (CNRM-ESM2–1) was used to assess the changes in the habitat suitability of Korean water under increasing sea temperature scenarios. Both sea turtle species seasonally used the East Sea and Jeju Sea as their main feeding grounds in Korean waters during summer and fall. This could be attributed to an oceanic front, which provides an abundant food source, formed in the East Sea when in contact with the North Korea Cold Current and the East Korea Warm Current. However, sea turtle occurrence decreased from early winter, when the sea temperature dropped below 15 °C, and only a few green turtles were found in the Jeju Sea during winter and early spring. The climate simulation model revealed that sea turtles will likely use Korean waters more frequently in the 2050s and 2100 s following continuously increasing sea temperatures. Our findings suggest that immediate management actions are necessary to conserve sea turtles and prevent additional damage to them in Korean water.