Habitat use of loggerhead (Caretta caretta) and green (Chelonia mydas) turtles at the northern limit of their distribution range of the Northwest Pacific Ocean

Abstract

Verifying habitats, including the foraging and nesting areas for sea turtles, enables an understanding of their spatial ecology and successful planning of their conservation and management strategies. Recently, the observation frequency and bycatch of loggerhead (Caretta caretta) and green (Chelonia mydas) turtles have increased in the northern limit of their distribution range, in the northern part of the East China Sea and East (Japan) Sea. We conducted satellite tracking to investigate the habitat use of seven loggerhead and eight green turtles from June 2016 to August 2022 in this area, where little is known about their spatial ecology. We applied a 50 percent volume contour method to determine their main foraging areas and analyzed 6 environmental variables to characterize their habitats. Loggerhead turtles mainly stayed in and used the East China Sea as a foraging area during the tracking period, while two individuals among them also used the East Sea as a seasonal foraging area. Most green turtles also used the East China Sea as a foraging area, near South Korea and Japan, with one individual among them using the lower area of the East Sea as a seasonal foraging area. Notably, one green turtle traveled to Hainan Island in the South China Sea, a historical nesting area. Our results showed that the two sea turtle species included the East Sea as a seasonal foraging area, possibly owing to the abundance of food sources available, despite its relatively lower sea temperature. Considering that loggerhead and green sea turtles were observed using the northern part of the East China Sea and East Sea more frequently than previously known and that the sea temperature gradually increases due to climate change, conservation and management activities are required for sea turtles in these areas.

Identifying the source rookery of green turtles (Chelonia mydas) found in feeding grounds around the Korean Peninsula

“…First, as previously described, one haplotype (3 of 40 turtles, Cmp49.1) was found in populations across Japan, IP, CWP, and SWP MUs, and was the main haplotype in the IP clade ( Jensen et al, 2019 ). Second, previous satellite tracking studies have shown that green turtles in Korean waters have migrated to Hainan Island in the Indo-Pacific Ocean ( Kim et al, 2022 , 2024 ). The genetic diversity of green turtles (0.77) in the study area was similar to the reported genetic diversity (0.65 – 0.88) in seven Japanese feeding grounds ( Nishizawa et al, 2013 , 2014 ; Hamabata et al, 2015 ), showing that protecting the feeding population in the Republic of Korea could be meaningful to conserve genetic diversity of green turtles in northwestern Pacific Ocean.…”

“…The Kuroshio recirculation may first transport OGA turtles to the southeastern shore of mainland Japan ( Hurlburt et al, 1996 ) and then follow the coastline westward to reach the Jeju Sea. On the other hand, in our previous satellite tracking study, green turtles released in the Jeju Sea in the Republic of Korea used across close feeding grounds including the Jeju Sea, the East Sea, and the South Sea in the Republic of Korea as well as sea nearby Kyushu Island in Japan ( Kim et al, 2024 ). These areas are abundant in seaweed, which is food for green sea turtles ( Kim et al, 2021 ).…”

“…Juvenile and adult green turtles are observed at an average rate of 5–10 turtles per year. Ecological and satellite tracking studies have been conducted to gain insights into the physical attributes, habitat preferences, and movement and migration patterns of green turtles ( Jang et al, 2018 ; Kim et al, 2022 , 2024 ). Such studies have shown that green turtles actively use the sea around the Korean Peninsula and migrate to breeding grounds in both the Japan MU and the IP MU.…”

“…Local feeding populations of these turtles are jeopardized by a variety of issues, including fishing, construction, and pollution ( Moon et al, 2009 ; Kim et al, 2017 ). Understanding the source rookeries and MUs of green turtles, for which the Republic of Korea has implemented various government conservation efforts ( Kim et al, 2022 , 2024 ; Moon et al, 2022 ), is crucial for the long-term conservation of sea turtles. The inclusion of relevant studies was crucial and required immediate attention.…”