Movement Patterns of Juvenile Loggerhead Turtles (Caretta caretta L. 1758) and Green Turtles (Chelonia mydas L. 1758) Hatched in Captivity and Released in the Korean Waters

Abstract

With most sea turtle populations declining, activities to conserve their habitat and nesting grounds and restore their populations are being implemented worldwide. To preserve the Northwestern Pacific populations, the National Marine Biodiversity Institute of Korea has been releasing artificially propagated sea turtles, but whether these individuals join the wild population remains unknown. The present study aimed to determine the movement patterns of artificially propagated juvenile loggerhead (Caretta caretta) and green (Chelonia mydas) turtles fitted with satellite transmitters on their carapaces and released in the waters of Jeju or Yeosu, Republic of Korea, between August 2018 and April 2022. Loggerheads traveled northward to the East Sea, whereas green turtles moved west or southwest. Two 36-month-old and two 48-month-old loggerheads moved toward their potential nursery grounds and toward their feeding grounds, respectively. Three green turtles with a curved carapace length (CCL) of <40 cm moved toward their nursery or feeding grounds, while three individuals (CCL > 45 cm) moved toward their inshore foraging areas. The travel paths were closely related to the direction of local sea currents. Our results implied that releasing artificially propagated sea turtles, considering their age and CCL, can positively contribute to the conservation of Northwestern Pacific populations.

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

“…To track sea turtles, we used Argos satellite transmitters (Wildlife Computers Inc., Redmond, WA, USA), SPOT-352A (before September 2019, lifespan: approximately 920 days; 72 × 56 × 22 mm; 129 g), and SPOT-375A (after August 2021, lifespan: 915 days; 99 × 55 × 21 mm; 152 g). The Argos transmitter has been widely used for satellite tracking of sea turtles until recently [ 15 , 16 , 22 , 44 ], although it could overestimate space use of sea turtles compared to more accurate approaches such as Fastloc-GPS [ 45 , 46 ]. We attached transmitters to the carapaces of each sea turtle using Aqua Mend Epoxy (PSI-Polymer Systems, NC, USA) and instant mix epoxy (Loctite, CT, USA).…”

“…We only used the higher accuracy data for a given day among the recorded daily points, excluding data in LC A and B. We excluded location data on land without a distinct signal from the transmitter or long-distance travel of over 100 km in a single day, which is impossible for sea turtles to travel [ 15 , 16 ]. Although LC 0, which has lower location quality, could overestimate home range size, the reduction in the number of location points due to the exclusion of LC 0 obscures the understanding the space use patterns of sea turtles [ 46 ].…”

“…To define the home range size of individual sea turtles during the tracking periods, we used the minimum convex polygon (MCP) and 95 and 50 percentage volume contours (PVC) based on kernel density estimation, often used to calculate the home range of sea turtles [ 14 , 16 , 41 , 50 ]. The MCP and 95 PVC represent general home range sizes, while the 50 PVC indicates the core area of each individual.…”

“…Despite the increased observation of loggerhead and green turtles at these distribution margins, studies on their habitat use are lacking. Several studies have shown that juvenile loggerhead turtles use the East Sea near South Korea and Japan as a route to the open Pacific Ocean, but few studies have been conducted on adult specimens [ 5 , 16 , 40 ]. In green turtles, some research showed that adults use the sea near Jeju Island [ 41 ], whereas juveniles use the Yellow Sea west of the Korean Peninsula [ 16 ].…”

“…Several studies have shown that juvenile loggerhead turtles use the East Sea near South Korea and Japan as a route to the open Pacific Ocean, but few studies have been conducted on adult specimens [ 5 , 16 , 40 ]. In green turtles, some research showed that adults use the sea near Jeju Island [ 41 ], whereas juveniles use the Yellow Sea west of the Korean Peninsula [ 16 ]. However, the detailed patterns of habitat use by adults have not been studied yet.…”