Aim: Quantifying the importance of habitat areas for conservation of highly migratory marine species with complex life histories can be challenging. For example, loggerhead turtles (Caretta caretta) nesting in Japan forage both oceanically and neritically after their reproductive period. Here, we aimed to quantify the proportions of turtles using these two contrasting habitats (foraging dichotomy) to suggest priority conservation areas.

Methods: We examined the occurrence of foraging dichotomy at three nesting sites (Ishigaki, Okinoerabu Islands, and Ichinomiya) based on stable isotope analysis of the egg yolks for 82 turtles and satellite tracking of post-nesting migration for 12 turtles. Moreover, we used the data of three other sites from previous studies (Yakushima Island, Minabe, and Omaezaki).

Results: Two neritic foraging grounds (East China Sea and the coastal area of the Japanese archipelago), and an oceanic ground (North Pacific Ocean) were identified. We found a latitudinal cline with respect to the occurrence of foraging dichotomy; >84% of the females nesting at southern sites (Ishigaki and Okinoerabu Islands), 73% at middle sites (Yakushima Island and Minabe), and <46% at northern sites (Omaezaki and Ichinomiya) were neritic foragers; the proportion of oceanic foragers increased at northern sites. Based on the annual number of nests in the entire nesting region of Japan, satellite tracking, and the latitudinal cline of foraging dichotomy, we estimated that 70% and 9% of annual nesting females in Japan utilise the neritic foraging habitat in the East China Sea and the coastal area of the Japanese archipelago, respectively and that and 22% utilise the oceanic habitat of the North Pacific Ocean.

Main conclusions: The East China Sea represents a critical foraging habitat for the North Pacific populations of endangered loggerhead sea turtles. Our findings emphasise the need for international management to ensure their protection.

Isotope data

We obtained egg samples from a total of 82 loggerhead turtles nesting at three sites: Ishigaki Island, Okinoerabu Island and Ichinomiya, Japan. All whole egg samples were frozen at −20 °C until analytical preparation. Following this, the egg yolk samples were dried at 60 °C for 72–96 h. Lipids were removed using a chloroform-methanol (2:1) solution and then ground to a fine powder. The carbon and nitrogen stable isotope ratios in the egg yolks were measured. Isotope ratios in lipid-free powdered yolks (~2 mg) were determined at the Atmosphere and Ocean Research Institute, University of Tokyo, using a mass spectrometer (IsoPrime100, Elementar, Germany) interfaced with an elemental analyser (Vario Micro Cube, Elementar, Germany). The δ13C and δ15N were expressed as deviations from the standard (Vienna Pee Dee Belemnite and N2 in air for δ13C and δ15N, respectively), as defined by the following equation: δ13C or δ15N = (Rsample/Rstandard −1) ×1000(‰), where R is 13C/12C or 15N/14N. The isotopic composition was calibrated against a commercial standard (L-Alanine AZ101-SS13, δ13C = −19.6‰ and δ15N = 13.7‰, Shoko Science, Japan). The analytical precision was estimated to be 0.2‰ for both δ13C and δ15N based on repeated analyses (n = 5) of in-house standard (mussel tissue standard for trace elements, SRM 2976, National Institute of Standards and Technology, USA) of which CN ratio is similar to egg yolk.

Satellite tracking data

To draw the migration trajectories, we used the location data with Argos location classes (B, A, 0, 1, 2, 3). The first two classes have no accuracy information assigned by Argos, and the remaining classes have reported accuracies of >1500, 500–1500, 250–500, and <250 m, respectively. However, accuracy has been measured on marine mammals at 10.3 and 6.2 km for class B and A and 4.2, 1.2, 1.0 and 0.49 km, respectively, for the remaining classes. Additionally, location data with excessive speed (>5 km h^-1) were omitted.