DOI: 10.5061/dryad.gmsbcc2nj
title: Latitudinal cline in the foraging dichotomy of loggerhead sea turtles reveals the importance of East China Sea for priority conservation

description: 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;
 &gt;84% of the females nesting at southern sites (Ishigaki and
 Okinoerabu Islands), 73% at middle sites (Yakushima Island and Minabe),
 and &lt;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.
description: 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 &gt;1500, 500–1500,
 250–500, and &lt;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 (&gt;5 km
 h<sup>-1</sup>) were omitted.