Latitudinal cline in the foraging dichotomy of loggerhead sea turtles reveals the importance of East China Sea for priority conservation

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.