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.Location: North Pacific Ocean. 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 utilize the neritic foraging habitat in the East China Sea and the coastal area of the Japanese archipelago, respectively, and that and 22% utilize 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 emphasize the need for international management to ensure their protection.

Satellite telemetry devices can record movement data of animals along with the environmental data. Such data are relayed remotely via satellite systems, but are constrained by the limited bandwidth availability. A satellite relay data logger (SRDL) that can abstract dive profiles and compress the data for transmission using a broken stick model (BSM) has been widely used in studies on dive behavior and physiology of marine animals. However, there is still uncertainty in the abstracted dive profiles. Here, we aimed to evaluate the certainty of abstracted dive profiles (via satellite communication) in terms of dive performance (dive depth, duration, and dive type) by comparing it with the actual dive data (from the retrieved tag) in a loggerhead turtle deployed with the SRDL throughout a 1.4-year foraging period. There was no significant difference in the maximum dive depth between the retrieved and satellite transmission data; however, there was a slight but significant difference in the dive duration. The dives from both datasets were classified into five types. Inconsistent dive classifications occurred in 1.7% of the data. There was no significant difference in the proportion of time spent diving between the retrieved and satellite transmission data for each type during the common recording period. In monthly scale comparisons, however, a significant difference was detected when the amount of data via satellite transmission was the smallest. Our results demonstrated that the dive data abstracted using BSM almost reconstructed the actual dive profiles with certainty in a loggerhead turtle, although slight inconsistencies were observed.

Life-history polymorphism in habitat use has been widely reported in aquatic migratory animals. Adult loggerhead sea turtles Caretta caretta in the North Pacific exhibit foraging-habitat polymorphism, showing a 2.4-fold greater reproductive output for neritic foragers than for oceanic ones. However, knowledge of how turtles in each habitat behave and what effect this has on reproduction remains limited. We combined horizontal movement and dive behavior analyses to explore foraging behavior and habitat utilization in adult female loggerheads in the North Pacific. Between 2016 and 2018, satellite relay data loggers were attached to 10 females nesting in Japan. The turtles were tracked for 219.2 ± 185.0 d (±SD) within the range of 10 to 521 d. Three foraging destinations were revealed, with different foraging tactics in each habitat: (1) the neritic East China Sea (ECS), (2) neritic coastal areas of mainland Japan (CMJ), and (3) oceanic North Pacific Ocean (NPO). ECS females mainly foraged on benthic prey with foraging-site fidelity; however, some turtles alternately preyed on benthos and plankton along with their seasonal migration. CMJ females also foraged on benthic prey with site fidelity, whereas NPO females foraged on plankton by drifting eastward with the oceanic currents. Dive-type analyses revealed that the proportion of time spent presumably foraging was similar among the 3 habitats, suggesting that the nutrient richness of prey items and the migration distance between foraging and breeding grounds may cause differences in reproductive outputs among different life-history types of loggerhead turtles.

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