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

Abstract

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.

A validation of abstracted dive profiles relayed via the Argos satellite system: a case study of a loggerhead turtle

“…1). She stayed in the East China Sea for 2 years with seasonal migration; she stayed in the northern part of the East China Sea during summer and the southern part during winter [13]. Then, on May 23, 2018, she came back to her initial nesting place at Okinoerabu Island, Japan, for nesting.…”

“…We conducted a survey during the main nesting season (from the end of May to the middle of June) from 2015 to 2018. To examine the post-nesting foraging habitat of loggerhead females, satellite relay data loggers (SRDL, Sea Mammal Research Unit, UK) were deployed on eight nesting loggerhead turtles using epoxy putty, two-component epoxy resin, and glass fiber cloth (Konishi Co., Ltd. Osaka, Japan) after nesting [13]. A female turtle (straight carapace length of 91.5 cm) deployed with an SRDL and released at Wanjo Beach on June 27, 2016, was recaptured at the same beach on May 23, 2018.…”

Biological traits, geographic distributions, and species conservation in aquatic ecosystems

“…Both Thoya et al (2022) and Okuyama et al (2022) identified sensitivity of marine vertebrates to stressors like overfishing or bycatch based on physiological and functional (e.g., diet) traits. Thoya et al (2022) used physiological tolerances of billfish species in the Indian Ocean to develop hypotheses for species distribution models, which they then used to relate the distributions of the Indian Ocean billfish community to fishing effort and stock status.…”

“…Okuyama et al (2022) used both satellite tracking and stable isotope analysis of diets to characterize within‐population trait variability of loggerhead turtles ( Caretta caretta ) that can inform conservation and management of this species in the north Pacific Ocean. These authors found that loggerhead turtles that nest on beaches in Japan disperse to forage in either the Pacific Ocean (oceanic) or in the east China Sea (neritic) and that these habitat choices are highly associated with differences in traits like diet, body size, and reproductive output.…”

“…Oceanic loggerhead turtles are planktivores that achieve smaller body sizes and lower reproductive output than larger, neritic benthivores that disperse and forage in the east China Sea. Okuyama et al (2022) identify the east China Sea component of the loggerhead turtle population as a conservation priority given the high reproductive output of these turtles and possible vulnerability to bycatch in a complex, cross‐political jurisdiction ecosystem.…”