Ontogenetic habitat shifts of green turtles (Chelonia mydas) suggested by the size modality in foraging aggregations along the coasts of the western Japanese main islands

Abstract

a b s t r a c tTo understand the life histories and ontogenetic habitat utilization of green turtles along the coasts of the western Japanese main islands, we collected size frequency and genetic data of green turtles captured by pound nets in three foraging grounds (FG): Nomaike (n = 38), Muroto (n = 93), and Kumano-nada (n = 31), and compared their natal origins among different size classes. Population genetic analyses based on an 820-bp fragment of mitochondrial DNA showed that the three FG were part of a single multiple-coast FG. Although turtles from all size classes originated mainly from rookeries in the Ogasawara Group, the size distributions clearly exhibited bimodality, with low occurrences of turtles in the 50-70-cm straight carapace length (SCL) range. The bimodal size distributions could not be attributed to demographic shifts in rookeries, because the number of female green turtles in Ogasawara has exhibited an increasing trend since 1979. We also examined whether factors such as seasonality and predation risk could have caused the size bimodality. There were, however, no strong relationships between sea-surface temperatures when turtles were captured and the sizes of the turtles (r 2 b 0.2), and it appeared that predation risk could not result in the size modality observed in the FG. Our results strongly suggest that after switching from a pelagic to a neritic lifestyle, the green turtles in the neritic FG along the western Japanese main islands undergo another ontogenetic habitat shift upon reaching~50-cm SCL. Here, we explore the possibility that developmental growth might stimulate a habitat shift, resulting in habitat differentiations by size and growth phase in the long-lived green turtle.

Dietary analysis of two sympatric marine turtle species in the eastern Mediterranean

“…If this is the case, then this is of great concern, and could represent a wider problem in the region. Alternatively, the observed absence of mid-size green turtles could reflect developmental transience as has been found in other populations (Hamabata et al 2015). Small juveniles may recruit to neritic developmental grounds, to later migrate to other coastlines as mid-size juveniles for the final stages of development prior to adulthood such as the wellknown adult foraging grounds off the coasts of North Africa and Turkey (Stokes et al 2015).…”

Overview of the population genetics and connectivity of sea turtles in the East Asia Region and their conservation implications

“…Foraging green turtles in the South China Region and Ryukyu Islands of Japan shared similar natal rookeries in Southeast Asia, Yaeyama Islands of Japan, Micronesia and the Marshall Islands in the West Pacific (Nishizawa et al, 2013;Ng et al, 2017;Hamabata et al, 2018). Several studies confirmed that northern foraging green turtles in mainland Japan were primarily contributed by Japanese rookeries in the Ogasawara and Ryukyu Islands (Nishizawa et al, 2013;Nishizawa et al, 2014b;Hamabata et al, 2015, Hamabata et al, 2016, Hamabata et al, 2018. Whereas the southern foraging aggregations around the Yaeyama and Ryukyu Islands were sourced from the Yaeyama and Ogasawara Islands, and various Pacific rookeries in the West Pacific and Indian Oceans and Southeast Asia (Nishizawa et al, 2013;Hamabata et al, 2018).…”

Latest and comprehensive mitochondrial DNA haplotype data on green and hawksbill turtles collected in their habitats in Japan

“…Haplotype data associated with nesting populations were collected from Yaeyama (Nishizawa et al, 2011), the central Ryukyus (Hamabata et al, 2009(Hamabata et al, , 2014, and Ogasawara Islands (Nishizawa et al, 2013;Hamabata et al, 2020). Haplotype data for the foraging aggregations were collected from Yaeyama (Nishizawa et al, 2013;Hamabata et al, 2023), Okinawajima (Nishizawa et al, 2013;Hayashi and Nishizawa, 2015;Hamabata et al, 2018), Nomaike, Muroto, Kumano-nada (Hamabata et al, 2015), Oita (Kudo et al, 2021), Kanto (Nishizawa et al, 2013), and Sanriku (Nishizawa et al, 2014). We expanded the existing haplotype dataset by including new samples from nesting sites and sites where turtles were present while foraging or in water.…”