Environmental characteristics of sandy beaches at the northern limit of the breeding range of the loggerhead sea turtle (Caretta caretta)

Mizoroki, Manami

渡部, 明美

Watabe, Akemi

猿田, 勇

Saruta, Isamu

大木, 清

Oki, Kiyoshi

藤田, 健一郎

Fujita, Kenichiro

岩瀬, 環樹

Iwase, Tamaki

岩瀬, レン有世

Iwase, Ariyo Ren

三次, 恵美子

Mitsugi, Emiko

平, 治隆

Taira, Harutaka

吉村, 智範

Yoshimura, Tomonori

吉田, 正人

Yoshida, Masahito

佐伯, いく代

Saeki, Ikuyo

The Ecological Society of Japan

気候変動

絶滅危惧種

繁殖

保全

climate change

conservation

marine organism

reproduction

threatened species

The loggerhead sea turtle (Caretta caretta) breeds in temperate zones, and females are thought to return to their natal beach to nest. The northern limit of the C. caretta breeding range encompasses sandy beaches on the Pacific coast in Fukushima, Ibaraki, and Chiba prefectures, Japan, where nesting is frequently observed, although less commonly than in other parts of its range. Recently, the loggerhead sea turtle population has been declining due to bycatch and habitat degradation. The species is listed as endangered (EN) on the national Red List of Japan, and the importance of these breeding sites may further increase under global warming. However, our knowledge of these northernmost breeding environments remains limited. Therefore, we investigated the ecological factors influencing the nesting, hatching, and emergence of C. caretta on sandy beaches at the northern limit of its breeding range. We selected 12 sites for field surveys, focusing on those where nesting had been observed, and recorded the size and geomorphological features of the beach, vegetation cover, sand temperature, sand particle composition, presence of artificial structures, and human activity. The average width, length, and slope of the 12beaches were 4004m, 38m, and 2.9°, respectively. The average vegetation cover was approximately 62%. Using the NASA Sea Level Projection Tool, we predicted that the beaches would decrease in area by approximately 24% by 2100 under the most advanced warming scenario (SSP5-8.5). Sand temperatures at 9 of the 12 study sites were measured during the summer and autumn of 2023; temperatures above 32°C, which are thought to reduce hatching success, were recorded at six sites. Based on sand temperature data from 16 nests at three sites where nesting events were observed during the study period, we predicted a hatchling primary sex ratio of >90% (maximum, 99.99%) female for 12 nests. Measured sand hardness was soft at all sites but one, and appeared suitable for C. caretta breeding. Artificial structures and artificial light sources were present at all study sites, and both recreational swimming and cars or heavy machinery were recorded at >80% of all sites. Our results reveal that sandy beach environments at the northern limit of the C. caretta breeding range are exposed to risks related to decreased habitat due to sealevel rise, increased sand temperatures due to global warming, and degradation of the nesting environment due to human activities. To minimise these risks, it is important to conduct monitoring surveys and conservation activities at the breeding sites. Mitigation of human impacts on potential breeding areas is also encouraged for beaches with intensive human activity.

渡部, 明美

猿田, 勇

大木, 清

藤田, 健一郎

岩瀬, 環樹

岩瀬, レン有世

三次, 恵美子

平, 治隆

吉村, 智範

吉田, 正人

佐伯, いく代