The first record of the appearance of green turtles in the winter in a summer-to-autumn habitat in northeast Japan

Recent increases in seawater temperature have been predicted to induce a poleward shift in the distribution of marine organisms. This study reports the first record of the winter appearance of green turtles (Chelonia mydas) in a habitat restricted to summer and autumn in northeast Japan. One individual was observed by a diver at Tomari-hama on the Oshika Peninsula (38°21′ N, 141°31′ E) on 28 December 2023. Another individual was incidentally captured in a fixed net near Tashiro-jima on the Oshika Peninsula (38°17′ N, 141°24′ E) on 18 April 2024 with a straight carapace length of 41.5 cm and body mass of 6.4 kg. Because previous bycatch surveys show that the earliest and latest appearances of green turtles in this area occurred in late June and late November, respectively, the occurrences reported in this study were two months earlier and one month later. The sea surface temperatures at the time of these observations were 15.9 and 16.0°C, respectively, which are 3.5 and 6.6°C higher than those of normal years. The unexpected winter appearance of green turtles implies an expansion of the habitable period for this species in response to climate change, and it, therefore, is emphasizes the need for continued monitoring surveys to collect additional sightings.