Summer-restricted migration of green turtles Chelonia mydas to a temperate habitat of the northwest Pacific Ocean

The foraging habitats of green turtles Chelonia mydas range from tropical to temperate areas. Previous studies have generally been biased toward tropical and sub-tropical areas; hence, available data do not accurately describe the species' foraging activity in temperate areas. To reveal seasonal patterns of habitat use in temperate areas, we conducted a by-catch survey, a mark-recapture study, and satellite tracking of green turtles along the Sanriku Coast, a temperate zone in the northwest Pacific Ocean. From July through November of 2005 to 2014, 78 green turtles were captured during a period of relatively high water temperatures (16 to 24°C). Straight carapace length (SCL) ranged from 36.8 to 85.6 cm (average: 49.4 ± 11.4 cm; n = 78), indicating that most of the turtles were juveniles. In the mark-recapture study, 14 of 72 tagged turtles were recaptured 5 to 426 d after release, 12 of which were recaptured south of the release point. Based on satellite tracking data, 3 turtles travelled more than 500 km to reach southern habitats, where water temperature was warmer (13 to 25°C) than along the Sanriku Coast (4 to 22°C). Our results revealed that the Sanriku Coast is a seasonally restricted habitat for juvenile green turtles, which migrate to southern habitats in winter, and that turtles in temperate areas migrated longer than those in tropical and sub-tropical areas. This is the first report of seasonal migration of juvenile green turtles to a temperate habitat in the northern Pacific Ocean.