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

To 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 (r2 < 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.