Size-related differences in feeding habitat use of adult female loggerhead turtles Caretta caretta around Japan determined by stable isotope analyses and satellite telemetry

Stable isotope analyses and satellite telemetry were used to investigate the relationship between the body size and feeding habitat of adult female loggerhead turtles Caretta caretta around Japan. A total of 149 females from 2 nesting sites, 600 km apart, 1 in Minabe and 1 in Yakushima, was examined by stable isotope analyses of the egg-yolk in 1998 and 1999. Since there were neither significant intra- nor inter-clutch variations in delta super(13)C or delta super(15)N egg-yolks from the same female, the isotope signatures of a single egg-yolk from any clutch in a nesting season could be used to represent those of its nesting female. Both isotopic values increased as the body size of the nesting female increased, irrespective of nesting site. Comparisons between isotopic values of loggerhead egg-yolks and prey items revealed size-related differences among both feeding habits (planktonic or benthic) and habitat areas (pelagic or neritic) of female loggerheads. In 1999, 5 females nesting in Minabe were tracked by satellite. After the nesting season, 2 females with low isotopic values migrated to the pelagic Pacific along the Kuroshio Current, whereas 3 with high isotopic values migrated to the neritic East China Sea. The former females were smaller than the latter. These results were consistent with inferences from stable isotope analyses and previous findings, indicating that the body size, habitat area, and feeding habit of female loggerheads around Japan are closely related. In addition, there were no significant differences in delta super(13)C or delta super(15)N between recruits and remigrants, implying that female loggerheads do not change their feeding habitats during the reproductive stage. We postulate that female habitat selection as a function of body size is closely related to the recruitment and settlement in immature loggerheads, which return to Japan after developmental migrations in the North Pacific.