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We investigated how trophic polymorphism observed at a loggerhead sea turtle (Caretta caretta) rookery affected the interannual variability of clutch frequency (the number of clutches laid by a female during a nesting season) and of the number of total located nests, based on comprehensive nighttime and daytime field surveys conducted at Yakushima Island, Japan, over a non‐consecutive eight‐year period. Taking into account the fact that lower trophic‐level sea turtles show greater variability in these parameters, we anticipated clutch frequency and the number of total located nests to vary more in low trophic‐level oceanic planktivores (secondary consumers) than in high trophic‐level neritic benthivores (tertiary consumers). Oceanic planktivores and neritic benthivores were classified by stable isotope ratios in egg yolks. Neritic foragers had higher clutch frequencies than oceanic foragers in any year. Contrary to our anticipation, however, both types of foragers showed similarly low interannual variability in clutch frequencies. No significant correlations were found between mean clutch frequencies and the estimated number of nests made by either oceanic or neritic foragers and the number of total located nests. Interannual fluctuations in the total number of nests made by loggerhead turtles, whether trophically monomorphic or polymorphic, seem to be driven more by the number of females than by clutch frequency; this suggests that the number of total located nests can be a good proxy for the population size of iteroparous animals at relatively high trophic levels. [ABSTRACT FROM AUTHOR]

Intrapopulational polymorphism in habitat use is widely reported in many animal species. The phenomenon has recently also been recognized in adult female loggerhead sea turtles Caretta caretta, with small females tending to inhabit oceanic areas (where water depths are >200 m) while presumably feeding pelagically and large females tending to inhabit neritic areas (where depths are <200 m) while presumably feeding benthically. In this study, dive recording satellite telemetry units were used to verify their foraging and diving behaviours in these habitats. Two females that nested on Yakushima Island, Japan, were tracked for 124 and 197 days. The small female wandered in the oceanic Pacific, and spent most of the time at 0–25 m depths regardless of day or night, implying that she foraged pelagically at the surface and shallow depths. Her mean dive durations were significantly longer at night than during the day. The large female moved into the neritic East China Sea, and spent most of the time over the continental shelf at 100–150 m depths during the day and at 0–25 m depths at night, suggesting that she alternated between diurnal benthic foraging and nocturnal resting within the depths where she could attain neutral buoyancy. Her mean dive durations were not significantly different between day and night. The increase in dive duration for both turtles coincided with a seasonal decrease in water temperature. The small female sometimes showed midwater dormancy at 0–25 m depths with a duration of >5 h that was in contrast with bottom dormancy by sea turtles inhabiting other regions. The diving behaviours observed during this study were consistent with their estimated main feeding habits, which demonstrated resource polymorphism in a marine reptile. [ABSTRACT FROM AUTHOR]