Trophically polymorphic loggerhead sea turtles show similar interannual variability in clutch frequencies: implications for estimating population size of iteroparous animals

Abstract

AbstractWe 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.