Estimating clutch frequency in the sea turtle Dermochelys coriacea using stopover duration

When direct counts are not possible, relative population size indices, based on animal activity (e.g. number of tracks, burrows, feces, calls per unit area), are sometimes the only tools available to investigate population size variations over time. In sea turtles, the number of clutches laid on a nesting beach in a particular season is commonly used as a relative index of population density. The factor of proportionality that links the relative index and population density is the number of clutches annually laid per female per season or clutch frequency. At Awa:la-Ya:lima:po Beach, French Guiana (South America), we estimated clutch frequency in the sea turtle Dermochelys coriacea using capture-recapture monitoring over an 8 yr period (1995 to 2003, 1997 excluded). We compared 2 methods: (1) Estimated Clutch Frequency (ECF), where Observed Clutch Frequency (OCF) was corrected by taking account for intermediate missed nests, and (2) Total Clutch Frequency (TCF) derived from recent advances in capture-recapture methodologies that were initially developed to estimate stopover duration in migratory animals. Compared to the first method, the latter enabled estimation of the time spent before first capture and after the last capture, as well as the capture probability. Mean annual clutch frequency derived from stopover duration was considerably higher than ECF (respectively 2.38 and 7.01 clutches female–1 yr–1). In nesting sites where survey effort is low and variable over years, using capture-recapture data analysis might enhance the reliability of conclusions about population size in marine turtles.