This project addresses major gaps in knowledge on vital rates such as age to maturity, survival, sex ratios, and population size (including the males)whcih have made it difficult to conduct meaningful population and risk assessments. Although vital rates are difficult to observe directly, genetic analysis provides a practical approach to understand these processes. Understanding the proportion of males to females in any population has important consequences for population demographic studies. Using hatchling and maternal DNA fingerprints, one can deduce the paternal genotypes ? from one to many fathers per clutch. The resulting genotypes represent individual males that are actively breeding in the population. This means that males can effectively be sampled without ever having seen them or having to catch them in the field. The nesting population on St. Croix is an important US Index Population for leatherbacks that has been intensively monitored using a variety of Capture-Mark-Recapture (CMR) methods since 1981 (Dutton et al. 2005). Due to the richness and consistency of the demographic data, this population offers unique opportunities for research and development of tools & approaches for getting at vital rate parameters that are needed to improve stock assessments in sea turtles, as identified in the recent NRC Report (2010). These approaches can then be applied to other populations, e.g. the critically endangered Pacific leatherback. We have developed non-injurious in-situ techniques to mass sample large numbers of live hatchlings for genetic fingerprinting as part of a long term CMR experiment, and also demonstrated the feasibility of using hatchling genotyping and kinship analysis to determine the genotypes and number of breeding males in the population (Stewart & Dutton 2011). We have sampled a total of 17,087 hatchlings between 2009-2011 as part of this project, will continue field effort in 2012 toward the goal of a minimum sampling of 50,000 hatchlings over the next 2-4 years. At an appropriate time in the future, we will use high throughput genotyping methods currently being developed in the next 2-4 years to create a database of individual hatchling identifications (?genetic tags?) that will be compared to those first time nesters sampled annually into the future. This project will also genotype a subset of the samples collected in 2011 to assess males in two consecutive seasons for a more accurate census of the number of males in the breeding population and to determine the extent of male fidelity and breeding periodicity. Objectives include 1) mass-tagging of leatherback hatchlings for Capture-Mark-Recapture (CMR) studies to determine age at first reproduction and age-specific survival rates and 2) application of kinship approaches to reconstruct parental genotypes from mother-offspring comparison to census males, determine operational sex ratios (OSR) of the breeding population, reproductive success of males and mating system.