Sex ratios of olive ridley sea turtles in the North Pacific high seas: implications for climate change research

Size-class distributions and sex ratio data provide critical information to assess the demography and reproductive potential of animal populations, such as sea turtles. Sea turtle sex is determined by incubation temperature, whereby warmer temperatures during a certain period of embryonic development produce more female hatchlings. Whereas hatchling sex ratios have been well-studied, sex ratios of sea turtle foraging aggregations are less known for most populations. Here we report on sex ratios of immature and mature olive ridley sea turtles Lepidochelys olivacea in the Eastern Tropical Pacific (ETP) and Central North Pacific (CNP) based on blood plasma hormone analysis, refined with Bayesian modeling, or gonad examination. Our findings established that (1) the commercial enzyme-linked immunosorbent assay used in the present study was appropriate to analyze testosterone concentration in olive ridley blood plasma to determine foraging ground sex ratios (via a Bayesian model); (2) size-at-maturity is generally larger in males than females in the ETP; (3) the overall sex ratio among all turtles was 1.2F:1.0M; and (4) the sex ratio of smaller-sized immature turtles from both study regions was female-biased (ETP, 1.6F:1.0M and CNP, 2.1F:1.0M). These are the first sex ratio estimates for olive ridleys foraging in the high seas of the North Pacific Ocean. The data can inform population models for species conservation, particularly those that contribute to the development of conservation plans that consider climate change projections.