Theoretical modeling and neritic monitoring of loggerhead Caretta caretta [Linnaeus, 1758] sea turtle sex ratio in the southeast United States do not substantiate fears of a male-limited population

Abstract

Sea turtles are among several hundred species whose sex is determined by incubation conditions during critical developmental periods. Consequently, these marine reptiles may be vulnerable to global climate change, and under the assumption of continued climate warming, numerous studies pose dire predictions for future populations based primarily on hatchling sex ratio data. Alternatively, as long-lived species that take decades to reach maturity, without inherent coping mechanisms for such change, sea turtles could not have persisted across geological epochs. Globally, loggerhead Caretta caretta [Linnaeus, 1758] sea turtles occupy temperate zones, with ontogenetic development that spans the entirety of gyres associated with respective ocean basins. The largest rookery for this species occurs in the Northwest Atlantic Ocean (NWA) population, where a 30-year cycle in annual nest counts is reported through 2018. Complementary studies document a lagged association between these annual nest counts and the Atlantic Multidecadal Oscillation (AMO); however, the underlying mechanism for this association remains elusive. Therefore, objective 1 evaluated the effect of AMO-mediated cohort resonance on the demographic structure of a theoretical neritic assemblage under variable cohort abundance and female proportion but stable annual survival during 165-year runs (i.e., extent of AMO data). For objective 2, blood samples were used to assign sex to 2217 loggerhead sea turtles captured by research trawling (2000 to 2019) on the inner continental shelf from St. Augustine, FL (29.9°N) to Winyah Bay, SC (33.1°N). Shorter oceanic duration of less female-biased cohorts from the AMO cold phase synchronized peak adult male and adult female co-occurrence during subsequent warm phases three decades later. Grand sex ratio predicted from testosterone was 67% female (n = 1484), with a slight temporal female decline. Our findings suggest greater population sex ratio plasticity than predicted solely from terrestrial nesting data.

Keywords: Atlantic Multidecadal Oscillation; climate; demography; sea turtle; sex ratio.