Accounting for emigration reveals high survival and bimodal size at departure from a loggerhead sea turtle (Caretta caretta) foraging area

Abstract

The life history of hard-shelled sea turtles includes several ontogenetic shifts in habitat use and these complex permanent emigration patterns can impact estimates of stage-specific population rates, including survival. We developed several multistate mark recapture models to estimate survival of adult and juvenile loggerhead turtles from a coastal bay in the northern Gulf of America (also commonly referred to as the Gulf of Mexico) while, in some cases, accounting for permanent emigration and transient individuals. Our mark-recapture dataset consisted of 228 individual turtles with 37 total recaptures from 2011 to 2024. Of the models we fit, those that incorporated emigration produced higher estimates for annual survival than models that did not, and higher estimates than what is commonly seen in the literature for loggerheads. All models suggested a major permanent emigration pulse at the typical size of sexual maturity (70 cm straight carapace length) and another major pulse at > 90 cm. This bimodal pattern of departure may reflect differences in size at sexual maturity among loggerheads, possible genetic variability within the assemblage, or both. To assess the models’ ability to effectively recover true parameter values, we developed a simulation study of 50 randomly generated independent data sets under our specified models of similar sample size to our study dataset. Simulation results suggested that models that accounted for permanent emigration and transient individuals produced relatively unbiased estimates of survival, while models that did not often underestimated survival rates. Mark-recapture studies that may exhibit emigration and suffer from low recapture rates would benefit from auxiliary data collection such as acoustic telemetry detections to better estimate true rates of emigration and survival. Obtaining unbiased estimates of true survival by accounting for processes like emigration can support effective conservation of endangered long-lived species like loggerheads.