Recovery of long-lived marine vertebrates can change local capture records by altering movement and site fidelity, making mortality difficult to distinguish from redistribution. We developed a behaviorally explicit framework to separate true mortality from changes in availability caused by movement and redistribution. We evaluated this problem for juvenile green turtles (Chelonia mydas) and loggerhead turtles (Caretta caretta) captured at the St. Lucie Nuclear Power Plant intake canal on Florida’s Atlantic coast using a multi-decade capture–mark–recapture record spanning 1977-2023. Hidden Markov Models classified individuals into latent habitat-use strategies (Nomadic, Resident, and Transient) based on capture timing and body size. Across both species, these strategies captured consistent differences in site fidelity and recapture structure that were not represented by size-only groupings. This behavioral framework was then applied to green turtles to estimate strategy-specific apparent survival and temporary emigration under Pollock’s Robust Design, with uncertainty in behavioral classification propagated using multiple imputation and Rubin’s rules. Apparent survival increased through time in all strategies, but temporary emigration also increased, especially in Nomadic and Transient turtles and increasingly in Residents, indicating that absence from the capture domain reflected changing availability rather than mortality alone. To evaluate long-term site-based change, we compared expected five-year local retention (from survival and temporary emigration) with observed retention patterns and a detection-only benchmark. Observed retention patterns over time were consistent with changing availability and movement, whereas a detection-only explanation did not fit the data. These results suggest an interpretation of long-term change at this foraging ground as shifts in availability and retention during regional green turtle recovery, consistent with redistribution within a connected coastal network.
Ryan Welsh
Dr. Kate Mansfield, Advisor
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