Integration of Physical Oceanography with Spatio-Temporal Patterns of Stranded Sea Turtles in North Carolina

Abstract

The conservation status of sea turtles warrants research on their mortality rates (Turtle Expert Working Group 1998). Stranded carcasses document mortality but represent an unknown fraction of total number of dead turtles at-sea (Murphy and Hopkins-Murphy 1989, Epperly et al. 1996). In addition to water temperature, tidal forcing, decomposition rates, scavenging rates, and the spatio-temporal distributions of turtles and mortality sources, wind and water current regimes probably play a major role in the stranding of carcasses on beaches. Fifteen years of hourly wind speed data, recorded off the North Carolina coast, were transformed into vectors, converted into wind stress magnitude and direction values, and averaged by month. Near-shore surface currents were then modeled for the South Atlantic Bight via a three-dimensional physical oceanographic model (Werner et al. 1999). Estimated currents and particle tracks were compared to the spatial locations of sea turtle carcasses stranded along ocean-facing beaches of North Carolina. The seasonal development of along-shelf flow coincided with increased numbers of recorded strandings in late spring and early summer. The model also predicted net offshore flow of surface waters during winter, typically the season with the fewest relative strandings. Modeled lagrangian drogues were retained in shallow (< 20m) bathymetric contours, indicating that turtles killed only very close to the shore may be most likely to strand. During seasons when net along-shelf flow was present, turtles were likely to have died "upstream" from the residual current. A reevaluation of oceanic drift bottle experiments may also provide a reasonable upper bound to describe how far carcasses could theoretically travel and how likely those carcasses could make landfall from points offshore. Though qualitative, this research a.) provides a starting point for more robust analyses and b.) demonstrates that stranding research requires an understanding of ocean physics in addition to sources of mortality.