Brian A. Stacy
National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Office of Protected Resources
The Deepwater Horizon (DWH) oil spill in the Gulf of Mexico (GOMX) was unprecedented in magnitude and duration and caused significant injury to the environment and wildlife, including sea turtles and habitat they rely upon for foraging, migration, reproduction, and shelter. Five species of sea turtle, all of which are threatened or endangered, inhabit the GOMX and are found within different areas and habitats, depending on species and life phase. Efforts to mitigate and document effects of the DWH spill on sea turtles spanned from the open ocean to nesting beaches. The origin of the spill distant from shore posed unique challenges as oil aggregated with convergence fronts that also accumulate diverse Sargassum communities utilized by small surface-pelagic juvenile turtles. An unprecedented rescue effort was undertaken to collect oiled turtles out of harm's way for care and treatment. A network of rehabilitation facilities with the southeastern U.S., bolstered by veterinarians and care staff from institutions nationwide, were an integral part of this operation and effected the successful treatment and release of almost all rescued turtles. Interventive measures also were undertaken on nesting beaches under threat from oiling and included the translocation of thousands of eggs that otherwise had minimal chance of survival.
These efforts to lessen losses from the oil spill to the extent possible were concurrent with and significantly informed the Natural Resource Damage Assessment, which is the legal framework used to assess and restore for oil spill impacts. A number of approaches were used to estimate the numbers of sea turtles within the vast spill footprint and to estimate mortality and lost reproduction resulting from both effects of the spill and clean-up efforts. A combination of direct field observations, enhanced effort to document and examine stranded sea turtles, vessel and aerial surveys, satellite-derived data, experimental exposure studies using freshwater surrogate species, and modeling were used. Two significant effects of the DWH spill on sea turtles that were most readily apparent and figured prominently in the damage assessment were mortality caused by physical fouling and deterrence of nesting due to shoreline response activities. Upper bound estimates of losses across all life phases were in the hundreds of thousands of turtles, including approximately 10 to 20% of all surface pelagic juvenile Kemp's ridleys. Information obtained during the DWH spill will greatly benefit preparedness for future spills, and also underscores the need for better information on baseline population information and the effects of oil exposure on sea turtles, especially potential long-term, chronic, and sublethal effects.
In addition, large numbers of dead, non-visibly oiled, neritic phase sea turtles were found within the northern GOMX concurrent with the DWH spill and during subsequent years, further heightening public concerns over effects of the spill. Following a comprehensive investigation, many of these strandings were attributed to fisheries interactions and other anthropogenic sources of mortality, including vessel strikes and entanglements. Observations related to DWH and subsequent efforts to understand its effects have highlighted the threats to sea turtles posed by various anthropogenic activities in the GOMX.