The 2010 Deepwater Horizon (DWH) oil spill posed a severe threat to surface-pelagic sea turtles because the surface convergence zones, which provide vital habitat by aggregating pelagic Sargassum and other floating material, also aggregated floating oil. Following the DWH spill, turtle rescue operations between 17 May and 9 September 2010 documented 937 juvenile sea turtles in the spill area and examined 574 captured turtles. Of the captured turtles, 81% were visibly oiled. Transect searches in convergence zones found Kemp's ridleys (51% of individuals), green turtles (37%), loggerheads (7%), hawksbills (2%), and unidentified sea turtles (2%). Linetransect methods estimated the density of all surface-pelagic sea turtles in surface convergence zones to be 3.32 km −2 (95% CI = 2.82-3.88), and the density of heavily oiled turtles to be 0.24 km −2 (95% CI = 0.15-0.39). Turtle densities and the areal extent of heavy oiling probability were used to estimate total number of turtles exposed to DWH oil. We estimate approximately 402 000 surface-pelagic sea turtles were exposed, and of those, 54 800 were likely to have been heavily oiled. Our estimates formed the basis of surface-pelagic juvenile sea turtle mortality estimates included in the DWH natural resource damage assessment.
The Deepwater Horizon (DWH) oil spill was unprecedented in extent and duration, and affected marine natural resources, including sea turtles, throughout the northern Gulf of Mexico. Consequently, US federal and state Trustees documented and quantified oil exposure and resulting injuries to sea turtles under the DWH Natural Resource Damage Assessment. At-sea rescue operations focused on surface-pelagic juvenile sea turtles, which were especially at risk to oil exposure within oceanic convergence zones, and provided direct observations of the degree that turtles in this young life stage were exposed to DWH oil. In contrast, locations of larger neritic juvenile and adult turtles were documented during aerial surveys, but because these turtles were not captured, their oiling status could not be directly evaluated. Both the rescue operations and aerial surveys were able to observe only a small fraction of sea turtles within the vast spill footprint. We developed a spatio-temporally explicit approach that used direct observations of oiled surface-pelagic juvenile sea turtles and satellite-derived surface oil distributions to statistically estimate the probabilities of oil exposure for all sea turtles that were present within the area of the DWH spill, but whose oiling status was unknown. Our results enabled an expansion of exposure and injury quantification across the entire DWH spill area and period. This approach was conceptually straightforward and used common geospatial and statistical techniques, making it applicable to other situations in which the full extent of oil exposure for marine natural resources must be estimated from an incomplete sample.
The Deepwater Horizon (DWH) incident was the largest offshore oil spill in the history of the United States, contaminating surface waters, the water column, deep-sea corals and benthos, nearshore and coastal ecosystems, and natural resources across 5 states and an ocean area of more than 112 000 km 2 in the Gulf of Mexico (GoM). Protected marine species -sea turtles and marine mammals, in particular -were a main focus of the DWH Natural Resource Damage Assessment (NRDA). The DWH spill overlapped in time and space with sea turtle and marine mammal habitats and life stages throughout the northern GoM. Thus, the DWH NRDA Trustees (2016; www.gulfspillrestoration.noaa.gov/restoration-planning/gulf-plan/) performed several activities to assess adverse effects of oil exposure on sea turtles and marine mammals to quantify the full extent and nature of the impacts to these taxa across the region. A synopsis of the Trustees' assessment activities and conclusions is presented in the DWH NRDA Programmatic Damage Assessment and Restoration Plan (DWH NRDA Trustees 2016). This Theme Section presents several of these specific sea turtle and marine mammal assessment activities and associated findings. This Overview provides a context for the Theme Section papers, introduces basic NRDA concepts and discusses generally why and how protected marine species were assessed in the DWH NRDA.
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