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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.

Common analytical methods for detecting and quantifying polychlorinated biphenyls (PCBs) in environmental samples rely on matching a pattern of peaks to a series of Aroclor standards. Due to differences in degradation, partitioning, and metabolism, the PCB pattern in environmental samples can be very different from the Aroclor standards, making identification and quantification of PCBs difficult. To identify an appropriate approach for PCB analyses, the U.S. Environmental Protection Agency Region 9, Biological Technical Assistance Group (BTAG), has considered PCB environmental fate and ecotoxicity, reviewed methods and laboratory capabilities for PCB analysis, and evaluated site-specific data from California and elsewhere. Results of this review indicate analytical methods for routine measurement of specific PCB congeners are currently available, and are cost-effective in comparison with Aroclor- based methods. Accordingly, BTAG has developed a core list of congeners and a phased approach for PCB congener-specific analysis in ecological risk assessments.