In 1979, as the oil from the Ixtoc II well blowout approached the U.S. coast, the Scientific Support Team from the Hazardous Materials Response Branch of the National Oceanic and Atmospheric Administration (NOAA) was advising the U.S. Coast Guard on protection priorities. The concept of ranking shorelines according to their oil spill sensitivity had recently been developed (Michel et al., 1978), and it was first applied in the days prior to oil landfall in south Texas. In 1980, the first Environmental Sensitivity Index (ESI) maps were produced for south Florida; by 1990, hardcopy ESI maps were available for most of the U.S. coastline.

Since 1990, updated maps have been produced using Geographical Information System (GIS) technology, with both hard copy and digital products available. ESI maps and databases are comprised of three general types of information (Fig. 2-1; Halls et al., 1997):

Shoreline Classification. The shoreline habitats are ranked according to a scale relating to sensitivity, natural persistence of oil, and ease of cleanup. A scale of 1 to 10 is used, with 1 being least sensitive and 10 the most sensitive. The classification system has been standardized nationwide, for estuaries, rivers, and lakes. The ranking scheme is based on extensive, empirical observations at oil spills, and it has become the basis for many spill response tools and strategies, such as protection prioritization, selection of response options, and determination of cleanup endpoints.

Biological Resources. The maps display the spatial and temporal distributions of oil-sensitive animals, habitats, and rare plants that are used by oil-sensitive species or are themselves sensitive to oil spills. There are seven major biological groups (marine mammals, terrestrial mammals, birds, fish, invertebrates, reptiles and amphibians, and habitats and plants), which are further divided into groups of species with similar taxonomy, morphology, life history, and/or behavior relative to oil spill vulnerability and sensitivity. The maps show the locations of the highest concentrations, the most sensitive life-history stages or activities, and the most vulnerable and sensitive species. The maps link to data tables that include species name, legal status of each species (state and/ or federal threatened or endangered listing), concentration at that specific location, seasonal presence and/or abundance by month, and special life-history time periods (e.g., for birds, nesting, laying, hatching, and fledging dates).

Human-Use Resources. The maps show four specific areas that have increased sensitivity and value because of their use: high-use recreational and shoreline access areas; management areas (e.g., marine sanctuaries and refuges); resource extraction locations (e.g., water intakes, subsistence areas); and archaeological, historical, and cultural resource locations.

Sensitivity maps are used to identify protection priorities in vessel and facility response plans, and they are used in area contingency plans as part of the Sensitive Areas Annex. ESI maps use a standard set of colors and symbology so responders from any region can use the maps readily. The concept of sensitivity mapping has been adopted internationally as a key component of oil spill contingency planning (Baker et al., 1995). Sensitivity atlases have been produced for such diverse areas as Australia, Mauritius, South Africa, the Gaza Strip, the North Sea, most of Canada, and the Sakhalin Islands.