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June 2003 Feature: NOS's Environmental Sensitivity Index Maps Help Scientists Protect Vulnerable Shorelines

This ESI map of the San Diego Bay area indicates seasonal nesting, laying, hatching and fledging information about the birds that frequent the area.

When an oil or chemical spill occurs, responders must decide quickly which coastal locations need the most protection. They need a systematic way to determine which areas are most vulnerable to the spill, and which areas can be protected with existing resources. Federal, state, and local responders rely heavily on color-coded Environmental Sensitivity Index (ESI) maps that serve as quick references to a threatened shoreline. ESI maps indicate a coastal area’s level of vulnerability to oiling. They also pinpoint the locations of plants, animals, and areas that people use that could be harmed by a spill. Scientists from NOS’s Office of Response and Restoration (OR&R) work with other state and federal partners to produce ESI maps long before an oil or chemical spill occurs.

ESI Map Features

ESI maps rank a shoreline according to its sensitivity level, the likely persistence of oil in the area, and expected ease of cleanup. Different types of shorelines are uniquely vulnerable to oiling because they show different degrees of substrate permeability. Oil mobility and the degree to which an area is exposed also affect ease of cleanup.


Gravel beaches like this one are considered less sensitive to pollution and are represented on ESI maps in blue and purple colors.

In the ESI maps, shorelines are color-coded and numerically ranked on a scale of 1 to 10. The most sensitive areas, such as tidal flats, swamps, and marshes, are indicated by warm colors like red and orange, and are given a ranking of 10. The least sensitive areas, such as rocky headlands and gravel beaches, are indicated by cool colors like blue and purple, and are given a ranking of 1. Areas of moderate sensitivity are green. Colored polygons denote habitats for plants and animals.

Depending on the season, oil spills can affect breeding sites, migrations, and life stages of different animals. ESI maps respond to this threat by classifying organisms into seven categories, which in turn are divided into sub-groups of organisms with similar oil sensitivities. ESI maps indicate where the most sensitive species live, and what they use the habitat for, such as spawning, or over-wintering.


This very sensitive marsh would be designated on an ESI map in a warm color like red or orange, and would be given a high numerical ranking.

Coastal zone managers and oil spill planners use ESI maps to identify the seasons when a particular species is most vulnerable, allowing them to establish protection priorities for a cleanup effort. Sensitive animals and habitats are denoted by special point symbols.

For example, a symbol of a seal identifies habitats used by gray and harbor seals. To protect certain endangered species from possible human disturbances, ESI maps do not show exact locations of sensitive habitats.

ESI maps categorize human use areas that are especially susceptible to oil spills. These include:

  • heavily used shoreline access areas, such as recreational beaches, marinas, boat ramps and diving areas;
  • officially designated natural resource management or protected areas, including national parks, marine sanctuaries and national wildlife refuges;
  • resource-extraction sites, such as aquaculture sites, subsistence and commercial fisheries locations, and surface water intakes; and
  • water-associated archaeological, historical, and cultural sites, including shipwrecks and lands managed by Native Americans. Like with endangered species and their habitats, ESI maps do not pinpoint precise locations of archaeological and cultural resources to protect them from vandalism.

In the Future


An ESI map of Point Reyes, Calif. Note the seal symbols along the bottom of the map to indicate gray and harbor seal habitat. Future ESI maps may incorporate dynamic geographic information system capabilities.

NOAA researchers are working with federal, state, and private industry partners to improve the ESI mapping system to extend beyond spill response. By incorporating geographic information systems information, higher quality and more frequently updated maps can be produced and distributed in various digital formats. In addition, OR&R is developing digital ESI databases for high-priority coastal areas, and establishing standards for ESI data users. These guidelines will help scientists exchange data, and provide a means for evaluating data quality. The guidelines also will permit the development of computer applications that display ESI maps.

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For More Information


Office of Response and Restoration

Environmental Sensitivity Index Maps

Environmental Sensitivity Index Map Educational Exercise


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June 2006: Wyckoff/Eagle Harbor: A Collaborative Cleanup and Restoration Effort


April 2006: Protecting Corals...Saving Ships


February 2006: Rebuilding Iraq Thru Global Positioning


September 2003: NOAA's National Geodetic Survey (NGS) provides the foundation needed for transportation infrastructure projects


July 2003: NOS Electronic Navigational Charts Improve Safety at Sea


June 2003: NOS's Environmental Sensitivity Index Maps Help Scientists Protect Vulnerable Shorelines


May 2003: NOS Sanctuaries Protect Nation's Maritime History


April 2003: NOS Products and Services Vital to the Nation’s Marine Transportation System








Scientists from NOS’s Office of Response and Restoration (OR&R) work with other state and federal partners to produce ESI maps long before an oil or chemical spill occurs.

















ESI maps indicate where the most sensitive species live, and what they use the habitat for, such as spawning, or over-wintering.



















OR&R is developing digital ESI databases for high-priority coastal areas, and establishing standards for ESI data users.

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