Coastal Services Center

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Monitoring Intertidal Oyster Reefs

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Having an up-to-date inventory of intertidal oyster resources is necessary to manage and protect this commercially and biologically important resource. State coastal managers and researchers are using airborne multispectral systems to update the original 1980s intertidal oyster database so that they can assess the current extent of the reefs and monitor any trends in the oyster population since the last inventory.

The Project: Mapping South Carolina's Intertidal Oyster Population

Intertidal oyster reefs are a major recreational and commercial resource and Eastern Oyster (Crassostrea virginica) are a key species in South Carolina's estuaries. Intertidal oyster reefs stabilize the easily erodible mudflats along the coast of South Carolina and provide valuable nursery habitat for fish and invertebrates. The South Carolina Department of Natural Resources (SCDNR) is responsible for oyster reef conservation and harvest regulation but does not have contemporary data to assess the state's intertidal oyster resources or to track changes in the oyster population. The SCDNR's current oyster reef inventory database was completed in the 1980s by field crews collecting in situ data. The agency suspects that coastal development, associated recreational boating traffic, and construction of docks, ramps, and marinas may have altered the extent and density of the state's oyster resources since the last survey. An up-to-date inventory is necessary to assess the extent of these impacts. The SCDNR is using digital aerial imagery to re-map the oyster reefs along the entire coastline of the state.

Mapping Oyster Reefs with Digital Aerial Photography

Field assessment techniques used to create the current data are labor intensive and take a long time to complete. So, in 2002 the SCDNR, along with the NOAA Coastal Services Center, evaluated several types of remote sensing data for oyster mapping. Aerial multispectral imagery acquired at 0.25 m spatial resolution was chosen as the best image source for oyster reef mapping in South Carolina. The imagery must be collected under extremely narrow flight windows that are constrained by lunar low tides and sun angles greater than 45 degrees above the horizon. It was determined that these were the best times to collect the imagery in order to minimize shadowing and maximize intertidal oyster reef exposure.

A variety of image processing methods to determine the perimeter and spatial characteristics of oyster reefs were evaluated. A combination of digital texture analysis to determine reef perimeter and spectral analysis to evaluate reef condition were found to be the most effective methods.

The Result

Approximately 1,500 square miles of digital multispectral aerial imagery are currently being collected to map South Carolina's intertidal oyster beds. This approach is viable because oyster reefs in much of the southeastern U.S. are intertidal, in contrast to other areas that are historically known for oysters, such as the Chesapeake Bay, most of North Carolina, and the Gulf of Mexico, where the oyster reefs are rarely, if ever, exposed.

This digital imagery will also be used for various other coastal applications in South Carolina, increasing the value of this imagery to the state. For example, the state's Ocean and Coastal Resources Management agency and several local government offices are planning to use the tide-coordinated imagery for dock permitting purposes.

For More Information

• SCDNR - Marine Resources Division
• NOAA Coastal Services Center's Benthic Mapping Project

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