Coastal Services Center

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Mapping Past and Present Oyster Reefs

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Louisiana is trying to revitalize its marshes and rebuild eroded land by diverting fresh water rivers and sediments into brackish bay ecosystems. Researchers are using side-scan sonar and a single beam acoustic subbottom profiler with highly precise positioning data to help assess the current status of oyster reefs in these areas.

The Project: Mapping the Seafloor and Sediments in Louisiana Using Several Acoustic Sensors

The State of Louisiana has been experiencing extensive marsh loss and erosion over the past decade. To help revitalize marshes and promote land-building, the state and U.S. Army Corps of Engineers have built controlled diversions of water from the Mississippi River to introduce sediment-rich freshwater from rivers into nearby bays, lakes, and marshes. While the marshes have benefited, the increased freshwater and suspended sediments may have put stress on oyster reefs that thrive best under brackish conditions.

Commercial harvesters leasing the oyster reefs in Barataria Bay and Breton Sound sued the State of Louisiana for the losses in oyster production caused by a nearby control structure, and won. In 2003, the state legislature began a lease relocation and compensation program to reimburse oyster lease-holders affected by the marsh restoration efforts and began oyster restoration projects.

In order to initiate the program, Louisiana needed to evaluate the extent and condition of existing oyster beds and identify sites suitable for restoration. Traditional mapping methods that involved probing the sediments with a pole or dragging a grate across the seafloor would not provide the detailed data sets needed to make the assessment in a timely way. In addition, Louisiana's bays are too turbid for mapping with aerial photography. As a result, researchers decided to use a side-scan sonar to map the bottom of the bay and a single beam subbottom profiler, a Chirp sonar, to link the shallow subsurface sediments to the side-scan sonar data.

Mapping the Seafloor with a Side-Scan Sonar and Single Beam Subbottom Profiler

Researchers used a dual frequency side-scan sonar to map the bay bottom. Side-scan sonar rapidly scans the bay bottom with sounds that are emitted and received along the boat's path. The reflected sound energy is used to produce an image of the seafloor. The image is created in real time from the variations of intensity of the reflected sound, as opposed to a traditional photograph which uses the variations of reflected light to create an image. The side-scan sonar system used in this study utilizes a satellite-corrected Geographic Positioning System (GPS) for accurate location of the data. It also corrects for boat speed and other potential sources of error. The strength and patterns of the reflected sound create shapes and textures that are interpreted to produce maps of the bottom. Coupled with ground verification data, such as quadrate sampling and cores, researchers can quantify bottom properties such as shell content.

Imagery derived from a subbottom profiler provides a cross-section view of the seafloor. This image shows the sediment layers in Barataria Bay.

The researchers also used a subbottom profiler to map the subsurface sediments and relate subsurface geology (i.e., features such as buried oyster reefs) to the current configuration of the bay bottom. Looking at the subsurface allows researchers to assess how the populations have changed over time and provides insight on how they may continue to change in the future. The surface data collected with the higher frequency acoustic signals helps researchers assess the status of current oyster reefs and find substrates suitable for oyster restoration projections.

The Result

Louisiana is gaining multiple benefits from the combined use of sonar systems. Not only is the state able to effectively apply the oyster lease relocation program and find new sites for establishing oyster reefs, it is developing a data set that can be used for general habitat characterization. In addition, because the data is spatially accurate, surveys can be repeated for future comparison studies of change over time.

For More Information

• Louisiana Sea Grant
• Louisiana State University
• NOAAOffice of Response and Restoration

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