Archive of Spotlight Feature Articles

Mapping the Bottom

Story By Elizabeth Coleman
Photos and figures courtesy of Harry Roberts and Charles Wilson

For generations, Louisiana oyster fishermen have "seen" the condition of their oyster beds by probing water bottoms with long poles, which helps them to evaluate sites for new oyster beds or assess damage after a storm or accident. It’s a slow process and far more art than science. But now a new tool, "an electronic cane pole," may make the job easier, faster, and more accurate.

In a Sea Grant-supported project, Dr. Charles Wilson of the LSU Coastal Fisheries Institute and Dr. Harry Roberts of the LSU Coastal Studies Institute are experimenting with an approach that combines side-scan sonar, a digital high-resolution subbottom profiler, and a geographic positioning system to accurately map the oyster beds on the bottom of Barataria Bay. The bay’s private-lease oyster harvest is traditionally one of the state’s most abundant, but now it’s in trouble. A detailed and reliable "picture" of the bottom of this shallow, dynamic bay may help to ensure the survival of a generations-old industry.

In an effort to stop the rampant erosion that is devastating Louisiana’s coastal marshes, the state built a freshwater diversion structure at Caernarvon on the lower east side of the Mississippi River and began building another at Davis Pond on the west bank. For two years, Caernarvon has been diverting nourishing, sediment-rich water from the river into adjacent bays, lakes, and marshes and now healthy marsh plants are recolonizing the area. But the extra freshwater has upset the range of salinities in which oysters thrive and they are dying on the reefs or falling victim to fouling by the freshwater hooked mussel.

Oyster fishermen holding leases in Barataria Bay and Breton Sound, the most profoundly affected areas, sued the state, citing enormous production losses directly caused by freshwater diversion. In response, the Louisiana legislature authorized an oyster lease relocation program for these areas, as well as for those that will be affected by the Davis Pond diversion structure.

A key component of the lease relocation program calls for evaluating the quality of existing oyster leases, but traditional appraisal methods provide only general classifications of bottom type. In poling, for example, the surveyor subjectively classifies the bottom as soft or firm mud, sand, or hard reef with or without oysters, according to the way the pole penetrates the bottom. Another technique is to drag a chain behind a vessel. As the chain grates on the bottom, it sends characteristic vibrations back to the operator who is holding it. The use of chains, however, can indicate only whether reefs and oysters are present.

Most lease evaluations have been done in response to litigation and depend primarily on the subjective opinions of private consultants. Testimony is frequently heard by judges and juries who may have little understanding of the true value of water-bottom characteristics for oyster leasing. Though there are about four million acres of coastal waters in Louisiana, currently there is no rapid, reliable method of evaluating oyster bottoms that is acceptable to both the state and the oyster industry.

"The landscape of coastal Louisiana is constantly being altered, so it’s important to be able to accurately and objectively evaluate the quality of oyster beds and the impacts on them, especially losses in value," says Wilson. "Because of freshwater diversion or activities like dredging and shoreline reconstruction, the state may often be faced with having to relocate oyster leases to comparable areas. It’s critical to be able to establish the character and value of the damaged beds."

Wilson and Roberts, along with Sea Grant oyster specialist, Dr. John Supan, and Walker Winans of the Coastal Studies Institute, have informally joined forces to establish the Oyster Geophysics Laboratory. Their first goal is to determine if combining bottom surface acoustic patterns from side-scan sonar and the sedimentary sections provided by the subbottom profiler can be used to identify oyster habitat, evaluate its quality, and select areas suitable for oyster reef relocation. They can then develop maps of Barataria and other Louisiana bays that will assist in lease evaluation and relocation for the Louisiana departments of Wildlife and Fisheries and Natural Resources, the U.S. Army Corps of Engineers, and the oyster industry.

Sonar, first developed in World War II to detect submarines, employs sound waves that reflect off submerged objects, indicating their size, location, depth, and hardness. When bounced off shallow water bottoms, the recorded sound waves can reveal details as small as crab traps. Wilson and Roberts’ side-scan sonar equipment - mounted on the bow of a skiff and suspended a foot deep in the water - sweeps up to 100 yards on each side of the boat. "It covers an area as big as a football field quickly," says Wilson, "and the electronic circuitry is so small that it can be used in water as shallow as two feet, which is necessary in Louisiana."

To give the researchers a true "3-D" picture, Roberts uses the subbottom profiler to provide information from the bottom surface to a depth of 10 to 14 meters. "We can view the bottom vertically, like the layers of a cake," says Roberts. "Combining sonar and the subbottom profiler is a revolutionary way to look at water bottoms."

The researchers are able not only to discriminate between different types of bottoms - for example, shell versus clay - but also to distinguish layers of sediment or shell that were deposited during specific time periods. Carbon dating of core samples tells them when the layers were formed. Thus they can better understand how oyster reefs form and change with environmental changes. "We can predict the future by looking at the past," observes Roberts.

Wilson sees the combined use of sonar and subbottom profiler as having wider applications than oyster reef mapping. The 1996 Sustainable Fisheries Act requires regional fisheries management councils to describe essential habitat for all life stages of the species they manage, identify activities that can damage these essential habitats, and make recommendations for habitat conservation. "At present," says Wilson, "there are no geological standards established for different kinds of essential fish habitats. With this equipment, we can develop an index of habitat types based on their acoustical signatures. It will be a useful tool to help establish and map habitat criteria for essential fish habitat."

[6/5/00]

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