| [Image: model oysterbreak] | | [Image: full-size oysterbreak] | | [Image: a “long-line” system] | | [Image: “French tube”] | | [Image: habitat for oysters and barnacles] | | [Image: close-up of a concrete and agricultural byproduct material] | | |
Steven G. Hall, Robert Beine, Matthew Campbell and Tyler Ortego
Loss of estuarine habitat in Louisiana, which contains 40 percent of the nation’s wetlands, has received national attention because of rapid erosion rates. One way to combat this trend is with engineered rock breakwaters and other heavy engineered structures, which have been placed offshore and onshore in critical areas. Unfortunately, these units are too costly for widespread use. Additionally, the heavy rock breakwaters tend to sink through poorly compacted mud sediments. Lower-density, bioengineered submerged breakwaters, such as oysterbreaks, can help address these issues and have the added benefit of contributing to habitat restoration and oyster production in Gulf Coast areas.
The oysterbreak design allows oysters to form a dense structure that dissipates wave energy, reduces erosion, restores estuarine habitat and enhances or maintains healthy oyster production. This technology uses agricultural byproducts, such as cottonseed, and safe concrete to encourage biologically dominated erosion- reduction reefs. Reef-building animals such as oysters can build mass on structures that encourage their growth. In this process, waterborne material is converted to biomass, which adds to the coastal area. At the same time, reduced wave energy behind the structure tends to cause additional material to be deposited. Recent studies have found a variety of aquatic organisms – including oysters, barnacles, worms, finfish, crabs, snails, algae and plants – increase the biomass on and adjacent to these structures. The increased biomass performs multiple functions, including reducing wave energy and providing habitat for spawn of oysters and other aquatic organisms.
A series of wavebreaks were tested in the LSU Coastal Studies Institute’s wavetank, an automated system where ¼-scale models of the oysterbreaks were tested with a series of waves approximating what might be expected in coastal Louisiana. The wave tank experiments concluded that wave transmission through the structure decreased as oyster growth occurred. The predictive model suggests that the oysterbreak can be used in field conditions. It showed that after one year of growth, an oysterbreak 22 yards wide has the capacity to reduce wave energy by 70 percent. These tests showed that oyster growth could significantly reduce wave energy, leading to reduced erosion and possible deposition of material to build land.
Assuming growth rates observed near Grand Isle, La., such reduced energy could be expected in one to three years under favorable growth conditions. One additional advantage of the oysterbreaks is their ability to provide habitat for other estuarine organisms, thus enhancing both the productivity and the protection value of these structures.
Applications include both the use of engineered reefs and engineered materials as cultch material (substances laid on oyster grounds to create a place for oysters to spawn). Engineered reefs are widely accepted for erosion reduction and habitat restoration. These can be made from a variety of materials, including concrete, natural oyster shell and agricultural byproducts such as cotton seed. Careful design and placement are necessary for optimal results.
The technology to produce engineered composite cultch material provides a cost-effective alternative to shells, which have a limited supply, and is more environmentally friendly than many other alternatives such as rock or concrete debris. This material can have many different enhanced growth characteristics compared to natural shell, limestone or concrete. First, these materials encourage oyster growth by providing attractants and possible food sources for young oysters, enabling aquaculture and enhancing coastal restoration. Second, oyster growth can be enhanced by appropriate food sources and surfaces in this material, and the oyster growth rates and harvestability can be improved by shape of the structure. Finally, this material can help jump-start growth of other organisms and thus restore or enhance estuarine ecology while providing a substrate for natural oyster fisheries.
Steven G. Hall, Associate Professor, Department of Biological & Agricultural Engineering; Robert Beine, Director, Department of Agricultural Chemistry, LSU AgCenter, Baton Rouge, La.; Matthew Campbell and Tyler Ortego, former students, Department of Biological & Agricultural Engineering, LSU AgCenter, Baton Rouge, La.
(This article was published in the spring 2007 issue of Louisiana Agriculture.) |