|Technical Aspects of Oyster Restoration|
Oyster restoration involves the construction of oyster reefs using oyster shell and larval oysters raised in hatcheries. Young oysters (spat) need a hard surface on which to settle and grow. Historically, spat settled on the shells of oysters in a nearby reef, but overharvest and the resulting susceptibility to sedimentation of oyster reefs has lead to a dramatic decline in the oyster population in the Bay. NOAA and others are looking at ways to access previously unavailable dredge shell sources, or develop appropriate, cost-effective alternative substrates for oyster restoration. Various management techniques are employed throughout the Bay including oyster sanctuaries (reefs that are protected from harvest) and managed reserves (where the reef is cleaned and closely managed for a set number of years without harvest).
Native oyster restoration in the Chesapeake Bay starts by creating hard surfaces (reefs) on which larval oysters can settle and grow. In low-salinity areas where there is inadequate natural spat set (mainly in Maryland), reef creation must be followed by planting hatchery-raised juvenile oysters (also called spat) that are attached to oyster shells (left photo above). Acoustic seabed surveying systems are used to identify oyster habitat and restoration planting locations that will maximize the survival of spat-on-shell. Preferable restoration sites are hard, geologically stable terraces, of generally uniform depth, of moderate to high rugosity (a measure of surface irregularity) with sand and/or oyster shell as the base. Restoration sites are typically located on historic oyster bottom. Hatchery-raised spat on shell are often washed off a specially built boat over suitable planting sites (see right photo above). To monitor the success of restoration, divers return to the planting area at regular intervals and collect a sample that is brought to the surface to be counted and measured. The number and size of spat per shell (left photo below) are counted and measured and the data recorded (right photo below) to help guide future restoration efforts. The oysters are then returned to the reef.
Young oysters (spat) need a hard surface on which to settle and grow. Historically, spat settled on the shells of oysters in a nearby reef but overharvest and the resulting susceptibility to sedimentation of oyster reefs has lead to a dramatic decline in the oyster population in the Bay and therefore hard substrate, for oyster spat to settle on. NOAA and others are looking at ways to access historic shell deposit sources, or develop appropriate, cost-effective alternative substrates for oyster restoration efforts.
Traditional oyster bottom repletion programs (those that condition harvest bottom to receive natural spatset) and restoration projects (those that rehabilitate formerly productive oyster bottom) have relied on supplies of oyster shell from either processing houses or deposits of 3,000- to 4,000-year-old buried fossil shell. For many years these sources provided ample supplies to meet oyster fishery repletion and restoration needs. In fact, oyster shell was once so abundant that much of it was used for roadbed, crushed calcium sources, fertilizer additives, and chicken feed. Dredging to reclaim buried shell deposits in Maryland alone was a 2- to 3-million bushel activity in many years.
The decline of the oyster industry in the Chesapeake Bay region has led to a decrease in the amount of shell available from shucking houses. Increased demand for dredged oyster shell has also depleted the historically used areas of large, buried deposits, which have been heavily used over the last 20 years. Other areas that have sizeable buried deposits of oyster shell have been identified, but access to these areas may be limited because of concerns about the environmental impacts and resource tradeoffs associated with shell dredging. A number of these sites also fall within traditional fishery management protection zones because they are spawning or nursery grounds for commercially important fish species.
Alternative materials have different uses in oyster restoration. They can be used to provide a base or a bottom type for larval recruitment in the absence of fossil shells, and they can be used as a medium to set spat on in a hatchery before being deployed to the natural environment. A lack of availability of natural or fossil oyster shell in the Bay has necessitated the investigation of alternative materials. Alternative material may come in the form of construction rubble, precast concrete structures, coal ash, limestone marl, recycled porcelain, or slag. Alternative substrate materials must go through detailed analyses to make certain that they don’t contain toxic materials that may leach into the environment or may otherwise compromise the Bay ecosystem. Tests are also conducted to evaluate whether or not oyster larvae settle on the substrate, the spat growth, and whether or not it is prone to fouling by other marine organisms. Before deploying structures that are effectively permanent and are potential navigational hazards, it is imperative that their chances of success be evaluated as accurately as possible. After deployment, regular monitoring is needed to evaluate the success of consecutive years' settlement events. In addition, the substrate should not subside or deteriorate in saline waters. The reef should have an expected life that would allow an oyster exterior to become established. Availability and cost—including cost of the material itself, any associated processing cost to clean, size, or prep the material, and transport and deployment costs—are also considerations. Special permits must be obtained in both Maryland and Virginia for placing anything but oyster or clam shell on historic oyster bottom.
This recent focus on identifying sources of concrete rubble suitable for use in oyster restoration has resulted in some innovative and unique uses for formerly productive structures. In Maryland, the former Memorial Stadium in Baltimore, once home to the Orioles and Colts, was slated for demolition, and much of the stadium rubble was re-used as the base for a large oyster reef restoration effort. In another example of innovative reuse, decking from the Chesapeake Bay Bridge was reused to create an oyster sanctuary in the Severn River. Planning is under way for even more of this recycling/reuse projects, where appropriate. Discussions are under way to use the concrete rubble from an old dam, which has blocked fish passage for hundreds of years but is now slated for removal. The concrete material, if planning and analysis deem it worthy, would be placed on the site of an historic oyster reef to restore oyster and fish habitat as part of the larger oyster restoration efforts.
Field trials have shown that oyster larvae will settle on virtually all hard substrates. Significant differences exist, however, in the setting density and subsequent survival of those oyster spat. This apparently results from the significant differences in surface area of the various substrates, both of the individual pieces, and of the interstitial space between piles or layers of the material. Monitoring suggests that the irregular surfaces and pore spaces of certain materials (natural oyster shell, stone, crushed concrete, and marl) also protect the oysters from predation. Materials that create a smooth pile without crevices (clam shell, surf clam shell, coal fly ash) are not as good a substrate for oysters as the others. Additional studies are under way to further evaluate and quantify the effect of reef rugosity (roughness or irregularity) on oyster survival and its benefit to benthic communities that develop on the restored reefs.