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 sedimentation of oyster reefs and a dramatic decline in the oyster population in the Bay have resulted in a shortage of shell for spat to settle on. NOAA and others are looking at ways to access previously unavailable dredge shell sources, or develop appropriate, cost-effective alternative substrates for oyster restoration and fishery repletion efforts.

The Current Substrate Shortage

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 those areas that have been heavily utilized 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 associated with 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 Material Considerations

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, reef balls, coal ash, limestone marl, recycled porcelain, or concrete that is cast in the general shape of a shell. 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 which 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, 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.

Performance of Alternative Reef Materials

Field trials to date have shown that oyster larvae will settle on virtually all hard substrates. In recent studies, concrete with an elevated lime content or with certain oil additives has been suggested to improve spat set. 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.

Examples of Alternative Substrates (from the top left)

1. dredge shell, limestone marl, coal fly ash, crushed concrete and hard clam shell
2. recycled porcelain
3. crushed concrete
4. limestone marl
5. dredge shell
6. reefballs

Average Cost (per cubic yard delivered) of Substrate Material

Substrate	Delivered Cost	Available
Dredged Oyster Shell	$10	Dwindling (access issue)
Surf Clam Shell	$15	High
Hard Clam	$21	High
Slag	$23	High
Shucked” Oyster shell	$25	High (VA only)
Stone (gabion 2-7”)	$26	Moderate
Crushed Concrete (2-8”)	$35	Intermittent
Coal Fly Ash	$40	High
Marl (marine limestone)	$50	High
Reefballs	$60	High