- Water quality. Poor water quality (i.e., pollution, sedimentation, eutrophication, sewage contamination and sudden changes in salinity due to massive inputs of freshwater) can prevent natural recruitment and increase natural mortality among oysters. This can threaten the long-term success of oyster restoration.
- Population and reproductive capacity. Historic over-harvesting, habitat loss and disease have pushed the oyster population below one percent of historic levels. At their current level of abundance, the Chesapeake Bay’s oysters cannot create enough offspring to support full population recovery.
- Hard bottom habitat loss. Without hard bottom habitat, oyster larvae have few suitable locations on which to settle. After decades of damage to natural reefs, a significant amount of hard bottom habitat has been lost.
- Shell loss. When oyster populations are low, the stable feedback loop of shell addition and shell loss breaks down. When shell loss exceeds shell addition, a negative feedback loop drives lower shellfish recruitment and habitat production. While restoration efforts increase habitat and replenish shell, increases in recruitment and/or oyster longevity will be necessary to ensure long-term restoration success.
- Reef connectivity. Historic oyster populations likely relied on river-wide networks of “source reefs” (which produced larvae) and “sink reefs” (which served as substrate on which larvae settled). The fragmentation, degradation and loss of habitat have likely broken this connectivity. Using data-driven reef placement to reestablish this dynamic process at an appropriate restoration scale will be critical to restoration success.
- Spat set variability. Spat set varies from location to location and year to year. Some areas (particularly those in low-salinity waters) may require intensive seeding and re-seeding with hatchery-raised spat if we are to rebuild stocks.
- Climate change. Rising concentrations of carbon dioxide in the atmosphere can contribute to the acidification of the Chesapeake Bay. Research indicates that lower pH levels reduce shell production rates and slow the shell calcification process among oysters and other bivalves, lowering the amount of natural shell available in the environment.
Innovative restoration techniques could increase the likelihood of success for oyster restoration projects. Incorporating experimental design concepts at the planning stage will allow for rigorous evaluation of restoration outcomes and allow adaptive innovation in reef design.
- Funding availability. Large-scale oyster restoration will require funds to support shell, alternative substrates and manpower.
- Shell and alternative substrates availability. High demand has limited the amount of natural shell available for oyster restoration. Alternative substrates (including crushed concrete, stone, fossil shell and fabricated reef structures) have been used with varying degrees of success. While these substrates may offer benefits over shell (including poaching deterrence and increased persistence over time), they may also interfere with some legal fishing practices and gear.
- Hatchery-raised spat availability. The availability of hatchery-raised spat-on-shell depends on funding and the capacity of the hatcheries themselves. A shortage in the spat supply can delay restoration work.
- Permitting. Several state and federal permits are required for reef construction to take place. The permit application and review process must be integrated into our partners’ restoration timelines. Unexpected issues during this process can cause delays and prevent restoration from moving forward as planned.
- Bottom leasing. Parcels of tributary bottom that are or can be leased for aquaculture are unavailable for restoration without reconciliation with leaseholders or state owners. This can limit the amount of bottom habitat available for restoration in select tributaries, particularly in Virginia.
- Designation of sanctuaries. Oyster sanctuaries, or designated areas where oyster harvest is not permitted, provide legal protection to restoration sites. This can protect the oysters within them and allow these oysters to serve as sources of larvae for those reefs that are open to harvest.
The illegal harvest of oysters has long posed a problem in the Chesapeake Bay. Reduced funding and manpower limit the Maryland Natural Resources Police and Virginia Marine Police forces’ ability to enforce harvest regulations for all fish species and protect oyster reefs. Poaching could threaten the success of oyster restoration in sanctuaries.
The navigation requirements of commercial, recreational and maritime safety vessels must be taken into account when selecting oyster restoration sites. Reefs must leave sufficient navigational clearance to allow for local vessel traffic, and/or be marked with appropriate navigational aids. This substantially reduces the potential areas where restoration can take place.
Oysters are valued as a source of food, a driver of local economies and a contributor to the health of the Chesapeake Bay. Public support for oyster restoration—especially from citizens who live near tributaries that have been selected for restoration projects—is critical to long-term restoration success.
Factors Influencing Progress
A range of factors—including resource availability, regulation enforcement and public support—impact the abundance of oysters in the Chesapeake Bay and the Chesapeake Bay Program’s ability to restore oyster reefs.