Maryland Department of Natural Resources

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Tred Avon Restoration

​Large-scale restoration efforts began in 2015 in Tred Avon sanctuary. The initial implementation phase of the restoration project was completed in 2021, with a total of 130 acres of reef restored. Monitoring is underway to gauge success.

The restoration plan for Tred Avon may be found here.


The oyster metrics are quantitative evaluation criteria used to determine the restoration success of an oyster reef and tributary.

  • A successfully restored reef should have:
    • A ‘minimum threshold’ of 15 oysters and 15 grams dry weight per square meter covering at least 30 percent of the target restoration area at six years post restoration;
    • A ‘target’ of 50 oysters and 50 grams dry weight per square meter covering at least 30 percent of the target restoration area at six years post restoration;
    • Two or more oyster year classes present; and
    • Stable or increasing spatial extent, reef height, and shell budget.
  • A successfully restored tributary is one where:
    • 50 to 100 percent of the currently restorable oyster habitat (CROH) has oyster reefs that meet the reef-level metrics above. Restorable habitat is defined as area that, at a minimum, has appropriate bottom quality and water quality for oyster survival AND
    • 8 to 16 percent of historic habitat (Yates Bars), and preferably more, has oyster reefs that meet the reef-level metrics above


Three-year and six-year monitoring efforts are used to assess the oyster metrics for each reef. 

From the 2019 Monitoring Report: ‘Two different types of gear were used to collect samples, depending on reef substrate type. Divers were used to collect samples from reefs with substrate materials that were not amenable to patent tong sampling (stone and fossil shell substrate reefs). Patent tongs were used to collect samples from all other reef types (seed only, mixed-shell base, reference, and premet reefs) because it is more cost efficient than using divers. Previous field comparisons on natural oyster reefs revealed no difference in sampling efficiency between oyster densities estimated using divers and those estimated using patent tongs. A similar field comparison on restored reefs showed that densities estimated using patent tongs resulted in statistically significantly smaller numbers of oysters than those estimated using divers. Monitoring results in this report show oyster densities and biomass relative to the established Oyster Metrics benchmarks (e.g., minimum threshold oyster density of 15 oysters per m2 to be considered successful). Because two different gear types were used for sampling, and results of research on the relative sampling efficiencies of those gears vary, it may not be appropriate to use data in this report to compare relative efficacy among reef treatment type.’ 

So far, at three-year monitoring (2019-2021) 86% of reefs met the minimum threshold success criteria for oyster density and 90% met the minimum threshold for biomass in Tred Avon. ​​No reefs have yet to receive six-year monitoring in the Tred Avon Sanctuary.  Oyster monitoring reports can be found here:

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Tred Avon Restoration Reefs

Below is an interactive map with the restoration reefs in the Tred Avon Sanctuary. The legend and layers are located in the upper right-hand corner. You can click on a reef to see more information. The information was last updated in March 2023.​ 

Restoration sites are identified as ‘seed only’ restoration (hatchery-produced spat-on-shell added to existing remnant reefs) or ‘substrate + seed’ restoration (adding reef-building substrate to the reef footprint, followed by planting with hatchery-produced spat-on-shell) or ‘premet’ (sites that already meet the Oyster Metrics target oyster density (50+ oysters per m2) and oyster biomass (50+ grams per m2) and did not receive initial restoration).

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