- Preliminary Habitat Assessment of Floating Oyster (Crassostrea virginica) Gardens (Delaware)
Community volunteers, known as “oyster gardeners,” are growing the eastern oyster (Crassostrea virginica) off private docks in coastal lagoons of southeastern Delaware, known locally as “Inland Bays,” to aid restoration efforts. Today in Delaware, the oyster gardening program is a fraction of the size of those in neighboring states but has expanded tenfold from its inception in 2003. Oyster aquaculture can provide many of the same services as natural oyster reefs, such as providing a hard substrate for colonization by marine epifauna, which accordingly attracts fishes and mobile invertebrates (e.g., O’Beirn et al. 2004). Many species of economic and ecological importance are considered habitat limited in the Inland Bays, particularly in juvenile refuge and forage areas. Floating oyster gardens can provide additional habitat at small scales while supplementing oyster spawning stocks without difficult and costly types of habitat modifications.
Oyster gardening activities are taking place along most of the Atlantic Coast into the Gulf of Mexico across an extremely wide gradient of conditions. The ecological impact of oyster gardening programs has received little attention and is absent from the literature with regard to Delaware’s Inland Bays. This study is the first to identify the macroepifauna that use floating oyster gardens as habitat. We also quantified the growth and survival of oysters and several water quality parameters. No natural reefs remain in the Inland Bays; descriptions of reef fauna in the region are for Delaware or Chesapeake Bay, and direct comparisons would not be appropriate. Our data will be used as a baseline as the oyster gardening program expands within the Inland Bays.
The Delaware Inland Bays have a surface area of 83 km2, into which a 777 km2 watershed drains. The average depth is 1.2 m. The wild oysters present are few and widely scattered (John Ewart, Delaware Sea Grant Marine Advisory Program, pers. comm.). Many seagrass (Zostera marina and Ruppia maritima) beds have also disappeared, and there has been a net loss of approximately 800 ha of tidal wetlands in the watershed over the past century (DIBEP 1993). These bays are experiencing eutrophication, high turbidity, periodically hypoxic conditions, poor tidal flushing, annual fish kills, harmful algae blooms, and low species diversity, especially in the 2 km2 of man-made dead-end canals (DIBEP 1993).
Oyster gardening sites are located throughout the three Inland Bays, almost exclusively on canals (Figure 1). By placing oysters in floating cages, oyster gardeners use these filter feeders to remove sediments and algae from the water column to improve local water quality and clarity while facilitating the removal of nitrogen and phosphorous. Once the oysters are 40–50 mm (1–2 years old), they are transplanted throughout the Inland Bays in rip-rap used for shoreline stabilization.
We randomly selected and sampled three replicate oyster gardens that were not cleaned or otherwise disturbed by the oyster gardeners between July and October 2007 in each of three embayments (Rehoboth, South Bethany, and Fenwick Island), for nine total study floats. Each study site consisted of a floating cage constructed of PVC pipe (ca. 10 cm diameter) and vinyl-coated wire mesh (14-gauge, 25 × 25 mm mesh), each with two 46 × 46 × 23 cm wire baskets. Floats were tied to docks located at the residences of volunteer oyster gardeners. Before deployment, all oyster gardens were power-washed to remove all fouling organisms, in part because the sessile invertebrates and algae that grow on hard marine surfaces can lead to oyster mortality when occurring at high densities. This initial cleaning also allowed epifauna subsequently associated with the oysters to be attributed to the spatial and temporal location of stocking and not to prior events in the culture process.
Three different floats in each embayment were sampled at monthly intervals for species richness and abundance such that each float was sampled only once. We selected 15% of the live and dead oysters and measured shell length to the nearest 0.1 mm. Epifaunal sampling was conducted with a net (3 mm mesh) supported by a square PVC frame measuring 61 × 61 cm. The net was quickly placed...