Offshore oyster reefs along the Big Bend coast of Florida have declined by 88% during the last 30 years, with the most likely mechanism being repeated die-offs due to predation and disease during high salinity periods, driven by episodic and increasing periods of reduced freshwater input to estuaries. These die-off events have led to a conversion from shell to sandbar substrate and rapid loss of elevation (about 3 inches per year). This process appears to be nonreversible, because oyster spat are unable to colonize sandy substrate. A pilot project conducted by University of Florida researchers and funded by grants from The Nature Conservancy, NOAA, and Florida Sea Grant was recently completed in which the assumption that oyster populations on these reefs are limited by substrate was evaluated. Durable hard substrate was placed at the Lone Cabbage Reef complex off Levy County to determine if reefs can become more resilient to periodic declines in freshwater flow by providing a persistent settlement site for naturally occurring oyster larvae. Durable substrate was added in the form of limerock cobbles and about 400 recycled clam aquaculture bags filled with cultch, live oysters, and associated fauna to eight paired treatment and control sites spaced along the highly degraded offshore reef chain.
Post-construction, elevation on the treatment reefs increased by an average of 6.3 inches. Average oyster density on treatment sites increased by 2.65 times on rock, 14.5 times on clam bags, and 9.2 times overall compared with control sites. Recycled clam bags contributed approximately 25% of the surface area on treatment reefs, but accounted for 52% of the oysters observed. Oyster densities on the treatment sites were between 89 times and 125 times those measured at a larger sample of nearby natural reefs, and exceeded the 89th percentile of reported densities at natural and restored reefs in the Gulf of Mexico. Total bird use was higher on treatment sites, but when controlled for elevation, all species but double-crested cormorants and bald eagles preferred control (sand bar) sites. These results indicate that (1) oyster recruitment can be strongly limited by available, durable substrate, especially in high-energy environments; (2) aquaculture by-product materials can play a significant role in the process of restoration; and (3) restoration of oyster reefs and other living shorelines may have impacts on bird community composition. The results of this pilot project were published in the most recent issue of the Journal of Shellfish Research. The article, which documents key effects on oysters, elevation, and bird usage associated with the addition of substrate, can be downloaded here.
Cedar Key Oystercatcher Habitat Restoration
Another restoration project in Levy County is being conducted by the Florida Fish and Wildlife Conservation Commission (FWC). In this project, the intent is to increase elevation of local oyster reefs used as roosting sites for the American Oystercatcher, which is a bird species of high conservation concern in Florida. The Cedar Key area provides excellent wintering habitat for oystercatchers, which rely on offshore oyster reefs for high tide roosts and inshore reefs for foraging on oysters and other invertebrates. Roughly 10% (1,100 birds) of the global population winters in Cedar Key, which is the largest concentration of wintering birds in Florida and second largest across the range of this species.
Although the Cedar Key area shows excellent promise as a long term conservation area for oystercatchers, the extent of oyster reefs in the area has declined by over 80% in the past 30 years. This current study assesses the benefits to oystercatchers of offshore reef restoration, which prior research has shown to be the limiting factor for oystercatchers in this critical wintering ground. The prospects for restoring these oyster reefs are good with locally adapted and tested methods, a history of successful permit applications, and strong interest from state and federal agencies and local aquaculture farmers. This process is likely to increase long term stability and resilience of the roost sites because it uses living reefs that can outpace sea level rise and withstand extreme marine weather events. The overall intent to increase the amount of quality high tide roost habitat available for the American Oystercatcher will help to increase overwinter survival of the species.
The enhancement of two offshore oyster reefs (Corrigan’s and Gomez) used by oystercatchers as high tide roosting locations began in May 2016. Construction materials include clam bags filled with substrate and live oyster spat that have been placed as a barrier revetment to buffer wind-driven wave energy. Limestone rock will be placed behind the revetment, where accretion is already taking place, to function as a larger hard-structure for oysters to attach and grow. Since construction began, deployed materials and their implemented designs have successfully withstood several storms including Hurricane Hermine. The ability for this design to show resistance to extreme physical conditions as well as oyster settlement halfway through its construction is a good indicator that oystercatcher roosting habitat will be improved through the use of this project’s restoration methods. Once construction is complete, long-term monitoring will continue so FWC scientists can collect necessary data to inform future oyster restoration designs to benefit birds and to track demographic information about the American Oystercatcher.