Offshore oyster reefs in the Big Bend Coast of Florida have declined by 88% during the last 30 y, 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 (ca 8 cm/y). This process appears to be nonreversible, because oyster spat are unable to colonize sandy substrate. It is hypothesized that the addition of durable hard substrate would make reefs more resilient to periodic declines in freshwater flow by providing a persistent settlement site for extant larvae. This article reports a test of the assumption that oyster populations on these reefs are limited by substrate, and documents key effects on oysters, elevation, and avian usage associated with the addition of substrate. Durable substrate was added in the form of limerock cobbles and recycled clam Mercenaria mercenaria (Linnaeus, 1758) aquaculture bags filled with cultch, live oysters, and associated fauna to eight paired treatment and control sites spaced along a highly degraded offshore reef chain. Elevation on treatment reefs increased postconstruction by an average of 16 cm. Mean oyster density on treatment sites increased by 2.65×on rock, 14.5×on clam bags, and 9.2×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 treatment sites were between 89×and 125×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 ofMexico. Total bird use was higher on treatment sites, but when controlled for elevation, all species but doublecrested cormorants [Phalacrocorax auritus (Lesson, 1831)] and bald eagles [Haliaeetus leucocephalus (Linnaeus, 1766)] 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 byproduct 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 avian community composition. Future research should be aimed at understanding whether durable substrate can also confer longer term resilience to oyster reef communities.
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Vol. 35 • No. 2