Restoration efforts with native Eastern oyster, Crassostrea virginica, in Chesapeake Bay have been extensive, and there has been a continuing search for reef structures that will be effective under a range of environmental conditions. We quantified population structure, density, abundance and biomass of Eastern oyster and hooked mussel, Ischadium recurvum, on a novel concrete modular reef deployed subtidally (~ 7 m depth) in the lower Rappahannock River during October, 2000. The reef provided 3-D architectural complexity, substrate stability, and extensive surface settlement area (nearly 75 m2 of reef surface over 5 m2 of river bottom). Upon deployment, the reef was neither seeded with oyster spat nor exploited. After 4 ½ years of deployment (May 2005), we took 120 stratified random samples over the reef. The reef had been colonized heavily by oysters and mussels, which recruited and survived at densities per m2 of reef surface area ranging from 28-168 for oysters and from 14- 2177 for mussels. These surface densities on the modular reef translate to 1085 oysters and 8617 mussels per m2 of river bottom, which are among the highest recorded for natural and restored oyster reefs. Hence, the reef supported about 10,000 suspensionfeeding bivalves per m2 of river bottom. The size structure of oysters indicated the presence of four year classes, with approximately half of all oysters more than two years old and therefore of reproductive age. Oyster density per m2 of reef surface area was positively correlated with mussel density up to 2000 mussels per m2 , after which oyster density declined somewhat. This reef apparently provides an architecture that is conducive for settlement, growth and survival of the Eastern oyster and hooked mussel in suitable subtidal habitats, and which should therefore be considered as a viable alternative reef structure for Eastern oyster restoration
