Oyster reefs are widely recognized as ecosystem engineers that provide a suite of goods and services, however, overharvesting, destructive fishing practices and environmental degradation have resulted in significant declines in oyster populations globally. In the northeastern United States, eastern oyster, Crassostrea virginica, populations have been reduced to near functional extinction. Effective restoration efforts require insight into how biological and physical gradients regulate oyster recruitment, survivorship and growth. We conducted a field experiment to quantify how aerial exposure duration, predator exclusion and vertical relief of created oyster habitats influence densities and growth of oysters and other sessile organisms in a soft-sediment, intertidal environment. In April 2015, three 0.25 m2 recruitment units of high (oyster shells positioned upright in concrete) and low (oyster shells cable-tied to vexar mats) relief were deployed at three intertidal elevations (+0, + 0.5, + 1.0 meters MLWL) at three replicate sites. Within each relief type and across all elevations, open, cage control and full cage treatments were randomly assigned. Monthly sampling was conducted to quantify sedimentation, predator abundances, and epifaunal settlement. Settlement units were sampled in November 2015 and densities, lengths and mortality of sessile organisms were recorded.
Results/Conclusions
Our preliminary results indicate that intertidal elevation is the most powerful predictor of oyster recruitment. Oyster density was significantly greater (>300%) at the lowest (+0 m) elevation treatment than at either +0.5 m or +1.0 m, which were not significantly different from each other. Appreciable sedimentation was present at the deepest elevation treatment. At low elevation, oyster density was negatively correlated with sedimentation. Surprisingly, oyster densities were not significantly different among vertical relief or predator exclusion treatment. Densities of other sessile organisms, such as the barnacle Semibalanas balanoides, were also significantly greater (>600%) at +0 m compared to higher elevation treatments, and were not significantly different among vertical relief or caging treatment. Densities of oyster and barnacle scars, which are indicative of predation events, and trap sampling indicate predation events and predator abundance, respectively, are greatest at our deepest elevation treatment. Further sampling, resuming in April 2016, will quantify overwinter survivorship and growth of juvenile oysters subjected to potential cold-stress from aerial exposure. Results from this study will provide managers with information on the physical (i.e. elevation, sedimentation) and biological (i.e. predator communities) characteristics conducive to oyster recruitment, survivorship and, potentially, more successful restoration effort outcomes.