Estuaries have been armored with artificial habitat to protect coastal infrastructure from erosion, but armoring has negative ecological impacts. Other shoreline protection strategies, such as eco-engineered seawalls and living shorelines, offer more natural, rugose substrata to native species while limiting coastal erosion. Concerns about recruitment of non-indigenous species (NIS) call into question whether eco-engineered structures can be better designed to foster native communities. In southern California, USA, we explored whether concrete reef balls that recruit native Olympia oysters, Ostrea lurida , could be engineered to avoid recruitment of the NIS, Magallana (formerly Crassostrea) gigas .
cover and rugosity to determine if there was a treatment that favored native species recruitment. We deployed four treatment types (n=7 replicates per treatment) embedded into 15 x 15 cm concrete tiles: two with surface shell (100% cover of crushed or large, rugose shell fragments) and two without shell (smooth or rugose concrete) at two sites in San Diego Bay and one site in Newport Bay, California, at two tidal elevations (0 and 0.6 m MLLW) from May to September 2018.
O. lurida recruited in generally higher percent cover and abundance than M. gigas onto all treatments across all sites at 0 m MLLW and treatments that combined 100% shell cover with high rugosity at two of three sites at 0.6 m MLLW, a tidal elevation to which O. lurida rarely recruits. Recruitment strength of both native and non-indigenous oysters showed remarkable context-dependency, varying across sites and treatments, but a generalized recommendation emerged: projects that utilize reef balls across the range of the O. lurida should explore adding shell cover and rugosity to the concrete to favor native O. lurida and discourage non-indigenous M. gigas.