Shell cover, rugosity, and tidal elevation impact native and non-indigenous oyster recruitment: Implications for reef ball design

Estuaries have been armored with artificial habitat to protect coastal infrastructure from erosion, but armoring can have 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 structures can be engineered 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 increase recruitment of O. lurida and discourage recruitment of non-indigenous Pacific oysters, Magallana (formerly Crassostrea) gigas. We modified 15 × 15 cm concrete tiles with added shell cover and rugosity and deployed four treatments: two with surface shell (crushed, large fragments) and two without shell (smooth, rugose) at two sites in San Diego Bay and one site in Newport Bay at two tidal elevations (0.2 and 0.8 m MLLW) from May to September 2018. O. lurida generally recruited in highest abundance and percent cover to 0.2 m MLLW relative to 0.8 m MLLW; at 0.8 m MLLW, shelled treatments with high rugosity favored higher cover and abundance of O. lurida relative to unshelled, low-rugosity treatments. In contrast, M. gigas percent cover was generally higher on unshelled treatments relative to shelled treatments, and rugosity never had statistically significant positive effects on their abundance or cover. Recruitment strength and percent cover of both oyster species showed remarkable context-dependency but a generalized recommendation emerged across sites with strikingly different recruitment strengths: deployment of lower-elevation reef balls would favor O. lurida recruitment, and addition of shell fragments and rugosity will increase native O. lurida recruitment at higher tidal elevations. Shell additions and rugosity may also discourage non-indigenous M. gigas recruitment and cover in some contexts. Importantly, given the now-global distribution of M. gigas, resource managers elsewhere may wish to explore such eco-engineering strategies to control the spread and space domination of this and other NIS.