LIVING SHORELINES REEF BALL DESIGN: SHELL COVER, RUGOSITY, AND TIDAL ELEVATION IMPACT FAUNAL RECRUITMENT IN SOUTHERN CALIFORNIA, USA

Estuaries have been armored with artificial habitat to protect coastal infrastructure from erosion, but armoring may 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. The Port of San Diego plans to build a living shoreline using concrete reef balls that recruit the native oyster, Ostrea lurida, but the Port wants to avoid recruitment of non-indigenous species (NIS), especially the non-indigenous oyster, Crassostrea gigas. I modified concrete tiles that acted as proxies for reef balls with added shell cover and rugosity to determine if there is a treatment that can achieve these goals. I deployed six treatment types embedded into 15 x 15 cm concrete tiles: two with surface shell (100% cover of crushed or large shell fragments), two without shell (smooth or rough concrete), and two reference treatments (50% shell cover and terracotta). Seven replicates per treatment were deployed at two sites in San Diego Bay and one site in Newport Bay, California, USA, at 0 and 0.6 m Mean Lower Low Water (MLLW) from May to September 2018. I used three-way ANOVAs, ANCOVAs, and PERMANOVAs to test for the effects of tile orientation, tidal elevation, shell cover, and rugosity on the recruitment of oysters and other fauna. O. lurida recruited in higher abundance and generally higher percent cover than C. 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. Some NIS recruited in lower abundance to 100% shell treatments and/or rugose treatments, although the effects were not consistent across species. Recruitment strength of both native and NIS varied across sites and treatments and should be tested on a site-specific basis prior to reef ball deployment, but I suggest projects that utilize reef balls in living shoreline designs in southern California should generally add shell cover and rugosity to the concrete to favor native species and discourage NIS.