Interspecific facilitation, elevation, and site influence survival and growth for intertidal Ostrea angasi restoration

Introduction The flat oyster Ostrea angasi previously formed extensive reefs throughout temperate regions of Australia. These reefs were overharvested and destroyed after European colonization and have been functionally extinct for >150 years. While large‐scale subtidal O. angasi restoration has occurred, there are no intertidal restoration attempts despite historical occurrence. Further, while facilitative relationships with subtidal foundation species have been explored, intertidal species have received little consideration. Objectives and Methods We investigated the feasibility of intertidal restoration by testing O. angasi survival and growth across intertidal elevations spanning the shallow subtidal to low intertidal across three sites in Victoria, Australia. Further, we tested if co‐occurring foundation species—seagrass ( Zostera muelleri ) and mangroves ( Avicennia marina )—can facilitate O. angasi survival and growth. Results Oyster survival varied between sites and elevations. At one site, survival probability increased with decreasing elevation. Mangroves facilitated oyster survival at one site, likely through reduced temperature and light conditions. However, all oysters at this site died within 5 months post‐deployment. At another site, only oysters deployed under mangroves survived the entire duration of the experiment. We found no evidence of seagrass facilitation on growth and survival. Conclusion This study provides the first evidence that O. angasi can survive in the intertidal. While shallow water restoration appears feasible, survival will likely depend on underlying site conditions. Exploring novel restoration strategies for functionally extinct shellfish ecosystems can help bring these ecologically, culturally, economically, and socially important ecosystems back into coastal areas.