Densities and zonation patterns of native and non-indigenous oysters in southern California bays

Worldwide, artificially armored shorelines can dominate available estuarine habitat. Increases in artificial substrata in southern California, USA, coincide with increased abundances of nonindigenous species. The US west coast Olympia oyster, Ostrea lurida, may be particularly sensitive to changes in natural habitat availability and recently experienced large-scale declines in abundance. Simultaneously, the non-indigenous and often invasive Pacific oyster, Crassostrea gigas, has been introduced along the US West Coast, including into southern California estuaries where its impact is unstudied. We recorded C. gigas and O. lurida densities from October 2010 through July 2017 throughout southern California on several habitat types and as a function of tidal elevation as a critical baseline for evaluating impacts of C. gigas. Ostrea lurida was present in higher proportions than C. gigas in hard substrate-dominated habitats, whether natural or human-introduced. A strong zonation pattern emerged; C. gigas achieved its maximum density above +0.4 m MLLW versus O. lurida at or below +0.2 m MLLW. These data reveal the optimal tidal elevation for placement of a restored O. lurida bed and establish a critical baseline and techniques for evaluating future changes in oyster density.