Densities and Habitat Distributions of Two Oyster Species in Alamitos Bay and Newport Bay, California

Several recent studies have found that man-made substrata in marine habitats harbor a higher ratio of non-native to native species compared to natural substrata. Alamitos Bay, CA is a highly urbanized area with little natural substrata available as habitat for sessile organisms.  Alamitos Bay urbanization could affect distributional patterns of California’s only native oyster, Ostrea lurida, and the non-native Japanese oyster, Crassostrea gigas, especially compared to nearby Newport Bay which has more natural substrata available as habitat. We explored whether there were differences in the densities of each species in the two bays and whether their distributions on natural versus human-introduced habitat types differed. We randomly placed replicate 0.25 m² quadrats (n=20-30) and used point-contact techniques to quantify percent cover of each habitat type and then counted oysters within quadrats to calculate densities at each site. Suitable hard substrata were available at all sites in both bays implying that habitat availability does not constrain oyster abundances for either species.  However, Ostrea lurida density was 5.4 times greater in Newport Bay than Alamitos Bay and native oysters were always more abundant than C. gigas in both bays at all sites.  The proportion of non-natives to total oysters was relatively invariant in Alamitos Bay but higher on human-introduced versus natural habitats in Newport Bay.  Conservation efforts to restore native oysters in Alamitos Bay may be beneficial for increasing the relative abundance of O. lurida relative to non-native C. gigas.