Adaptive Restoration Reveals Potential Effect of Tidal Elevation on Oyster Restoration Outcomes

Abstract

Oyster restoration practitioners augment habitat with hard substrata to increase oyster recruitment, but success can be complicated by non-indigenous species recruitment and physical processes such as sedimentation. From June 2012 to June 2014, we examined changes in shell cover and recruitment of native Ostrea lurida and non-indigenous, global invader Crassostrea gigas onto a constructed oyster bed in Alamitos Bay, CA, USA relative to an un-augmented control mudflat. Due to 72% shell loss after 1 year and informed by a concurrent study on elevational distributions of both oyster species, we augmented the oyster bed with additional shell on the lower-elevation, seaward side. One year later, O. lurida densities were significantly greater on the lower-elevation bed relative to the control and 19.4 times greater than reference populations throughout Alamitos Bay. Crassostrea gigas recruited in densities equivalent to reference populations. Adaptively restoring the bed via shell additions at a lower elevation achieved higher shell retention, increased O. lurida densities, and increased the ratio of native versus non-indigenous oysters. Our increased understanding about the role of tidal elevation in targeted restoration success of a native relative to a non-indigenous oyster species will inform future restoration designs.