Feasibility of restoring native oyster reefs in highly modified urban estuaries

Abstract Introduction Urbanized estuaries are highly altered systems characterized by modified shorelines, degraded marine habitats, and abundant non‐native species. In South Australia's largest urbanized estuary, community‐based restoration initiatives aim to restore native flat oyster reefs ( Ostrea angasi ) lost over a century ago. However, extensive modification of the estuary and ubiquity of non‐native Pacific oysters ( Magallana gigas ) create uncertainty on the feasibility of restoration. Objectives This project assessed whether hand‐held restoration units (shell gabion baskets) deployed by community volunteers can facilitate recruitment and habitat formation by O. angasi , and whether specific depths limit recruitment of M. gigas . Methods Initiated by a fishing conservation charity, this community‐led project deployed 70 restoration units to assess site suitability and restoration efficiency for larger‐scale reef restoration. Restoration feasibility was evaluated by (1) quantifying oyster recruitment and biodiversity 1 year after deploying units, and (2) deploying settlement substrates across a depth gradient to identify depths that favor native oysters while limiting non‐native oyster recruitment. Results Restoration units supported high O. angasi recruitment (mean ± 1 SE = 1144 ± 223 per unit) and associated species diversity (24 species across seven classes) after 1 year. Oyster recruitment was significantly influenced by depth for both oyster species but in opposite directions: M. gigas dominated at shallower depths, while O. angasi increasingly dominated with greater depth. Conclusions Results demonstrate that shell gabion units can effectively facilitate native O. angasi recruitment at depths that limit non‐native M. gigas while also generating positive biodiversity outcomes in a highly modified estuary.