Shellfish reefs have been lost from bays and estuaries globally, including in the Swan-Canning Estuary in Western Australia. As part of a national program to restore the ecosystem services that such reefs once provided and return this habitat from near extinction, the mussel Mytilus galloprovincialis was selected for a large-scale shellfish reef construction project in this estuary. To assess the potential filtration capacity of the reef, estuary seston quality, mussel feeding behavior, and valve gape activity were quantified in the laboratory and field during winter and summer. In general, estuary water contained high total particulate concentrations (7.9–8.7 mg L⁻¹). Standard clearance rates were greater in winter (1.9 L h⁻¹; 17 °C) than in summer (1.3 L h⁻¹; 25 °C), the latter producing extremely low absorption efficiencies (37%). Mussel valves remained open ~97% and ~50% of the time in winter and summer, respectively. They often displayed erratic behavior in summer, possibly due to elevated temperatures and the toxic microalgae Alexandrium spp. Despite numerous stressors, the reef, at capacity, was estimated to filter 35% of the total volume of the estuary over winter, incorporating 42.7 t of organic matter into mussel tissue. The reefs would thus make a substantial contribution to improving estuary water quality.

Shellfish reefs around the world have become degraded, and recent efforts have focused on restoring these valuable habitats. This study is the first to assess the efficacy of a bouchot-style reef, where mussels were seeded onto wooden stakes and deployed in a hypereutrophic estuary in Australia. While >60% of translocated mussels survived one month, after ten months, only 2% remained alive, with this mortality being accompanied, at least initially, by declining body condition. Mussel survival, growth, body condition and recruitment were greater on the top section of the stake, implying that the distance from the substrate was important. More fish species inhabited the reefs (31) than unstructured control sites (17). Reefs were also colonised by a range of invertebrate species, including 11 native and six non-indigenous species. However, the number of individuals declined from 4495 individuals from 14 species in December 2019 to 35 individuals representing 4 species in March 2021, likely due to hypoxic bottom water conditions following unseasonal rainfall. Although the bouchot-style reefs were unable to sustain mussels and other invertebrates over sequential years, this approach has the potential to be successful if deployed in shallow water or intertidal zones, which are largely exempt from biotic and abiotic stressors characteristic of deeper waters in microtidal estuaries.

Shellfish reefs are among the most imperiled marine ecosystems, and restoration efforts are increasing worldwide. In the Swan–Canning Estuary (Western Australia), four shellfish reefs were constructed using limestone boulders and seeded with Mytilus galloprovincialis. Survival was lower than expected, potentially reflecting summer water temperatures exceeding its thermal tolerance. To identify shellfish species most likely to persist under present and future conditions (+1 °C), we developed thermal-suitability models combining seasonal temperature profiles with species-specific realized thermal distributions. Sixteen shellfish species were assessed. Species with intermediate thermal optima performed best: Saccostrea glomerata and Trichomya hirsuta remained within their modelled ther-mal-suitability ranges for >95% of the time. In contrast, M. galloprovincialis remained within its modelled thermal-suitability range for 70.8–74.0% of the time under present conditions, and 63.3–65.4% following warming. Cool-affinity species, including Ostrea angasi and Mimachlamys asperrima, were increasingly constrained by high summer temperatures, whereas warm-affinity species, such as Saccostrea cucullata, were limited by low winter temperatures. These results indicate that the persistence and future species composition of shellfish reefs in warming estuaries will depend strongly on their thermal tolerance and highlight the value of incorporating climate projections into restoration planning.