From shell to shelter: investigating small reef fish abundance on degraded seafloor restored with mussels and shell material

Abstract Introduction Reef‐forming shellfish facilitate biodiversity through providing structural complexity in benthic habitats. Globally, reef‐forming shellfish have been overharvested to near extirpation, with a corresponding loss in biodiversity. Mussel shell material, an aquaculture by‐product, has the potential to rehabilitate ecosystem services and support recovery in degraded ecosystems. In oyster restoration, relief from the seafloor influences species abundance and diversity; however, little is known about relief in the context of mussel restoration. Objectives To assess whether mussel and shell habitats of varying relief influence the abundance of small reef fish. Methods Mussel shell was deployed at three reliefs (high: 0.54 ± 0.18 m; medium: 0.42 ± 0.12 m; and low: 0.26 ± 0.03 m), both with and without live mussels. Small reef fish abundance was measured on the shell habitats and adjacent unaltered seafloor after 1 month (2024) and 1 year (2025) using standard monitoring units for the recruitment of fish. Results Shell relief had no influence on small fish abundance and diversity remained consistent across all treatments, reliefs, and sampling periods. However, after 1 year, there was a 2‐fold higher fish abundance on shells alone and 5‐fold on shells with live mussels compared to the unaltered seafloor, with a 2‐fold higher abundance on shells with live mussels compared to shell alone. Conclusion These results demonstrate that mussel shells, both alone and together with live mussels, offer a cost‐effective, scalable approach to enhance small fish abundance on degraded benthic habitats, with relief having no influence on outcomes.