Patch design principles for oyster reef restoration: Lessons from fish habitat use in natural remnant systems

Over 85% of oyster reef habitat has been lost or significantly degraded globally due to anthropogenic climate change, urbanization, and pollution. These systems provide critical habitat for fish communities, yet the relationship between oyster reef patch characteristics and fish assemblages remains poorly understood, limiting the effectiveness of restoration efforts. To address this gap, we conducted an extensive 3-year survey (2019-2021) of fish assemblages on natural remnant oyster reefs across three estuaries in temperate New South Wales, Australia, using unbaited remote underwater video systems. Sampling was conducted seasonally during summer and winter, providing insights into temporal variability and seasonal influences on fish communities and their associated relationships with reef patch characteristics. This study shows that oyster reef patch characteristics are important drivers of fish abundance, richness, and diversity, with relationships varying across spatial and temporal scales. While there were significant estuary-specific patterns and seasonal variability, some patterns were observed across the systems. Patch shape influenced fish assemblage metrics in contrasting ways, with circular patches supporting greater abundance, while elongate patches supported a greater richness and diversity of fish. Edge habitat was consistently important, with complex edges and higher perimeter-to-area ratios positively associated with fish assemblage metrics. Patch connectivity exhibited contrasting effects: greater network isolation supported fish abundance, while greater connectivity promoted diversity. These findings have practical applications for conservation and management. By identifying which patch sizes, shapes, and configurations maximize fish abundance or diversity, managers and restoration practitioners can strategically design restoration projects to achieve greater ecological outcomes per dollar spent. This knowledge helps avoid investing resources in sub-optimal designs, streamlining project implementation towards configurations more likely to achieve restoration goals. Specifically, restoration practitioners should prioritize creating elongated patches with complex edges and maintain high connectivity among patches to maximize fish habitat value. These recommendations are timely, given the global surge in oyster reef restoration efforts driven by their ecological and economic importance and dramatic decline.