Design at depth: A rapid ecological protocol to shape conservation potential of human-made reefs by linking structural features with presence and activity of marine life

Human-made reefs (HMRs) of various origins are proliferating worldwide with claims of conservation benefit, but ecological assessment protocols are often limited to single structure types, impeding learning and comparison for future design. We developed a novel, low-cost protocol for the rapid ecological assessment of HMRs and applied it to 70 subtidal structures in Cozumel, Mexico, including shipwrecks, concrete debris, coral restoration modules, sculptures, infrastructure and rock piles. Some of these structures were designed for conservation, while others were not. Drawing on existing methodologies for coral reefs, rocky reefs and single HMR types, we undertook standardised data collection on (1) physical characteristics such as size, structural complexity, depth, and materials, and (2) marine life including “general” metrics of richness and abundance, as well as “targeted” metrics of conservation relevance such as presence of endangered or invasive species. We used a generalised linear mixed effects model to explore associations between HMR characteristics and marine life, identifying relationships between species richness and abundance and structural factors including depth, structure size, and substrate type. We also evaluated the influence of conservation-focused alterations, such as coral planting and rock addition, and compared HMRs created for coral restoration with similarly sized natural reefs. We identified 78 species of fish and five species of mobile invertebrates across HMRs, with behaviours such as sheltering and feeding observed across multiple species and structure types. This method provides a transferrable tool for assessing the ecological performance of diverse HMRs, with data collection and analysis acting as a preliminary proof of concept for its use. However, results represent a snapshot based on a single daytime survey and should be interpreted in light of uneven representation of structure types and potential under-detection of small or nocturnal species. We reflect on practical challenges and opportunities encountered during its application, and implications for sustainable reef design and management in a rapidly changing seascape.