Effect of a coastal defence upgrade on fish assemblages in a tropical-temperate transition zone

Coastal defence structures, such as breakwaters, are being upgraded to accommodate sea level rise and climate uncertainty, yet the ecological impacts of these modifications remain poorly understood. We used a Multiple-Before-After-Control-Impact (M-BACI) design to assess how upgrading an established breakwater influenced fish assemblages in a tropical-temperate transition zone in New South Wales (NSW), Australia. Across 3 years of visual surveys at the breakwater and nearby natural reefs, we recorded 24373 individual fish from 155 species and 65 families. Assemblages were dominated by Labridae (wrasses, 16%) and Pomacentridae (damselfishes, 9%). Notably, 73 species (47%) were only observed on the natural reefs, while 20 species (12%) were uniquely observed on the breakwater. No statistically significant changes were detected in fish abundance, assemblage composition, or functional group richness following the upgrade. However, patterns in non-metric multidimensional scaling and boxplots suggested greater divergence between breakwater and reef sites post-upgrade for herbivorous, reef-associated, and tropical species. These directional trends may reflect early-stage responses to increased structural complexity, but high variability and short post-upgrade monitoring limit inference. Our findings suggest that well-designed upgrades can avoid major disruption to existing fish communities, while highlighting the need for longer-term monitoring to detect ecological shifts and inform sustainable infrastructure planning.