Distinct cross-shelf gradient in mesophotic reef fish assemblages in subtropical eastern Australia

The hypothesized importance of mesophotic (30 to 150 m depth) reefs justifies the recent trend in scientific scrutiny of these systems. However, to enable assessment of complex concepts such as connectivity and reef dynamics, baseline assemblage characteristics must first be established. This study used baited remote underwater video (BRUV) technology to investigate the assemblage structure of predatory and scavenging fishes across 4 mesophotic reef bands at ca. 30 to 82 m depth, at 2 locations 25 km apart in subtropical eastern Australia. We aimed to quantify patterns in predatory and scavenging fish assemblage structure at these reefs across the continental shelf and relate this to putative structuring environmental variables. Strong cross-shelf gradients were identified in species richness and overall assemblage composition. While the pattern of latitudinal affiliation did not change across the shelf, predatory and scavenging fish assemblages at non-adjacent reefs were statistically distinct (PERMANOVA interaction term p = 0.012), and best (but not well) explained by depth alone (BIOENV rho = 0.396). A high proportion (15 to 45%) of the fish species at each reef band were found only within that band. These cross-shelf trends contrasted with those described from more complex shelf topography such as at the Solitary Islands (250 km south), and did not match published patterns of epibenthic assemblage structure. Our results highlight the need for detailed information on mesophotic reef assemblage structure to support marine conservation and reserve design initiatives, rather than relying on generalised trends from the literature.