Depth Zonation and Environmental Characteristics of Shallow and Mesophotic Coral Reef Fish Assemblages in Comoros

Abstract

Mesophotic coral reef ecosystems (MCEs) support unique biodiversity and can act as refuges for species that also occur on heavily impacted shallow coral reefs. However, we still know relatively little about the physical and environmental structure of MCEs and the species that characterise them, especially within the Western Indian Ocean. To address this gap, we surveyed fish and macrobenthic assemblages between 10 and 100 m depth at three islands of the Comoros Archipelago using Baited Remote Underwater Video Systems (BRUVS). We also recorded temperature and photosynthetically active radiation (PAR) using a Conductivity–Temperature–Depth (CTD) probe. The structure of the fish assemblages was influenced by substrate type, relief and depth. More diverse fish assemblages were found on higher relief reefs, while low-diversity fish assemblages were associated with shallow sand and deeper low-relief sand/reef mosaic habitats. The reef fish assemblages delineated into three groups, corresponding to three depth zones: the photic zone (12–45 m), upper-mesophotic zone (45–62 m) and lower-mesophotic zone (63–98 m). The upper–lower mesophotic break aligned with the theoretical bottom of the euphotic zone at PAR1%surface. Macrobenthic assemblages comprised scleractinian corals on photic reefs, which transitioned at PAR3.5%surface to assemblages of non-light-dependent Octocorallia (soft corals and fans), Gorgonia (sea whips) and Antipatharia (black corals) on the mesophotic reefs. Scleractinian corals were found deeper in Comoros, which may confer greater resilience to climate change. The photic and lower-mesophotic reefs supported unique fish assemblages of depth specialist species, while the upper-mesophotic zone appeared to be a transition zone for fish assemblages. The presence of numerous large depth-generalist fishery species at depths > 50 m suggests that the mesophotic zone may be providing a depth refuge for fishery species that are overfished on shallow reefs. Our findings highlight potential resilience in the coral reef ecosystems of Comoros and illustrate the importance of mesophotic coral reefs in conservation planning.