Abstract

Mesophotic reefs (>30m depth) are relatively undisturbed ecosystems compared to their shallow water counterparts, often supporting high reef fish biomass including targeted fishery species such as Snapper and Grouper. However, owing to the specialist skills and equipment needed to reach mesophotic depths, many of these reefs are completely unexplored. Here, using a combination of tech diving and closed-circuit rebreathers, we conducted fish point transects and produced photomosaics to quantify fish and benthic communities on the mesophotic reefs of Grand Cayman at four sites, reaching depths of up to 50m. Macroalgae was the dominant benthic component across the four sites, however, we found significant differences between all sites in terms of benthic composition. For fish communities, invertivores consistently showed the highest species richness, with omnivores showing the highest abundance. Meanwhile, macrocarnivores showed the highest levels of fish biomass, despite having the lowest abundance of all trophic guilds at three of the four sites. Lastly, all sites showed significant differences in the trophic composition of fish communities, and their species composition. The high biomass of macrocarnivores suggests these mesophotic reefs may support important stocks for fisheries, while high omnivore abundance could suggest generic trophic strategies are favoured on the mesophotic reefs around Grand Cayman. Ultimately, quantifying these ecological communities is the first step in understanding the stability of fish communities on Grand Caymans mesophotic reefs, and the role deeper reefs perform for supporting local fisheries.