Habitats and fish communities at mesophotic depths in the Mexican Pacific

Aim: Mesophotic ecosystems, found at the limit of light penetration in the ocean, are rich in biodiversity and harbor unique ecological communities. However, they remain among the least studied habitat zones on earth due to high costs and technological limitations. Here, we characterize mesophotic communities in two marine reserves across a range of habitat types, depths, and temperatures using submersible technologies, with the goal of understanding the processes that structure these communities across biogeographic regions. Location: The Bay of La Paz and the Revillagigedo Archipelago, Mexico. Taxa: Fish and algal species. Methods: We used a small and inexpensive Remotely Operated Vehicle (ROV) to conduct roving-swim surveys of major habitat types in depths from 20–100 m. With the resulting binary data of fish species presence, we used generalized linear mixed models and canonical correspondence analysis and to determine whether biogenic habitat, depth, and/or temperature best explained species richness and community structure across reef and non-reef substrate. Results: We identified 81 species, including new depth records for nine fish species and a new geographic record for one fish species. Our surveys included large undocumented rhodolith beds (free-living coralline algae) and mesophotic algal communities, in addition to diverse communities of soft corals and sponges. Fish species richness was positively associated with rocky substrate and warmer water, and reef fish communities differed significantly by depth, temperature, and biogenic habitat. Main conclusion: Our results highlight the importance of biogenic habitat in structuring communities across gradients of depth and temperature. We also demonstrate the effectiveness of a small and economical ROV for conducting mesophotic surveys in remote regions. Our methods and results provide a framework that can be used to greatly increase the biogeographic and taxonomic scope of mesophotic research, especially for readily identifiable taxa such as fish.