DOI: 10.5061/dryad.pnvx0k6j3
title: Habitats and fish communities at mesophotic depths in the Mexican Pacific

description: 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.
description: All surveys were conducted using a small (roughly 1
 m<sup>3</sup> and 11 kg) and highly portable ROV (BlueRobotics
 BlueROV2) capable of diving to 100 m and equipped with four 1500 lumen
 lights, two cameras that filmed in 1080 p resolution (one 8MP Raspberry Pi
 v2 Camera and one GoPro HERO3 or HERO4), a Measurement Specialties
 MS5837-30BA sensor that measured depth and temperature, and a 250 m tether
 (see Appendix S1 in Supporting Information for image of ROV). At each
 site, the ROV operator would descend the ROV directly to the benthos and
 begin roving swim surveys. The duration of surveys ranged from 7 to 65
 minutes, depending on weather and ocean conditions, battery duration, and
 cruise schedules. Video, depth, and temperature were continuously recorded
 and saved for later analysis. Footage from the GoPro and Raspberry Pi
 cameras were synchronized for each dive and cut into 5-minute transects,
 starting at the time the ROV reached the benthos. In each video, all fish
 were identified to the lowest possible taxonomic level using Robertson and
 Allen (2015). Biotic and abiotic habitat characteristics of each transect
 were also noted. Biogenic habitat represented the dominant foundation
 species group (<i>sensu</i>Dayton 1972) that contributed to
 three-dimensional habitat, including coral, unidentified filamentous
 algae, rhodolith beds (also termed maërl, comprised of colonies of
 slow-growing, free-living coralline algae [Foster 2001]), turf (broadly
 encompassing encrusting organisms such as algae, bryozoans, hydroids, and
 crustose coralline algae), and burrows in sand (see Appendix S2 for
 representative images of habitat classifications). Substrate
 classifications included reef (continuous hard substrate or large
 semi-continuous boulders), and non-reef (small patches of unconsolidated
 hard substrate over sand or sand). Depth (m) and temperature (°C) for each
 transect were averaged and rounded to the nearest integer.
description: This dataset only records the presence of a given species in a
 transect, not the abundance.