Abstract

The development of advanced acoustic and optical imaging techniques along with autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs) has enabled high-resolution benthic mapping and biological characterization of mesophotic coral ecosystems (MCEs) over large spatial scales. Underwater vehicles can be grouped into tethered and untethered vehicles. Tethered vehicles allow the pilot on the ship to remotely control the vehicle using live video streaming. Tethered vehicles include ROVs, towed sleds, and drop cameras. Untethered vehicles, particularly AUVs, rely on autonomy for navigation, obstacle avoidance, and situational awareness. Untethered vehicles also include manned submersibles and Lagrangian buoys. Hybrid remotely operated vehicles combine the capabilities of both AUVs and ROVs into one vehicle. Acoustic mapping using side-scan or multi-beam sonars can offer much broader coverage than optical imaging and can be useful for habitat mapping. However, acoustic data generally have lower spatial resolutions and are more difficult to interpret since similar acoustic returns can correspond to more than one bottom type. Optical imaging from still and video cameras at close range delivers high-resolution data that are easy to interpret. Increased precision in underwater navigation enables time series of georeferenced optical and acoustical imagery for fine-scale detection of changes over longer temporal scales of years to decades. This chapter provides a summary of the advantages and disadvantages of both tethered and untethered vehicles for studies of MCEs, including their various imaging modalities, as well as illumination, positioning, navigation, and imaging considerations.