Comparison of recent survey techniques for estimating benthic cover on Caribbean mesophotic reefs

Highly divergent estimates of benthic cover of sponges have been reported for Caribbean mesophotic reefs (90-100 m) based on quadrat point-intercept data collection using 2 methods: visual surveys conducted in situ by technical divers, and analyses of photographs taken by unmanned underwater vehicles (UUVs). The second method has been criticized for potential errors from image distortion caused by variable camera angle relative to the substratum, but without a broader comparison of both methods. We find that studies that have used the UUV-based method are advantageous for a number of reasons, most importantly: (1) access to the full mesophotic zone, (2) higher sample replication, and (3) reduced likelihood of sampling bias. For tech diving surveys conducted at 91 m, i.e. the deepest depth reported using this method but only mid-way through the mesophotic zone, studies have reported particularly high sponge cover (~80 vs.