For quantitative comparisons, only images from depths of 350, 450, and 600 m were analyzed for each feature. Initial analyses included surveying every other image on each transect for trawl or drag marks and the proportion of soft substrate, totaling over 54,000 images analyzed. For quantitative comparisons, images along a transect that were <75% soft substrate were then used to count benthic megafauna, totaling 22,188 images analyzed. From these, all of the visible megafauna were counted in every other image to avoid duplicate counts. The primary benthic megafaunal taxa observed included cnidarians, sponges, and echinoderms. The height above the seafloor that the AUV must be flown over rough terrain and the angle of the camera make identification to the species level unreliable, so instead, we used a morphotype classification that allowed for a consistent level of resolution of the observed fauna. These categories included “wire coral,” “antipatharian fan,” “octocoral fan,” “scleractinian fan,” “scleractinian bush,” “sea pen,” “Eguchipsammia,” “encrusting zoanthid,” “stalked crinoid,” “unstalked crinoid,” “brisingid,” and “sponge.” Urchins were also present on most features, but the abundances made counting them time prohibitive; thus, they were not included. The gold coral, Kulamanamana haumeaae, common in precious coral beds in the NHWI (4, 2931), was notably absent from all included transects. By coincidence, extensive areas of live reef as shown in Fig. 4F also did not occur in the AUV images on the targeted depth transects in either treatment.

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