A total of seven seamounts in the NWHI and ESC were surveyed in 2014 and 2015 using the AUV Sentry. On the basis of the trawling history, the seamounts were categorized as Recovering or Still Trawled. Sites that were once actively trawled, but were protected with the establishment of the US EEZ in 1977 (21), were placed into the Recovering treatment and included Academician Berg, Bank 11, and SE and NW Hancock Seamounts. Sites that are still actively trawled, including Kammu, Yuryaku, and Koko Seamounts, were placed into the Still Trawled treatment (table S1).

AUV photo surveys with a length of ~30 to 40 km were designed to include replicate 1-km transects at depth intervals of 50 m from depths of 200 to 700 m. This survey design was then replicated on two to three sides of each seamount to reduce effects of within-seamount variability on comparisons between treatments. The depth range of 200 to 700 m was chosen to encompass the full range of depths that were part of the historic trawl and coral fisheries, which were concentrated at 300 to 600 m (21). The AUV flew at a height of ~5 m above the seafloor at a rate of 0.45 to 0.65 m/s, taking photos at a rate that ensured a continuous visual (photo) survey of the seafloor. Images were taken with a down-looking digital still camera, and each individual AUV image covered approximately 12 m2 of seafloor. Observations were made from all of the >536,000 dive images, with a subset used for the quantitative analyses as described below. Additional qualitative observations and images were obtained on dives with the Pisces IV and V submersibles, which returned to the same sites in 2016 and 2017, as well as to Colahan Seamount.

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