The Salas y Gómez and Nazca ridges are underwater mountain chains that stretch across 2,900 km in the southeastern Pacific and are recognized for their high biodiversity value and unique ecological characteristics. Explorations of deep-water ecosystems have been limited in this area, and elsewhere globally. To characterize community composition of mesophotic and deep-sea demersal fauna at seamounts in the region, we conducted expeditions to Rapa Nui (RN) and Salas y Gómez (SyG) islands in 2011 and Desventuradas Islands in 2013. Remote autonomous baited-cameras were used to conduct stationary video surveys between 150–1,850 m at RN/SyG (N=20) and 75–2,363 m at Desventuradas (N=27). Individual organisms were identified to the lowest possible taxonomic level and relative abundance was quantified with the maximum number of individuals per frame. Deployments were attributed with associated environmental variables (temperature, salinity, dissolved oxygen, nitrate, silicate, phosphate, chlorophyll-a, seamount age, and bathymetric position index [BPI]). We identified 55 unique invertebrate taxa and 66 unique fish taxa. Faunal community structure was highly dissimilar between and within subregions both for invertebrate (p < 0.001) and fish taxa (p = 0.022). For fishes, dogfish sharks (Squalidae) accounted for the greatest dissimilarity between subregions (18.27%), with mean abundances of 2.26 ± 2.49 at Desventuradas, an order of magnitude greater than at RN/SyG (0.21 ± 0.54). Depth, seamount age, broadscale BPI, and nitrate explained most of the variation in both invertebrate (R² = 0.475) and fish (R² = 0.419) assemblages. Slightly more than half the deployments at Desventuradas (N=14) recorded vulnerable marine ecosystem taxa such as corals and sponges. Our study supports mounting evidence that the Salas y Gómez and Nazca ridges are areas of high biodiversity with high conservation value. While Chile and Peru have recently established or proposed marine protected areas in this region, the majority of these ridges lie outside of national jurisdictions and are under threat from overfishing, plastic pollution, climate change, and potential deep-sea mining. Given their intrinsic value, these areas should be comprehensively protected using the best available conservation measures and thereby ensuring that the Salas y Gómez and Nazca ridges remain a globally unique biodiversity hotspot.

Data collection

Remote autonomous benthic landers with baited camera systems were used to conduct stationary video surveys at RN/SyG and the Desventuradas. These systems include high-definition cameras (Sony Handycam HDR-XR520V 12 megapixel) encased in a borosilicate glass sphere that are rated to 11,000 m depth. The camera is fixed at a 45° declination from the horizontal plane and has a 35 mm equivalent focal length of 29.8 mm, which results in a horizontal 62.27° and a vertical 37.53° angular field-of-view. Lighting at depth was achieved through a high-intensity LED array directed using external reflectors, and depth gauging was conducted using an external pressure sensor. The anchor line extends 2 m from the seafloor to optimize the field of view to both capture large mobile scavenging megafauna and allow for the identification of organisms. The deep-sea camera system is positively buoyant, resulting in an ascent rate of 0.5 ms⁻¹, and weighted with a 22 kg external weight, resulting in a descent rate of 1.5 ms⁻¹. The weight is released by burn wire, activated using onboard battery voltage, and the camera system was located for recovery by communication of an onboard VHF transmitter and locating antennae, with backup location via communication with the ARGOS satellite system. Up to three deep-sea camera systems were simultaneously deployed, 22 February – 4 March 2011, at 20 stations between 150–1,850 m depth (X  = 1,104.6 ± 433.6) along the island slopes of SyG and RN and the slopes of two seamounts west of SyG (Fig 1). At Desventuradas, 27 successful deployments were conducted, 10 – 26 February 2013, at 75 – 2,363 m depth (X  = 755.8 ± 649.2) (Fig 1). Cameras were deployed with a minimum distance of 500 m from another deployment location, and each camera was baited with chopped frozen fish and deployed for 1 to 5 hours with an average footage per deployment of 112 min at Desventuradas and 129 min at RN/SyG. Deployments targeted geological features and specific depths based on available bathymetry. Sampling design aimed to represent a gradient of depths away from a feature (e.g., island, seamount), and thus deployments generally occurred in a line radiating out from the shallow depths of a feature to deeper waters. The substrata for each camera deployment were classified as hard or soft, based on video footage. Seafloor type was defined as soft if the seafloor observed during the deployment consisted of ≥80% sediment (sands and muds), and hard if <80% of the seafloor observed during the deployment was composed of sediment. Hard habitats included rock, cobbles, pebbles, gravel, biogenic material (including live coral), coral rubble, and shell material.

Biological data processing

Annotations were made from the video footage based on standardized taxonomic nomenclature according to the World Register of Marine Species (WoRMS). Video files were stored in Tator, which is a cloud-based collaborative video annotation platform developed by CVisionAI (http://cvisionai.com/). Frame grabs of representatives of each species were taken in Tator for archival and identification purposes. Individuals were identified to the lowest possible taxonomic level using the Hawaii Underwater Research Laboratory (HURL) database ([62]; http://www.soest.hawaii.edu/HURL/HURLarchive/guide.php), the NOAA Office of Ocean Exploration Benthic Deepwater Animal Identification Guide (https://oceanexplorer.noaa.gov/okeanos/animal_guide/animal_guide.html), FishBase, regional guides and reports of trawl data from the Salas y Gómez and Nazca ridges, and input from regional taxonomic experts. The maximum number of individuals of each species in a single video frame (MaxN), rather than a total tally per deployment, was recorded to ensure that individuals were not double counted. Taxa were classified as vulnerable marine ecosystem (VME) taxa based on (Ardon et al. 2014, Davies et al. 2015).