Deep-sea biodiversity at the extremes of the Salas y Gómez and Nazca ridges with implications for conservation

Abstract

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 region, 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, broad-scale 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 and 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 its intrinsic value, this region should be comprehensively protected using the best available conservation measures to ensure that the Salas y Gómez and Nazca ridges remain a globally unique biodiversity hotspot.

Fig 1. Deep-sea camera system deployments on the Salas y Gómez and Nazca ridges.

Dotted lines depict exclusive economic zones (EEZs). Red dots depict camera deployment sites. Bathymetric data source: GEBCO Compilation Group (2020) GEBCO 2020 Grid (doi:10.5285/a29c5465-b138-234de053-6c86abc040b9).

Fig 2. Representative and unique taxa observed with deep-sea camera systems around Rapa Nui and Salas y Gómez islands.

(A) Hexanchus griseus (B) Hydrolagus melanophasma (C) Chromis mamatapara* (D) Parapristipomoides squamimaxillaris (E) Homolidae, Yaldwynopsis sp. (F) Geryonidae (G) Rexea sp. (H) Polymixia salagomeziensis*. *endemic species.

Fig 3. Representative and unique taxa observed with deep-sea camera systems at Desventuradas Islands.

(A) Nemadactylus gayi*, Scorpaena orgila*, Squalus sp. (B) Pterygotrigla picta (C) Hydrolagus sp. (D) Lithodidae (E) Antimora rostrata (F) Squalus sp. *endemic species.

Fig 4. Principal Components Analysis (PCA) of environmental variables at camera deployment sites.

Data were Box-Cox transformed prior to analysis. Des. = Desventuradas.

Fig 5. Vulnerable marine ecosystem indicator taxa, including deep-sea corals and sponges, observed on deep-sea camera deployments along the Salas y Gómez and Nazca ridges.

(A) Sala y Gómez—552 m (B) Desventuradas– 1,714 m.

Fig 6. Environmental variables at the deep-sea camera system deployment locations.

(A) Crust age (younger → older: yellow → red) was extracted for the nearest seamount to each deployment location. (B) Nitrate levels; bubbles depict nitrate levels at deployment location and depth (low → high nitrate: small/light → big/dark). (C) Absolute broad-scale bathymetric position index (BPI); bubbles depict absolute BPI values at deployment location (low → high BPI: small/light → big/dark), basemap depicts absolute BPI values of region (low →high: dark blue → gray). (D) Depth of each deployment; bubbles depict depth at deployment location (small/light → big/dark: shallow → deep), basemap depicts bathymetry (deep → shallow: dark blue → white). Purple lines in Panel D are 500 m contour lines. Bathymetric data source: Global Multi-Resolution Topography (GMRT) [80, www.marine-geo.org].

Fig 7. Distance-based redundancy analysis (dbRDA) ordination to investigate the relationship between the environmental variables and invertebrate assemblages.

Bray-Curtis similarity matrix based on ln(x+1) transformed MaxN higher level invertebrate taxa. Symbol sizes are proportional to depth.

Fig 8. Fish assemblage characteristics (taxa richness, MaxN, diversity, and evenness) at Desventuradas and RN/SyG on hard and soft-bottom habitats.

Box plots showing median (black line), mean (red line), upper and lower quartiles, and 5th and 95th percentiles.

Fig 9. Distance-based redundancy analysis (dbRDA) ordination to investigate the relationship between the environmental variables and fish assemblages.

Bray-Curtis similarity matrix based on 4^th root-transformed MaxN higher level fish taxa. Symbol sizes are proportional to depth.