Synaphobranchid Swarms at Seamount Summits: Biological Hotspots in the Abyssal Ocean

The abyssal ocean, 3000 to 6000 meters, is generally characterized as a food limited, low density community, especially for larger fauna such as abyssal fishes. Although they are not highly abundant and thus challenging to study, abyssal fishes are often top predators as well as scavengers in the benthic community and thus play a critical role in the ecosystem. Using baited camera systems and remotely operated vehicle surveys, we investigated the differences between the abyssal scavenging communities of the abyssal plains and seamounts of the western Clarion Clipperton Zone, an area threatened by impending deep-sea mining activities, to test the seamount refuge hypothesis. Because of their complex topography, abyssal seamounts in the Clarion Clipperton Zone will likely be spared from direct manganese nodule mining activities and thus may act as a refuge for impacted abyssal plain communities; however, this assumes these communities have substantial similarities. We found stark community differences between abyssal seamounts and the surrounding plains with two of our three seamounts hosting high relative abundances of synaphobranchid eels (Ilyophis sp.) and generally lower numbers or a complete absence of the abyssal plain dominant rattails (Coryphaenoides spp.). Additionally, one seamount hosted the highest relative abundance of fishes ever recorded in the abyssal ocean by an order of magnitude, with 115 individuals (Illyophis sp.) recorded in a single video frame and none present at deployments on the surrounding abyssal plain. This is evidence for a previously undiscovered biological hotspot at abyssal depths in an area threatened by deep sea mining activities, and it challenges the idea that fishes are only found at low densities in the abyssal ocean. Potential mechanisms supporting high fish biomass on the seamounts will be explored.