Distribution, ecology, and behavior of midwater eels (Anguilliformes: Nemichthyidae and Serrivomeridae) from remotely operated vehicle surveys in the eastern Pacific Clarion-Clipperton Zone

Ecological studies of many pelagic, deep-sea animals are often limited due to their remote habitats and sampling constraints. In the eastern tropical Pacific, numerous studies have been published about the distribution of demersal fishes across the region, yet studies for midwater taxa are lacking. Here, we utilized visual survey transects from an area targeted for polymetallic nodule mining to examine the drivers behind the distribution of two mesopelagic predators, snipe eels (family Nemichthyidae) and sawtooth eels (family Serrivomeridae). Additionally, we compared the vertical distributions of their prey and examined whether the two eels displayed different behaviors in situ. Time of day was not a statistically significant driver of eel distribution, although the shallowest observations were during the night, indicating that some in the populations of Nemichthyidae and Serrivomeridae had migrated weakly to the upper 200 m. Eel distributions were found to be affected by both depth and dissolved oxygen, with Nemichthyidae observed in shallower and lower oxygenated waters and Serrivomeridae in deeper and more oxygenated ones. Two prey taxa (families Euphausiidae and Sergestidae) were abundant at shallower transect depths, which likely drives the distribution of Nemichthyidae, as pelagic crustaceans constitute their primary prey. In terms of behavior, Serrivomeridae were more active and were oriented primarily vertically, suggesting that Nemichthyidae may be less active or under lower predation pressure due to the hypoxic environment in which they occur. This work highlights the importance of visual surveys in obtaining in situ data as a complement to traditional sampling methods, particularly for collecting data on poorly understood animals.