Addressing Data Gaps in Deep-Pelagic Fauna: A Case Study of an Apex Predatory Fish Family in the Meso- and Bathypelagic Domains

Prior to the Deepwater Horizon Oil Spill in 2010, very little was known about what lives in the deep-pelagic Gulf of Mexico, especially at the depths surrounding the wellhead. Only a handful of surveys had ever sampled the water column below 1000 m anywhere in the world, and none in the area of the oil spill. After the spill occurred, two consecutive projects were conducted between 2010 and 2019 to survey the deep-pelagic (meso- and bathypelagic zones; 200-1500 m depth) fauna of the Gulf of Mexico. The first project, associated with the 2010-2011 NRDA, employed two types of midwater trawls to collect deep-pelagic fauna: a 10-m2 MOCNESS (MOC10) and a commercial-sized, high-speed rope trawl. The second project, DEEPEND, used the MOC10 and followed the same sampling scheme as the first. Here, we present results of a monographic treatment of a top meso/bathypelagic predator fish family, Chiasmodontidae (swallowers), including species composition, abundance, biomass, and size distribution. Over 1,200 individuals were collected representing 17 species from four genera. The dominant species were Chiasmodon pluriradiatus/asper, Pseudoscopelus altipinnis, P. scriptus, and Dysalotus alcocki. Six species collected are new records for the Gulf of Mexico including Kali colubrina, K. parri, P. aphos, P. cordilluminatus, P. scutatus, and Chiasmodon braueri. Species of Pseudoscopelus were collected in the mesopelagic (above 1000 m) while species of Dysalotus and Kali were collected in the bathypelagic (below 1000 m). Species of the genus Chiasmodon were collected throughout the water column. Comparisons of these results between the two gear types highlights a major data gap with respect to larger-sized predatory deep-sea fishes. Given the importance of top-down control by apex predators in open ocean systems, filling these data gaps is an essential step to gain a more comprehensive understanding of deep-pelagic ecosystems, earth’s largest by both volume and animal number