The Deepwater Horizon Oil Spill (DWHOS) necessitated a whole-water-column approach for assessment of disturbance in the epipelagic (0–200 m), mesopelagic (200–1000 m) and bathypelagic (>1000 m) biomes. The deeper regimes collectively form the largest integrated habitat in the Gulf of Mexico (GoM). This habitat received the initial oil/methane discharge, plus millions of liters of dispersant, and contained persistent deep (~1100 m) plumes of oil and dispersant. By some estimates, only half of the discharged oil and none of the methane reached the ocean surface. This demonstrated that DWHOS had an extensive and persistent deep-pelagic component. Before the DWHOS we had only a basic knowledge of the deep-pelagic GoM. Consequently, the large-scale, NOAA-supported Offshore Nekton Sampling and Analysis Program (ONSAP), was implemented as part of the natural Resource Damage Assessment process. Here we provide preliminary results of ONSAP field campaigns in 2011 that sampled the pelagic fauna that inhabit the uppermost 1500 m. During these campaigns a minimum of 717 fish species were represented from the 328,560 specimens collected via midwater trawling. Ongoing analyses are likely to discover and define additional species, as hard-to-identify taxa are resolved. This diversity encompasses one-half of the fish species currently known for the GoM. Also, 60 of the species are new records for the GoM, including numerous undescribed taxa. Fish species richness by functional group and numerical dominance will be presented. These data emphasize that the diversity of the deep-pelagic GoM fauna, like the World Ocean proper, has been historically underestimated.