An advanced sensor platform for acoustic quantification of the ocean twilight zone

Abstract

The ocean twilight zone (OTZ) is the vast, globe-spanning, layer of water between 200 and 1000 m depth—home to diverse communities of small fishes, cephalopods, crustaceans, and gelatinous organisms. Yet, little is known about the biology, abundance, biomass, and distribution of these organisms. The OTZ is difficult to sample due to a combination of organism patchiness and avoidance, and difficulties capturing fragile species. Recent evidence suggests that the global OTZ biomass may be sufficient to commercially harvest. Furthermore, much of this biomass performs daily vertical migration (DVM) and may play a potentially critical role in regulating Earth’s climate through the export of carbon to the deep ocean. Deep-See, an advanced senor platform, was developed to fill the technological void for characterizing the OTZ. This towed vehicle integrates wide-band, split-beam acoustics (1–500 kHz) with optical, environmental, and eDNA sensors that can address many of the challenges associated with sampling in the OTZ. Data from the inaugural cruise in August 2018 highlight that (i) a surprisingly high abundance of organisms can be found outside the dense sound scattering layers and (ii) the target strength of many organisms that perform DVM changes with the depth, which is critical to estimate biomass.