Identification and characterization of deep sea mesopelagic scattering layers in the Gulf of Mexico using multiple-frequency, broadband acoustic backscatter

Acoustic backscatter data were collected using multiple-frequency (18 kHz and 70 kHz broadband and narrowband, 38 and 120 kHz narrowband) scientific fisheries echosounders from the surface to 1500 m in the Gulf of Mexico in August 2015 as part of the GOMRI DEEPEND project. By combining acoustic backscatter spectra, 10 m² MOCNESS net tows, and acoustic scattering models; scattering layers were categorized into broad, taxonomic groups (e.g. swim-bladdered fish, fish without swim bladders, crustaceans). In some locations, targeted net tows were used to sample a particular backscattering layer revealing (in some cases) the specific species responsible for the scattering layer. The vertical movements of these layers relative to hydrographic water column properties (primarily temperature and dissolved oxygen) were quantified. Additionally, the biomass of the various taxonomic categories was calculated from the acoustic data and compared with previous estimates from net tows and the literature. We also assessed the benefits and limitations of broadband echosounders for deep-sea mesopelagic surveys.