Distribution of deep-water sound scattering layers and diel vertical migration of mesopelagic fish in the Whittard Canyon, NE Atlantic

The mesopelagic zone plays a crucial role in the global carbon cycle and supports a key area for developing sustainable mesopelagic fisheries due to its significant fish biomass. Active acoustic scattering techniques are particularly well-suited for synoptic studies of fish and zooplankton distribution given that organisms scatter sound differently as the frequency changes. We estimated the distribution of sound scattering layers, the diel vertical migration (DVM) and the abundance of Mueller's pearlside (Maurolicus muelleri), a key mesopelagic fish in the Whittard Canyon deep-sea submarine canyon system in the NE Atlantic using established active acoustic techniques (EK60 echosounder). Environmental data were collected from 50 CTD stations. Environmental DNA samples were obtained at various depths at each CTD station to address a ground truth component of backscatter and to explore potential interactions between deeper scattering layers and other mesopelagic species. EK60 backscatter data was processed using Echoview 14 and unique multi-frequency acoustic discrimination algorithms. The most pervasive phenomenon observed acoustically was a regular DVM evident along a series of stationary and transects throughout the canyon and the interfluve/channel system. Two strong backscatter signals were encountered in mesopelagic (650-700 m, average Nautical Area Scattering Coefficient S_a =~2568 nmi²/m²) and pelagic (45 -70 m, average S_a = ~59 nmi²/m²) at 18kHz and 38kHz in nighttime transects. Moreover, stationary data illustrate a prominent signal at nighttime in pelagic waters (25-75 m, average S_a = ~716 nmi²/m²). Non-migratory scattering layers were noticeable in deep mesopelagic zones between 800-1000 m. However, a significant inter-canyon variability of deep scattering layers was observed, with stronger layers found in two of the four canyons surveyed. The characterisation of the sound scattering layer variability probably reflected the heterogeneity in hydrographic regimes within the multi-channel canyon system. The present study will advance our understanding of the function of migrating mesopelagic fish in carbon cycling and their ramifications on carbon fluxes and sequestration. A comparison of the dynamic Whittard Canyon system and the adjacent non-canyon shelf-edge areas (based on historical acoustic data) has identified the canyon region as a possible hotspot for continental margin carbon cycling.