Evening choruses in deep waters are associated with mesopelagic diel vertical migrations

Active and passive acoustic observation methods offer an effective approach to studying deep-sea fauna where direct monitoring is particularly challenging. Some of these mesopelagic organisms are part of Deep Scattering Layers (DSLs) which are recognized as being among the largest biomass aggregations of the planet. Current quantitative estimates of this biomass vary by an order of magnitude and it is essential to improve monitoring methods in the face of emerging initiatives to exploit this key ecological resource. In this study, we employ a combination of passive and active acoustic datasets to describe concurrent temporal patterns of DSL migration and changes of the soundscape off two volcanic islands of the subtropical NE Atlantic. We report a chorus centred at 2.5 kHz, matching those previously documented in the Pacific and Indian Oceans, and observed here for the first time in the North Atlantic. This chorus event coincides with the upward migration of organisms from deep scattering layers to surface waters. Furthermore, the maximum received sound levels of this chorus are positively correlated with the measured acoustic backscatter at 38 kHz of the DSL migrating to <150m depth. These results suggest that calibrated Passive Acoustic Monitoring (PAM) measures of chorus intensity could be applied globally as a cost-effective and powerful indicator of migrating organisms from DSLs, potentially reflecting biomass through their associated acoustic backscatter.