Recording acoustic data in the twilight and midnight zones at close range

The twilight (200-1000 m depth) and midnight (1000-2000 m depth) zones are still nowadays one of the most unexplored places on earth, despite hosting one of the most diverse and biomass-rich environments. Active acoustics is one of the most employed techniques to investigate the twilight zone but vessel- borne echosounders are limited in depth due to signal loss with time. To overcome this limitation new acoustic devices are being progressively designed and deployed at depth. Since 2014, the Spanish Institute of oceanography (IEO-CSIC) and the Institute of Oceanography and Global Change (IOCAG-ULPGC) have been collaborating in the fine-tuning of a new acoustic instrument attached to the rosette able to endure pressures at 6000 m depth. A recent publication presented results from the initial surveys and some recommendations. This article provides further descriptions and recommendations.