Harnessing the potential of a silent mesopelagic observation system integrated with active acoustics for mesopelagic faunal studies

Over the past six decades, extensive research has explored how marine animals interact with oceanographic sampling equipment, highlighting the impact of light and sound emissions on fauna behavior during surveys. Anthropogenic disturbances—such as sound, light, and hydrodynamic forces—affect quantitative observations in the twilight zone, but the underlying mechanisms remain unclear. To investigate, we developed a "silent" tethered mesopelagic observation system for depths up to 1000 m. It integrates multi-frequency echosounders, dimmable lights, and a sensitive low-light camera to track animal behavior at various spatiotemporal scales. Exclusively battery-powered, the system minimizes acoustic output in the water by eliminating the need for noisy ship engines. In July 2023, we conducted its first test dive in Bermuda, while opportunistically documenting mesopelagic fauna responses to ∼5-minute interval light changes during a new moon phase. Assimilation of multiple perspective echosounder data at 38-kHz revealed a weak positive correlation (R = 0.2657) between swimming direction and distance from the system for detected animals within 20 meters. Animals closer to the system (0–10 m) were more likely to swim towards it, while those farther away (10–20 m) swam in the opposite direction, regardless of light presence. These findings underscore the need for more comprehensive future experiments.