Evidence of vertical stratification in marine environments: insights from passive acoustic monitoring in French Polynesia

The stratification of species distribution in marine environments is well-documented, reflecting the influence of depth-related factors such as light availability, temperature, pressure, and habitat structure on the organization of marine communities. This study aimed to determine whether this stratification can also be highlighted using Passive Acoustic Monitoring (PAM), a technique proven effective in the study of various marine habitats. Sounds were recorded at 300 m in July 2022 off Moorea Island (French Polynesia) using a SNAP acoustic recorder. These data were compared to both those collected at shallower depths at Moorea Island (20, 60, and 120 m) and to data from other Polynesian islands obtained during a previous, non-concurrent study. Fish sounds in the recordings were manually identified. Richness and abundance of sounds from 20 m to 300 m depth were compared to assess whether fish communities in the altiphotic reef, upper mesophotic reef, lower mesophotic reef, and rariphotic zone could be distinguished based on the sounds they produce. Comparisons between zones over a 24-h period revealed for the first time the existence of acoustic stratification, with a fourfold decrease in species richness, from the altiphotic reef to the rariphotic zone, along with a general decline in sound abundance. However, this general pattern was not uniform in terms of abundance throughout the day, as sound abundance increased with depth at dusk from 950 sounds per hour at 20 m to 3600 at 300 m. It further confirms that diel activity patterns were also evident in light-deprived zones. This first exploration of vertical acoustic stratification based on fish sounds highlights the undeniable contribution of PAM to studies on the ecology of deep-sea communities.