Diversity of mesophotic coral reefs fish fauna through passive acoustics

Since it is not an easily accessible environment for humans, few is known on the deeper part of coral reefs (mesophotic coral ecosystems). In this study, fish sounds were used as a proxy to study Polynesian coral reefs. The aim of the study was to determine how the acoustic fish biodiversity varied depending on the depth and the type of island (atoll vs. high island), two features known to impact species richness and diversity. The link between benthic cover and both the acoustic α-diversity and acoustic fish communities was established at −20, −60 and −120 m in three atolls and three high islands. More than 45 different fish sound types were described. The number of some sound types increased with depth while the opposite was observed for other sound types. In most islands, acoustic α-diversity (Shannon index based on sound types) falls between −20 and −60 m but not between −60 and −120 m supporting the existence of a transition zone between two distinct fish communities. The highest similarities between acoustic communities were found between −60 and −120 m, the lowest between −20 and −120 m. Overall,
acoustic community composition was mainly driven by the depth due to benthic coral cover differences, and to a lesser extent, by the type of island. These results show fish sounds exhibit a bathymetric stratification. Moreover, differences can reflect different habitat features. It opens new perspectives in the study and monitoring of mesophotic coral ecosystems using passive acoustics.