Passive acoustic monitoring of fish diversity in mesophotic coral ecosystems, potential global-change refuges

peer reviewedMesophotic Coral Ecosystems, the deeper part of coral reefs (below −30/40 m), remain largely unexplored mainly because difficult to access by humans. In this study, fish sounds were used as a proxy to study Polynesian mesophotic 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 community composition was established at −20, −60 and −120 m in three atolls and three high islands of French Polynesia. More than 45 different fish sound types were described. In most islands, acoustic α-diversity (Shannon index based on sound types) decreased 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 depth likely 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