COS 124-3 - Mesophotic communities: diel patterns and environmental vertical gradients of fish sounds

Abstract: Mesophotic coral ecosystems (MCEs) represent the deepest areas of tropical coral reefs, but they remain largely unexplored due to the challenges associated with accessing them. However, passive acoustic monitoring (PAM) methods have shown promise as a suitable approach to studying fish communities in this unique habitat. This study aimed to investigate the diversity of fish sounds in relation to depth and diel cycle in the Polynesian islands. Additionally, we examined the link between benthic cover and fish sound diversity.

Our findings indicate that acoustic fish α-diversity generally decreased between depths of 20 and 60 meters, but remained consistent between depths of 60 and 120 meters. The composition of fish sound types was primarily influenced by depth, likely due to variations in benthic coral cover, rather than the type of island. We also conducted a detailed analysis of the diel cycle of the two most abundant frequency-modulated fish sounds. Results revealed a significant increase in sound activity at sunset, with sound occurrences concentrated at the beginning and the end of the night. Furthermore, the presence and number of sounds varied across different studied archipelagos, highlighting potential differences in vocal activity or bathymetric distribution of the emitting species.

These findings demonstrate a clear bathymetric stratification of fish sounds and their ability to reflect distinct habitat characteristics. This study opens up new possibilities for monitoring mesophotic coral ecosystems through passive acoustics. Additionally, the characterization of diel cycles and bathymetric differences, considering the geographical distribution of these sounds, represents a significant advancement in identifying the species responsible for this sound production.