DEEPLIFE in Coral Reefs (2023)

Contrarily to shallow-water coral reefs, mesophotic coral ecosystems remain largely unknown because they are not easily accessible to humans. Biodiversity associated with corals is therefore also little known, and only sparse information is available on fish communities at these depths.

In order to fill this gap, the present work aimed to characterize the diversity of fish communities in mesophotic coral reefs and determine how this acoustic fish diversity varies depending on the depth and the type of island in six Polynesian islands: three atolls (Rangiroa, Raroia and Tikehau) and three high islands (Bora Bora, Mangareva and Moorea). For this purpose, passive acoustics (study of sounds) was used.

The fieldwork was realized by deep divers from UNDER THE POLE. At each island, three different depths (−20, −60 and −120 m) were sampled simultaneously on the external slope of the reef. At each depth, an underwater long-term acoustic recorder was deployed during 72 h to record fish sounds. In addition, for each island and depth, photo-quadrats (30 quadrats per depth) were realized to characterize the benthic cover (e.g., sand, living scleractinian coral, dead coral, gorgones, black coral, etc.).

Results of the present study show that Polynesian underwater soundscapes are rich, from shallow-water coral reefs to deeper reefs. Fish sounds exhibit a stratification driven by depth, with maximum acoustic diversity at -20 meters. Diversity then decreases down to -60 meters, and is equivalent between -60 and -120 meters. The depth of -60 meters appears to be a transition zone since certain acoustic communities of fish from -20 and -120 meters are found there. This zone is explained by a progressive transition from one assemblage of coral species to another one at this depth. Thus, fish sound diversity reflects indirectly different benthic features. In addition to the differences related to depth and associated benthic cover, part of the acoustic variability is explained by the type of island (high islands or atolls).

In conclusion, acoustic monitoring of fish assemblages reveals subtle differences in benthic cover. The present study opens perspectives in the monitoring of mesophotic coral ecosystems using passive acoustics, which is a promising tool to follow temporal changes in the depths.

Summary of the article "Fish sounds of photic and mesophotic coral reefs : variation with depth and type of island."

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Authors - Raick X., Di Iorio L., Lecchini D., Gervaise C., Hédouin L., Under The Pole Consortium, Pérez-Rosales G., Rouzé H., Bertucci F., Parmentier E.