DEEPHOPE in Scientia Marina (2023)

Despite their ecological importance, mesophotic coral ecosystems (MCEs)—found between roughly 30–40 meters and 170 meters below the surface—remain largely unexplored due to the challenges posed by such depths. Yet research has already shown that MCEs host fish communities quite distinct from those found in shallower waters, making them a compelling subject for further study. Among the mesophotic fish species, some are “vocal”: the characteristic sounds they produce allow researchers to identify and distinguish them. Passive acoustic monitoring has therefore emerged as a powerful tool for studying biodiversity at these depths.

In the present study, researchers focused on acoustic data collected by divers from UNDER THE POLE across six islands in French Polynesia (see: Fish sounds of photic and mesophotic coral reefs: variation with depth and type of island, Raick X. et al., Coral Reefs, 2023). Two sounds that dominated the soundscape at 120 meters could not be linked to any known fish species in that study. The scientists therefore examined the timing and locations of these recordings in greater detail to gain more insight into these unknown sounds.

The results revealed that both sounds were primarily produced at sunset and during the night, and were almost entirely absent during the day. While dominant at 120 meters, these sounds were also detected at 60 meters, though less frequently. Additionally, their abundance and daily patterns varied from one island to another.

The researchers concluded that the fish producing these sounds are likely nocturnal species, characteristic of MCEs, and most abundant around 120 meters depth. However, the differences observed between islands suggest a variability that remains poorly understood—potentially linked to island type (high island vs. atoll) or geographic differences among the studied archipelagos. Future studies will help clarify the geographic variation in these sounds and ultimately identify the fish species responsible.

Summary from the article "Diel cycle of two recurrent fish sounds from mesophotic coral reefs."

Read the full article via the link below. 

Auteurs  - Raick X, Collet P, Under The Pole Consortium, Lecchini D, Bertucci F & Parmentier E.