Fish biophony in a Mediterranean submarine canyon.

Although several bioacoustics investigations have shed light on the acoustic communication of Mediterranean fish species, the occurrence of fish sounds has never been reported below -40 m depth. This study assessed the occurrence of fish sounds at greater depths by monitoring the soundscape of a Mediterranean submarine canyon (Calvi, France) thanks to a combination of Static Acoustic Monitoring (three stations, from -125 to -150 m depth, 3 km from coastline) and of hydrophone-integrated gliders (Mobile Acoustic Monitoring; from -60 to -900 m depth, 3-6 km from coastline). Biological sounds were detected in 38% of the audio files; ten sound types (for a total of more than 9.000 sounds) with characteristics corresponding to those emitted by vocal species, or known as produced by fish activities, were found. For one of these sound types, emitter identity was inferred at the genus level (Ophidion sp.). An increase of from 10 to 15 dB re 1 μPa in sea ambient noise was observed during daytime hours due to boat traffic, potentially implying an important daytime masking effect. This study shows that monitoring the underwater soundscape of Mediterranean submarine canyons can provide holistic information needed to better understand the state and the dynamics of these heterogeneous, highly diverse environments.