Mémoire

Abstract

Tropical coral reefs host 25% of marine biodiversity, yet cover less than 0.3% of the ocean's surface. While numerous studies have been carried out in the altiphotic reefs (i.e. the part of the reefs extending from the surface to a depth of 30 - 40 m), research on the mesophotic reefs (i.e. extending from a depth of 40 m to the depth at which hermatypic corals are no longer found) of French Polynesia is rarer. Concerning the rariphotic zone just below, it remains almost entirely unexplored. Passive acoustics represents an inexpensive and non-invasive way of monitoring vocal species, sometimes in areas inaccessible to man, both day and night and over long periods of time. The aim of this thesis is therefore to provide a preliminary description of the soundscape of the rariphotic zone on the north side of Moorea Island (French Polynesia). More specifically, we described the three components of this biophony (benthic invertebrates, fish, and cetaceans) and analysed the diversity and diel patterns of fish sounds. The results showed a very silent high-frequency band (> 2 kHz) characterised by a peak at 6 to 9 kHz, between 6 pm and 6.30 pm, produced by unidentified benthic invertebrates. The low-frequency band (< 2 kHz) was characterised by a predominantly diurnal anthropophony and a biophony composed of fish and mysticete sounds. We also noted a reduction in the number of fish sound types compared with what is known in Polynesia at 20, 60 and 120 m. There were variations in acoustic diversity according to depth, but also patterns of abundance for sounds that were recorded at various depths. This work represents an initial contribution to the study of the biophony of the rariphotic zone, shedding some light on an area that is still largely unknown.