Passive acoustic monitoring of a tropical marine soundscape: characterisation of fish acoustic communities in northern Brazil

Abstract

Sound is widely used by organisms to communicate, perceive their surroundings and mediate ecological interactions, and even more efficient for information transmission in the aquatic environment, where it propagates approximately five times faster than in air. Many fish species are known to produce sounds in different behavioural contexts, including reproduction and feeding, and, depending on the species, may form conspicuous choruses that elevate background sound levels. The soundscape integrates biological, geological and anthropogenic sounds and provides a valuable framework for investigating ecological processes, which can be explored using passive acoustic monitoring, a non-invasive approach based on autonomous underwater recordings collected over extended temporal and spatial scales. The present thesis examined the marine soundscape of the Barreirinhas Basin, on the northern coast of Brazil, with a particular focus on fish acoustic communities, through the application of multiple analytical approaches to acoustic recordings. Specifically, the objectives were to characterise the marine acoustic environment by identifying major sound sources, to document and describe fish acoustic diversity through the identification of putative fish sounds, to investigate temporal patterns of fish sound production and chorusing activity, and to examine how fish choruses are organised within the local acoustic space by analysing temporal and spectral relationships among choruses, in line with the Acoustic Niche Hypotheses. Long-term recordings from sampling sites revealed a complex soundscape, within which a diversity of 36 putative fish sound types and four choruses were identified and characterised. Fish choruses were a prominent and recurrent component of the soundscape, exhibiting clear diel and seasonal patterns. Analyses of sound pressure levels and third-octave band levels demonstrated that chorusing activity was a major driver of temporal variation in specific frequency bands, whereas other ecoacoustic indices showed limited sensitivity to fish chorusing activity in this environment. Within the local acoustic community, multiple fish choruses co-occurred predominantly at night, differing in diel and seasonal activity but overlapping at hourly scales, while maintaining spectral segregation of their central frequencies. Taken together, these results demonstrate that fish choruses represent a key ecological axis structuring the marine acoustic community in this region, and that changes in the temporal or spectral organisation of chorusing activity, as well as shifts in the balance between biological and anthropogenic sounds, may provide early indications of changes in acoustic community organisation or increasing anthropogenic influence within the soundscape. Overall, this thesis provides the first integrated characterisation of fish sounds within a marine soundscape along the northern coast of Brazil, establishing a baseline for future monitoring and highlighting the applicability of passive acoustic monitoring as a framework for tracking ecosystem dynamics and supporting conservation and management efforts.