Abstract
The Acoustic Complexity Index (ACI) is increasingly applied to the study of biodiversity in aquatic habitats. However, it remains unknown which types of acoustic information are highlighted by this index in underwater environments. This study explored the robustness of the ACI to fine variations in fish sound abundance (i.e. number of sounds) and sound diversity (i.e. number of sound types) in field recordings and controlled experiments. The ACI was found to be sensitive to variations in both sound abundance and sound diversity, making it difficult to discern between these variables. Furthermore, the ACI was strongly dependent on the settings used for its calculation (i.e. frequency and temporal resolution of the ACI algorithm, amplitude filter). Care should thus be taken when comparing ACI absolute values between studies, or between sites with site-specific characteristics (e.g. species diversity, fish vocal community composition). As the use of ecoacoustic indices presents a promising tool for the monitoring of vulnerable environments, methodological validations like those presented in this paper are of paramount importance in understanding which biologically important information can be gathered by applying acoustic indices to Passive Acoustic Monitoring data.
Highlights
Since the mid-20th century, human-induced ecosystem changes have been occurring with increased rapidity[1]
Regarding the Acoustic Complexity Index (ACI), it remains unknown if this index highlights the abundance of sounds or the number of sound types, and if it is possible to discern between these two types of information
As the ACI calculates the absolute difference between two adjacent values of intensity (Ik and I(k+1)) in a single frequency bin (∆fi), and adds together all of the dk encompassed in the recording analysis window, our hypothesis was that the ACI is sensitive to variations in both sound abundance and sound diversity, but may not discriminate between the two
Summary
Since the mid-20th century, human-induced ecosystem changes have been occurring with increased rapidity[1]. A narrow low-frequency band is mostly occupied by temporally patterned fish vocalisations, and very frequent unpatterned broadband sounds originating from benthic organisms are superimposed to different levels of environmental noise Despite these differences, an increasing number of studies have applied the ACI to aquatic environments in order to gain information about diversity or ecological state[22,23,24,25,26,27,28,29,30,31,32]. The ACI was evaluated in a controlled experiment and in natural conditions (field recordings; wild fish communities)
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