“To be, or not to be”: critical assessment of the use of α-acoustic diversity indices to evaluate the richness and abundance of coastal marine fish sounds

Passive acoustic monitoring can be used to assess the presence of vocal species.
Automatic estimation of such information is critical for allowing diversity monitoring over
long-time spans. Among the existing tools, α-acoustic indices have been originally designed to assess the richness/complexity of terrestrial soundscapes. However, their use in marine environments is impacted by fundamental differences between terrestrial and marine soundscapes. The aim of this study wasto determine how they vary depending on the abundance and sound type richness of fish sounds. Fourteen indices used in terrestrial environments were tested. Indices were calculated on files from three sources: a controlled environment (playback of artificial tracks in a pool), in-situ playbacks (playback of natural soundscapes), and a natural environment (only natural sounds). The controlled experiment showed that some indices were correlated to sound abundance but not to sound type richness, implying that they are not capable of distinguishing the different types of fish sounds. With in-situ playbacks, indices were not able to capture differences, neither in terms of sound abundance nor sound type diversity. In the natural environment, there was no correlation between most of the indices and the abundance of sound. They were impacted by mass-phenomena of biological sounds (e.g., the Pomacentridae sounds in shallow reefs) that cannot inform on fish acoustic diversity. Indices are appropriate when soundscapes are divided in bands. In contrast to terrestrial environments, frequency-bands in coastal marine soundscapes do not provide ecologically relevant information on diversity. Overall, indices do not appear suitable to infer marine fish sound diversity.