Assessing the soundscapes of the critically endangered grey nurse shark (Carcharias taurus) in rocky reef habitats

Soundscapes are vital components of marine ecosystems, yet increasing anthropogenic noise is altering natural acoustic environments. This study explores the relationship between soundscape characteristics and hotspots of abundance for the Critically Endangered grey nurse shark (Carcharias taurus). Using SoundTrap hydrophones (ST600 and ST300), acoustic data was recorded at three aggregation and three non-aggregation sites off Port Stephens, Southeast Australia, between May and July 2023 to accumulate a total of over 1000 h of sound for the entire study. Using code developed for soundscape analysis, a multi-factorial analysis of root mean square sound pressure levels, peak sound pressure levels, impulsiveness, periodicity, and uniformity across broadband (0.1-24 kHz), fish (0.1-1 kHz), and invertebrate (2-4 kHz) frequency bands was undertaken. Our results indicate that aggregation sites, that recorded a low proportion of files with boat noise present, generally exhibited lower levels in the broadband and invertebrate bands than for non-aggregation sites, and there were no clear trends observed within the fish band. The lower broadband sound levels could be an indication that C. taurus are attracted to quieter areas. The differences detected within the invertebrate band may reflect variations in habitat characteristics, or ecological dynamics, such as reduced invertebrate activity or trophic interactions affecting prey availability. Clustering of sites was observed in t-distributed stochastic neighbour embedding (t-SNE) analyses, however, there were no significant distinctions between aggregation and non-aggregation sites. Individual sites exhibited unique sound profiles at sunrise and sunset, most notably, aggregation sites had a higher proportion of recordings with humpback whale (Megaptera novaeangliae) songs. There was a higher proportion of files with boat noise present at the non-aggregation sites (<15 %) compared to aggregation sites (<5 %), therefore, boat noise was not considered a key soundscape contributor in this study. This research provides initial insights into the role of soundscapes in C. taurus habitat selection and acts as a primer for further research integrating acoustic telemetry and biodiversity assessments to enhance conservation strategies.