Rapidly obtained ecosystem indicators from coral reef soundscapes

Present-day coral reef ecosystem monitoring techniques can be costly, labor-intensive point measurements that can potentially introduce intractable sampling bias and error. Here we report correlations between visually obtained ecological assessment metrics and concurrently recorded aspects of the underwater biological soundscape over coral reefs at 23 sites spanning the Hawaiian Islands archipelago. Sites from the 'cool tropics' oceanographic habitat grouped along a principal component defined by an acoustic sliding scale: from protected or more remote sites at which lower frequencies were more dominant, to degraded sites which produced soundscapes dominated by higher frequency sound. Positive correlations between ambient biological sounds below 2 kHz and the density of benthic invertebrates (Pearson's rho = 0.61-0.76), their predators (rho = 0.65-0.8), organism-scale bathymetric complexity (rho = 0.6-0.62) and the quantity of crustose coralline algae (rho = 0.6-0.62) suggest a connection between this part of the spectrum, habitat complexity and the density of benthic fauna. A positive correlation was also observed between the daytime soundscape in the 2 to 20 kHz band and the proportion of benthic fleshy macroalgal cover (rho = 0.63-0.7), an indicator of reef degradation. This study indicates the possibility of rapid, inexpensive and spatially integrative remote sensing of the ecological state of coral reefs. Such quantitative methods could be used to ecologically assess vast areas of reef habitat autonomously in near real-time and could be important for remote in situ detection and characterization of subtle but significant ecological changes brought about by climate change and other more localized anthropogenic impacts.