Using Acoustic Indices to Determine Changes in Biodiversity off the Coast of Cape Hatteras, NC

Abstract

Monitoring of environmental health is an essential but often difficult and time-consuming task typically accomplished using environmental health indicators such as biodiversity. Large scale habitat degradation and climate change particularly in coastal environments have made it vital to monitor these environments and observe changes in biodiversity that can indicate ecosystem health. Collecting passive acoustic data enables researchers to examine environments over longer temporal and spatial scales compared to traditional biodiversity surveys, and various biodiversity indices like the Acoustic Complexity Index have been developed to relate the frequency variability observed in these audio files to biodiversity in the environment. This project attempted to apply the Acoustic Complexity Index to acoustic data collected off the coast of Cape Hatteras in 2014 with an array of five hydrophones to distinguish any spatial or temporal patterns in biological diversity or composition. Results showed significant differences over time at each hydrophone location, as well as variation between locations within each month. Complexity values increased in the spring at the hydrophones located closer to the coast, while the hydrophones further offshore showed increased values in the fall. The primary cause of the extreme peaks in complexity observed throughout the study site were determined to be the result of increased fish vocalizations, commonly attributed to Atlantic spadefish (Chaetodipterus faber), Channel bass (Sciaenops ocellatus), Grey triggerfish (Balistes capriscus) and Black drum (Pogonias cromis). The increase in the average complexity values observed in the fall at the hydrophones further offshore were explained by the migration of large whale species such as the North Atlantic right whale (Eubalaena glacialis), humpback whale (Megaptera novaeangliae), fin whale (Balaenoptera physalus) and minke whale (Balaenoptera acutorostrata) as they travel between breeding and feeding grounds. Results of this analysis also revealed the need for further research examining the ambient noise levels in this region and how they are changing over time. Noise pollution is a growing concern that was observed in some of the recordings used in this project, but metrics designed to evaluate sound levels would be a more appropriate way to determine sources of noise pollution and changes over time.