Soundscape analysis is a potentially powerful tool in ecosystem monitoring. Ecoacoustic metrics, including the Acoustic Complexity Index (ACI) and Acoustic Entropy (H), were originally developed for terrestrial ecosystems and are now increasingly being applied to investigate the biodiversity, habitat complexity and health of marine systems, with mixed results. To elucidate the efficacy of applying these metrics to marine soundscapes, their sensitivity to variations in call rate and call type were evaluated using a combination of field data and synthetic recordings. In soundscapes dominated by impulsive broadband snapping shrimp sounds, ACI increased non-linearly with increased snapping rate (∼100–3500 snaps/min), with a percent range of variation (∼40–50%) that exceeds that reported in most studies. H, however, decreased only slightly (∼0.04 units) in response to these same snap rate changes. The response of these metrics to changes in the rate of broadband snapping was not strongly influenced by the spectral resolution of the analysis. For soundscapes dominated by harmonic fish calls, increased rates of calling (∼5–120 calls/min) led to decreased ACI (∼20–40% range of variation) when coarse spectral resolutions (Δf = 94 or 47 Hz) were used in the analysis, but ACI increased (∼20% range of variation) when a finer resolution (Δf = 23 Hz) was employed. Regardless of spectral resolution used in the analysis, H decreased (∼0.20 units) in response to increased rates of harmonic calling. These results show that ACI and H can be modulated strongly by variations in the activity of a single sound-producing species, with additional sensitivity to call type and the resolution of the analysis. Variations in ACI and H, therefore, cannot be assumed to track call diversity, and the utility of these metrics as ecological indicators in marine environments may be limited.

During May 2015, passive acoustic recorders were deployed at eight subtidal oyster reefs within Harris Creek Oyster Sanctuary in Chesapeake Bay, Maryland USA. These sites were selected to represent both restored and unrestored habitats having a range of oyster densities. Throughout the survey, the soundscape within Harris Creek was dominated by the boatwhistle calls of the oyster toadfish, Opsanus tau. A novel, multi-kernel spectral correlation approach was developed to automatically detect these boatwhistle calls using their two lowest harmonic bands. The results provided quantitative information on how call rate and call frequency varied in space and time. Toadfish boatwhistle fundamental frequency ranged from 140 Hz to 260 Hz and was well correlated (r = 0.94) with changes in water temperature, with the fundamental frequency increasing by ~11 Hz for every 1°C increase in temperature. The boatwhistle call rate increased from just a few calls per minute at the start of monitoring on May 7th to ~100 calls/min on May 10th and remained elevated throughout the survey. As male toadfish are known to generate boatwhistles to attract mates, this rapid increase in call rate was interpreted to mark the onset of spring spawning behavior. Call rate was not modulated by water temperature, but showed a consistent diurnal pattern, with a sharp decrease in rate just before sunrise and a peak just after sunset. There was a significant difference in call rate between restored and unrestored reefs, with restored sites having nearly twice the call rate as unrestored sites. This work highlights the benefits of using automated detection techniques that provide quantitative information on species-specific call characteristics and patterns. This type of non-invasive acoustic monitoring provides long-term, semi-continuous information on animal behavior and abundance, and operates effectively in settings that are otherwise difficult to sample.

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