Using Ecological Acoustic Recorders (EARs) to Monitor Coral Reef Ecosystems

The PIFSC and HIMB have begun to study the effectiveness of EARs to establish patterns of near shore biological activity by a wide variety of marine organisms, large and small, in tropical island habitats. Systematic monitoring of sound-producing species on coral reefs and along island slopes will help to document the occurrence of various species, their distribution patterns, and changing levels of activity. The same passive acoustic tools are being used to monitor human activity. The EAR program seeks to:

  • Establish long-term temporal trends in the acoustic activity of coral reef-dwelling organisms like fish and snapping shrimp, and correlate these with seasonal and non-seasonal changes in physical variables on the reef, such as temperature, currents and tidal cycles.
  • Apply passive acoustic listening methods to determine levels of human activity in remote locations, such as blast fishing and vessel visitations to restricted areas.
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Distinctive diurnal patterns of sound production were recorded for damsel fish (Dascyllus albisella) and an unidentified fish in Kaneohe Bay, Oahu. Vertical bars show averages over a 10-day period in May 2005. Thin lines above bars represent one standard deviation.
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An EAR was deployed to detect the presence of vessels at a site within the National Park of American Samoa.

Preliminary findings from deployment of EARs reveal that the pulses of sound produced by snapping shrimp, which represent the dominant source of biological acoustic signals on coral reefs, exhibit clear diel trends -- peaks of activity consistently occur around times of sunrise and sunset. At frequencies below 2 kHz, many fish sounds occur which also exhibit distinct temporal variability. Some fish are acoustically active mostly during daylight hours, while others produce sounds primarily before sunrise and just after sunset. Cetacean sounds also occur at some locations.

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This EAR recording shows a regular diurnal pattern of sound production by snapping shrimp over a 10-day period on a coral reef off Kaena Point, Oahu.

Studies are underway to determine the influence of significant episodic events such as coral bleaching, crown-of-thorns infestations, disease outbreaks, and storm damage on the patterns of ambient sound on coral reefs.

Because many of the sounds on coral reefs can be detected automatically using EARs, the activities of acoustically active species can be monitored by examining the complex mixture of sounds recorded (the sound field). The results obtained to date indicate that acoustic monitoring of the ambient sound field is a promising means of tracking biological activity, particularly at locations where traditional surveys are impractical.