• Northwestern Hawaiian Islands Connectivity
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  • Science Terms Glossary

    Passive acoustic monitoring of biological activities

    The Research Problem

    The use of passive acoustic tools to record biologically produced sounds in aquatic environments is increasingly becoming a tool of choice for assessing and monitoring biological activity in habitats where conventional survey methods are impractical or not cost effective. This is because many species of fish, invertebrates and marine mammals regularly produce sounds for communication, displaying and/or environmental sensing. In addition, acoustic monitoring is an effective means of detecting human activities in a marine habitat, such as vessel traffic and fishing. Passive acoustic monitoring is therefore a cost-effective means of logging both biological activity and human presence at a location and thus help gauge the effectiveness of management practices.


    Technology to monitor both biological and anthropogenic sounds in marine environments was developed through a joint effort between the Hawai‘i Institute of Marine Biology (HIMB) and the Pacific Islands Fisheries Science Center’s Coral Reef Ecosystem Division (CRED). The Ecological Acoustic Recorder (EAR) is a digital, low power acoustic recording system designed to intermittently sample the ambient sound field between 20 Hz and 40 kHz. Recordings are made on a user-specified schedule and are also initiated on a start trigger set to specific acoustic energy thresholds, such as vessel engine noise.

    An effort has been underway to monitor long-term trends in biological and anthropogenic activities in the Papahānaumokuākea Marine National Monument using EARs since 2006. Four EARs were first deployed in the summer of 2006 at French Frigate Shoals, Pearl & Hermes Atoll and Kure Atoll. In 2008 three additional EARs were placed at Maro Reef, Lisianski Island, Midway Atoll and a second unit was deployed at Pearl & Hermes Atoll. In 2010 four deep EARs were placed in waters ranging from 123 m to 405 m deep off Nihoa, French Frigate Shoals, Lisianski Island and Kure Atoll, bringing to 12 the total number of EARs deployed in the Papahānaumokuākea Marine National Monument.


    One notable finding to date is that humpback whale (Megaptera novaeangliae) song consistently occurs throughout the Northwestern Hawaiian Islands, which suggests an over-wintering pattern similar to that found in the Main Hawaiian Islands (Lammers et al, 2011). Previously, it was believed that humpback whales do not winter in the Northwestern Hawaiian Islands, but rather only pass through on their way to the Main Hawaiian Islands.

    Other research

    Another area of investigation is the spatial and temporal variability of reef-associated sounds. Monthly variations in acoustic activity are revealing trends across months and years in the stability of the ambient sound field. This information is being used to gauge the response of the overall ecosystem to stressors such as storms, disease, ocean acidification and climate change.


    As data continue to be collected, more information is emerging about the unique characteristics of the Northwestern Hawaiian Islands. The ability to acoustically “observe” the activities of a wide range of organisms 24 hours a day, 7 days a week and 365 days a year provides an unparalleled perspective on the activities taking place in the remote marine ecosystems of the Monument. The data obtained through this non-intrusive, non-extractive effort are expected to continue yielding valuable information about the Monument’s overall health and stability to managers and stakeholders for years to come.

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