Passive Acoustics in the Pacific Islands

Using underwater recording devices to listen to whales and dolphins in the Pacific Islands.

A spectrogram of a rough-toothed dolphin whistle

A spectrogram of a rough-toothed dolphin whistle.

At the Pacific Islands Fisheries Science Center, the Cetacean Research Program uses a variety of passive acoustic approaches to advance its assessment capabilities and to examine the relationships between cetaceans and their environment. The use of passive acoustics to gather critical data and to augment visual surveys is growing more important. Passive acoustic data are used to augment ship-based line-transect surveys, to help delineate cetacean populations, to assess occurrence and seasonality, and to estimate the density, abundance, and trends of cetacean populations within the Pacific Islands Region.

Passive Acoustics for Population Assessment

Whales and dolphins spend most of their time under the surface of the water, where visual monitoring methods are ineffective. Additionally, sound travels much farther in water than light, and cetaceans commonly use sound rather than vision to communicate with each other and to find food. Using passive acoustic tools, we can listen to the sounds made by whales and dolphins, their ambient environment, and any human-generated noise. These data augment our visual surveys for cetaceans and provide an opportunity to examine temporal and spatial changes in the density and behavior of some species at time scales that we can not monitor using other methods. The data also provide insight into the acoustic environment, the soundscape, and changes within that environment that might affect cetacean populations.

Our passive acoustics program incorporates a broad diversity of instrumentation- bottom-mounted long-term acoustic recording devices, miniature recorders for use on mobile or dynamic platforms, drifting medium-term acoustic recording devices, towed hydrophone arrays during ship-transect surveys, and many novel approaches to addressing our challenging mission. Passive acoustics is particularly well-suited to helping us understand three important elements of population assessment:

  • Species occurrence and range: What species occur in the area, how are they distributed, and when are they there?
  • Population abundance and trends: How many animals are in the population? How does that abundance change over time?
  • Population stresses: What natural processes or human-caused threats may be impacting the population? Are threats constant or does the severity vary? Can the threat be reduced?

Instrumentation

Pacific Islands Passive Acoustic Network

HARP being recovered onboard the NOAA Ship

HARPs are long-term acoustic recorders used to record a very broad frequency range, allowing us to hear more cetacean species. A HARP package is deployed at a specific location, then recovered after months to years to obtain its recorded data and identify the species detected by the HARP (Photo: NOAA Fisheries).

The Pacific Islands Passive Acoustic Network (PIPAN) is designed to provide long-term monitoring of cetaceans in remote regions or areas of special interest. PIPAN sites are currently monitored using bottom-mounted passive acoustic data recorders called HARPs (high-frequency acoustic recording packages). HARPs are generally deployed at each site once or twice per year, recording sounds from 10 Hz to 100 kHz, and sometimes higher frequencies. They must be recovered to retrieve the data. The resulting data record includes vocalizations from both odontocetes and mysticetes, as well as ambient sounds (wind, rain, other organisms) and human-generated noises, such as echosounders, ships, and sonar.

To date, PIPAN includes HARPs in 13 locations across the Pacific Islands region, including several that were deployed as early as 2006. We have HARPs off the Kona coast of Hawaii Island (a 12-year record!), at Pearl and Hermes Atoll in the Northwestern Hawaiian Islands, at Wake Atoll, and off Saipan, Tinian, and Pagan in the Mariana Islands (see map). These long-term data sets allow us to assess the occurrence, seasonality, and long-term changes in distribution of species, as well as any changes in vocal behavior or distribution in response to increasing human sound sources or changing environmental conditions.

Locations of HARPs in the Pacific Islands Region to monitor cetacean occurrence and seasonality. Green locations are active as of October 2018, and red locations are currently inactive HARP locations.

Locations of HARPs in the Pacific Islands region to monitor cetacean occurrence and seasonality. Green locations are active as of October 2018 and red locations are currently inactive HARP locations.

Towed Hydrophone Arrays

Passive acoustic tools also prove valuable in real-time applications. We conduct visual and acoustic cetacean surveys to estimate the distribution and abundance of each encountered species. During the survey, we use an acoustic array to listen for vocal cetaceans. The acoustic array consists of a series of hydrophones (underwater microphones) that we tow behind the ship (a "towed array") while it is moving along the survey trackline. The research team aboard the ship monitors the hydrophone signals with headphones and watches a visual representation of the sound—a spectrogram—for sounds that are at higher frequencies than can be detected by the human ear.

Using these techniques and a suite of processing software, the acoustic team can determine when a whale or dolphin is nearby, what species it is based on various characteristics of its vocalizations, and where it is relative to the ship. The location is estimated based on the geometry of the array itself—we can get a bearing to the vocal animal based on the difference in the time that a given sound is heard at each hydrophone (plus some trigonometry). A series of intersecting angles provided as the ship moves along the track provides a location. Acoustic detections provide an opportunity to collect additional data from the group and for some species, an opportunity to examine dive behavior and abundance based on information in the acoustic data.

The acoustics team deploys and retrieves the 300 m hydrophone array from behind the ship at least twice a day.

The acoustics team deploys and retrieves the 300 m hydrophone array from behind the ship at least twice a day (Photo: NOAA Fisheries).

The acoustics team monitors the towed hydrophone array and detects cetaceans in real-time.

The acoustics team monitors the towed hydrophone array and detects cetaceans in real-time (Photo: NOAA Fisheries).

Medium-term Drifting Recorders

Autonomous stationary acoustic recorders, such as those in the PIPAN, have been broadly used to understand the distribution and seasonality of cetaceans throughout the world’s oceans. The Cetacean Research Program has recently started using a new type of free-floating autonomous recorder, the DASBR, to listen for cetaceans during cetacean surveys. DASBRs are designed with two hydrophones forming a short vertical array at ~150 m depth that allows for computing the depth and distance of the sound source. With these measurements, we can determine how far the vocal animals are from DASBR, and the collection of those measurements may allow us to estimate the abundance of the species we’re acoustically detecting. Because the instruments are drifting, we can deploy them at any bottom depth, and we can sample an area in a more random and representative way than is possible with bottom-mounted stationary recorders. We track the instruments’ movements using Iridium satellite locators. Although our primary goal is to estimate abundance of deep-diving whales, DASBRs record across a broad frequency range that enables us to detect the occurrence of most cetacean species, from baleen whales to the highest-frequency delphinid sounds.

A floating DASBR just before pickup. The satellite transmitter is located in the top, and the hydrophone array hangs vertically from a line below it.

A floating DASBR just before pickup. The satellite transmitter is located in the top, and the hydrophone array hangs vertically from a line below it (Photo: NOAA Fisheries).

The NOAA ship Oscar Elton Sette crew successfully retrieves a DASBR after a month long deployment.

The crew of the NOAA Ship Oscar Elton Sette successfully retrieves a DASBR after a month-long deployment (Photo: NOAA Fisheries).

A Unique Approach: Recorders for Monitoring Longlines

False killer whales take fish and bait from the Hawaiʻi longline fishery, often causing fishermen to lose money, and sometimes whales to be hooked, entangled, or even killed. Observations of these interactions are rare, as they generally occur underwater, at night, and on gear potentially miles from the fishing boat. False killer whales make specific vocalizations that are easily detected and identified, which makes sound a great way of investigating whale–fishery interactions.

Bayless_Longline Graphic.tif

Using modified HARPs designed specifically for deployment on longline fishing gear, we can monitor the sounds associated with these types of interactions. The package is directly attached to the mainline of the gear and then placed overboard to be activated by a saltwater switch, recording all animal, gear, and vessel sounds in the vicinity of the recorder.

By monitoring the sounds associated with these interactions, we hope to better understand what factors are attracting animals to the vessel (an acoustic cue, perhaps?) and help to develop strategies to prevent these interactions. This research supports the work of the False Killer Whale Take Reduction Team as they strive to identify measures that can deter animals from fishing gear or reduce the level of injury to false killer whales that get hooked while attempting to take fish from hooks.

Figure of hydrophone mounted within the cylindrical pressure case of a longline HARP.

Figure of hydrophone mounted within the cylindrical pressure case of a longline HARP (NOAA Fisheries).

Read more about how we use passive acoustic technology to study cetaceans: