Technologies for Passive Acoustic Research | NOAA Fisheries

We record sounds in the ocean to learn about marine mammals and other ocean species, working with partners around the globe.

We use technologies to conduct research. Our Passive Acoustic Cetacean Map shows some of our cetacean (whale, dolphin, and porpoise) detections from different projects and technologies.

We deploy hydrophones in the ocean to record underwater sound over time (hours to years) and space (within a few kilometers to ocean-wide). We choose the appropriate acoustic technology based on the species and research goals. For example, bottom-mounted recorders are the most common because they can record continuously for longer periods of time. This makes them ideal for studying the long-term migratory patterns of marine animals. Data downloaded from the retrieved recorders are used to track seasonal changes in sound production and distribution of individuals and populations. Alternatively, real-time devices provide nearly instantaneous data on marine mammal whereabouts. Our main real-time technologies are autonomous gliders and moored surface buoys. Real-time data contributes to quick decisions about protection and management.

With the proper permitting, we can place acoustic tags on individual animals to get information about their foraging behavior, vocalizations, and how they respond to surrounding sounds. This information helps create a better picture of animal behavior beneath the water’s surface and identifies species-specific vocalizations (or lack thereof) in relation to their movements.

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A bottom-mounted mooring is a stationary passive acoustic recorder. We anchor these to the seafloor in a particular location to record sounds. We recover them after a set amount of time, which can be several months or years. These recorders collect long-term data on vocally active species and surrounding sounds in a given area. There are many types of bottom-mounted recorders, each built to handle the different combinations of recording period, water depth, and species we want to hear.

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These slow-moving acoustic recorders drift along with the local currents in which they are deployed. The surface component sits above the water, with a recording device below the water. Data collection can be stored, or transmitted in real-time using a satellite or VHF transmission. Some drifting buoys collect and store acoustic data and GPS tracks, so we can determine where a signal was received. Drifting buoys with GPS components emit their current location over a satellite network, allowing us to deploy and recover them whenever needed (typically weeks to a few months).

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A drop hydrophone is usually suspended from the side of a boat or from land to record underwater sounds. It is connected directly to a recording device and sounds can be heard as they are recorded. This approach is simple and is mostly used for opportunistic data collection.

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A glider, or autonomous underwater vehicle, can be programmed to travel along a given path and outfitted with different instruments, including passive acoustic recording packages. They can transmit data in near-real time using a satellite or cellular link and store the data onboard until it is retrieved. There are several different types of gliders; the main ones are battery- or wave-powered. Battery-powered gliders travel up and down the water column in a vertical zig-zag, using buoyancy and an onboard motor to maintain course. Wave-powered gliders are propelled by the motion of waves.

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A moored surface buoy is a stationary mooring that has a surface float that houses electronics that allow data transmission. The surface float has a long downward-facing cable that is anchored to the bottom. We can attach various instruments, including passive acoustic recorders, to the cable or anchor system. The data can be transmitted in near-real time using a satellite link or VHF antenna. It can also collect and store data onboard for retrieval later.

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Telemetry tags are attached to an animal that send information to strategically placed receivers. The tags are either implanted in or attached externally to marine animals (invertebrates, fish, marine mammals) in order to track their movements. When a tagged animal is in the vicinity of a telemetry receiver, it detects their presence and stores it on the receiver's internal memory for later analysis. In most circumstances, telemetry receivers are deployed as an array at selected locations to not only detect the tagged animal at a single site but also detect it at multiple locations over time. A good example is a tagged salmon smolt that is migrating downriver to the sea as it matures. Its progress can be tracked with telemetry receivers installed along the river. Over time, telemetry data can provide information about an animal’s site fidelity, habitat preference, and migration patterns.

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A towed array consists of a long electronic cable with a series of hydrophones and other sensors encompassed within it. It collects passive acoustic data in real time. A linear array (most common) has the hydrophones arranged in a straight line and is towed horizontally behind a vessel or suspended from a stationary mooring or vessel. Volumetric arrays have hydrophones arranged along three axes (x,y,z) to be able to locate sound sources and measure their depth below the surface.

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NOAA towed arrays in the Pacific Islands

Last updated by Northeast Fisheries Science Center on 05/06/2024