Chapter 5: Acoustical Oceanography
If you’ve ever seen the movie Star Trek IV: The Voyage Home (Nimoy 1986), you undoubtedly would recognize the deep bass rumblings and high-pitched wailings of humpback whales. The sounds were recorded in the 1950s and 1960s by navy engineer Frank Watlington (1916–1982), who worked at a top-secret navy listening post on Bermuda. Watlington’s job was to listen for Soviet submarines using an underwater microphone, or hydrophone. Hydrophones work just like an abovewater microphone except they’re waterproof. But instead of submarines, he heard the eerie sounds of humpback whales. Word of the sounds reached American biologist Roger Payne (b. 1935), who recognized repetitive patterns in the vocalizations, which he called songs (Payne and McKay 1971). In 1970, Payne released these recordings on an LP, Songs of the Humpback Whales. It became the best-selling nature album of all time (Rothenberg 2014).
Whale songs are not the only sounds you will hear in the ocean. Underwater sounds come from a surprising variety of sources: bubbles, calving icebergs, rainfall, waves, underwater volcanoes, shrimp, crabs, fish, whales, dolphins, and ships, among many others. This is no “silent world,” as suggested by the title of Jacques Cousteau’s 1956 underwater documentary, Le Monde du Silence. In fact, the ocean is quite noisy, rivaling the soundscapes you might hear in a tropical rainforest or a southern bayou.
Two subdisciplines of underwater acoustics have emerged in modern times: active acoustics, the use of artificially produced sounds to study ocean properties and detect underwater objects; and passive acoustics, the study of underwater sounds using listening devices (i.e., hydrophones). Oceanographers in the twenty-first century employ tools and techniques in both types of underwater acoustics to study the ocean (e.g., Howe et al. 2019). And now their arsenal includes acoustic tools attached to the newest recruits in oceanographic research—marine animals. These approaches belong to the field of acoustical oceanography, the application of sound and its properties to study the ocean.
Acoustical oceanography applies to a broad range of oceanographic disciplines. It’s no surprise. In a medium largely opaque to light, sound offers the ability to “see” what otherwise might remain hidden. Listen closely to the sounds around you. Experience the world as whales and dolphins do. Enter the world of acoustical oceanography.
5.1 Anatomy of a Sound Wave
Before we dive into how oceanographers use sound to study the ocean, we need to know something about the nature of sound itself. Dictionaries define sound as vibrations that you can hear. But we must take a broader view because many underwater sounds fall outside the range of human hearing. Sounds originate through vibrations of an object that travel as pressure waves through air, water, and even solids. Thus, we may define sound as “a mechanical disturbance that propagates as a pressure wave through an elastic medium” (modified from britannica.com).
Sound waves move through the air by compression and expansion—more commonly referred to as rarefaction—of the air molecules. The completion of one compression–expansion motion of air molecules is termed a cycle, or oscillation. We can represent this cycle as a sine wave, a graphical representation of the sine function, a concept from trigonometry. A sine wave has a sinusoidal appearance—that is, it’s a line with regularly spaced arches. The top of the arch represents the crest of the wave and the bottom of the arch the trough. The horizontal distance between successive crests (or troughs) is called the wavelength. The height of the crest above the undisturbed baseline (the zero line) is called the amplitude. With sound waves, amplitude determines the “loudness,” or the intensity of the sound we perceive. Amplitude is a measure of the amount of energy a wave carries. Higher-amplitude waves sound louder. Lower-amplitude waves sound softer. Slam a key on a piano, and you get a high-amplitude sound (i.e., loud). Touch the same key gently, and you get a low-amplitude sound (i.e., soft).
Sound waves travel in a forward direction. Thus, they are called progressive waves, waves that move outward from their source. Progressive waves represent the transfer of energy between molecules in the medium through which they travel. The ocean swell you observe from a pier at the ocean is another kind of progressive wave.
The forward movement of progressive waves adds a time element to their description. We define wave speed as the distance a wave travels over a given period of time. Wave speed is related to two other wave variables. Wave period represents the interval of time between successive crests. Wave frequency indicates the number of cycles in a fixed interval of time (usually a second). You may be familiar with different frequencies of radio waves used to transmit sounds via radio, television, smartphones, and other devices.
Frequency is generally reported in units of hertz (Hz; 1 cycle per second) or kilohertz (kHz; 1,000 cycles per second). Human hearing typically ranges from 20 hertz to 20 kilohertz (but this varies by individual); this range is called audible (or sonic) sound. At the low end of the range (low-frequency sounds), you’ll find a bass drum or the rumble of a truck. At the high end of the range (high-frequency sounds), you’ll find crickets and light rain. Scientists define sounds outside the range of human hearing, such as those produced by humpback whales and other animals, as infrasonic or subsonic (frequencies less than 20 Hz) or ultrasonic (frequencies greater than 20 kHz).
5.2 Factors Affecting the Speed of Sound
The speed of sound in a fluid such as water or air depends on the fluid’s density; the denser the fluid, the faster sound travels. Because seawater is 800 times more dense than air, sound travels about 4.4 times faster in the ocean versus in the air, about 4,921 feet per second (1,500 meters per second ) versus 1,115 feet per second (340 m/s), respectively. But these speeds are not constants. Factors that affect the density of seawater (or air) can also cause variations in the speed of sound.
When we increase the temperature, salt concentration (salinity), or pressure of seawater, we increase the speed of sound. Now, in a typical ocean, temperature is highest at the surface, then rapidly decreases and stays relatively constant with increasing depth. So, based on temperature alone, the speed of sound would be highest at the surface, then slow down and remain constant with greater depth. However, salinity (usually) and pressure (always) increase with depth. These two factors alone would cause sound to speed up. The opposing effects of the three combined—temperature slowing sound and salinity and pressure speeding it up—create a curved sound speed profile in a typical ocean. Near the surface the speed of sound is fast (because temperature is high), but as temperature decreases, the speed of sound slows down. At around 3,281 feet (1,000 m), temperature remains constant, and the effects of increasing salinity and pressure cause sound to speed up. The result is that the sound speed reaches a minimum at around 3,281 feet (1,000 m) in the ocean. Above and below 3,281 feet (1,000 m), the speed of sound increases. Oceanographers refer to this depth as the sound minimum layer.
5.3 The SOFAR Channel and SOSUS
The subsurface minimum in the speed of sound has enormous implications for animals (and humans) who use sound for communication in the ocean. The sound minimum layer has an unusual effect on sound traveling at these depths. It causes sound waves that move shallower or deeper than the sound minimum layer to bend back into the layer. This bending of the waves—called wave refraction—prevents the sound waves from leaving the sound minimum layer. This “channeling” of sound waves allows them to travel hundreds to thousands of miles through the deep ocean. Discovery of this property led to recognition that it could be helpful in underwater communications. The US Navy equipped life rafts with small underwater explosives that could be detonated in the channel to assist ships in finding sailors or pilots lost at sea during World War II. Henceforth, the sound minimum layer became the Sound Fixing and Ranging, or SOFAR, channel. Because the SOFAR channel is so efficient at propagating sound over long distances, scientists speculate that marine mammals, such as blue whales, use it to communicate (Tsuchiya et al. 2004).
In the 1950s the US Navy installed arrays of hydrophones to detect noisy submarines and surface vessels in the SOFAR channel. The project, known as the Sound Surveillance System, or SOSUS, successfully detected Soviet submarines until the Soviets learned about SOSUS and started making their submarines quieter. Declassification of the SOSUS recordings in the 1960s gave researchers a golden opportunity to study underwater sounds, including the mysterious ones that puzzled hydrophone specialists like Frank Watlington.
5.4 Echolocation and Sonar
As scientists developed tools for listening to underwater sounds, they also worked to perfect tools using “echoes” for finding underwater objects such as icebergs and submarines, a process known as echolocation. The principle is familiar to anyone who has created their own echo against a wall. You make a loud noise. A split second later, an echo comes back to you. Echolocation—finding an object using sound—originated in animals, such as bats, birds, and marine mammals, who use echolocation to find food and sonically visualize their environment (Brinklov et al. 2013; Surlykke and Nachtigall 2014). Humans can use echolocation to determine water depth. By accurately measuring the time between the start of the sound and detection of the reflected echo, the distance to the seafloor can be estimated. Of course, the distance traveled by the reflected sound waves is twice the distance between the source and the seafloor. Knowing the time gives you the total distance traveled by the sound; dividing by two gives you the one-way distance to the seafloor, the water depth.
Today we know echolocation devices as sonar, an acronym for sound navigation and ranging. If you watch submarine movies, you’re probably familiar with the ping of active sonar. This range-finding tool produces a pulse of sound to locate an object through its echo. Modern sonar can employ multiple signals of various frequencies such that, at close range, the sonic image, when displayed in three dimensions, looks almost as detailed as a photograph.
5.5 Types of Sonar and Their Applications
Active sonar tools support a field of science known as geophysics, the study of solid Earth through observations of its physical properties. Marine geophysics, exploration of solid Earth beneath the ocean, has contributed significantly to our understanding of plate tectonics, formation of petroleum reserves, and much more. Of course, sonar also ensures safe navigation for anyone who ventures forth in a vessel in shallow waters and provides fishers an indispensable tool for finding fish. In that regard, sonar has evolved into a tool with a surprising variety of applications.
One of the earliest applications of sonar was determining the depth of the seafloor. Before sonar—since at least the sixth century BCE—mariners relied on a device called a sounding line (or lead line), which consisted of rope (or wire) and a lead weight on which a piece of candle wax (made from animal fat) was placed (e.g., Hawley 1931; Oleson 2000). The Challenger expedition carried miles of rope to sound for depth. The line and weight were lowered over the side of a ship while keeping track of the arm-lengths of rope deployed, a unit of measurement that came to be known as a fathom, equal to 6 feet (1.8 m). Of course, if you lowered hundreds (or even thousands) of feet of rope, you had to raise it to the surface to retrieve it. If you’ve ever worked with ropes in the gym, you can imagine the workout from raising a few miles of rope from the seafloor! Once at the surface, the weight was inspected for the presence of sediments in the wax, a sign that it had reached the bottom.
Because soundings refer to depth measurements, oceanographers adopted echosounding to refer to depth measurements using sound. However, the first device, invented by German physicist Alexander Behm (1880–1952), set out to detect icebergs following the sinking of the Titanic in 1912. Eventually, these instruments were turned downward to measure seafloor depth. They came to be known as fathometers, a word invented to describe the device patented by Canadian physicist Reginald Fessenden (1866–1932). Swarthmore College physicist Harvey Hayes (1878–1968) developed the first working fathometer in 1922. This instrument was deployed that same year off the coast of Southern California. This device helped to produce the first map of seafloor depths—what is known as a bathymetric chart. Comparisons of fathometer readings and sounding lines made by the US Coast and Geodetic Survey proved so favorable that the navy placed the instruments on all its ships soon afterward (e.g., Adams 1942; Hersey 1977; D’Amico and Pittenger 2009).
In 1957 echosounder data compiled by American oceanographers Bruce Heezen (1924–1977) and Marie Tharp (1920–2006) revealed the presence of a continuous range of undersea mountains running down the middle of the North Atlantic (e.g., Heezen et al. 1959). The Heezen-Tharp bathymetric map of the North Atlantic uncovered the Mid-Atlantic Ridge, an early piece of evidence supporting plate tectonics theory (see Chapter 8). Nowadays, powerful and much-improved fathometers can be found on nearly every vessel afloat.
5.5.2 The Deep Scattering Layer
Almost immediately after fathometers became available to the general public, vessel operators and fishers put them to use. But the instruments produced strange signals, ghost reflections, and other aberrations in the soundings. Military sonar operators noted the appearance of “false bottoms” in their fathometer displays. The phenomenon became known as the deep scattering layer (or sound scattering layer), a depth layer at which sound waves were reflected in different directions. More curious, the layer moved up and down at different times of day. Scripps biological oceanographer Martin Johnson (1893–1984) was among the first to propose that the layer was composed of organisms. The curious motions of the layer resulted from diel vertical migration, the daily movement of organisms up and down in the water column. An experiment on a research cruise in 1945 confirmed his hypothesis (Johnson 1948).
Modern instruments and automated processes have improved tools for observing organisms acoustically. Acoustic oceanographer D. Van Holliday (1940–2010) and biological oceanographer Rick Pieper (b. no data) spent decades studying zooplankton using acoustical methods (e.g., Holliday and Pieper 1995). The latest generation of acoustic instruments can now be deployed on moorings (De Robertis et al. 2017), towfish (Wiebe et al. 2002), remotely operated vehicles (ROVs; Dunlop et al. 2020), and autonomous underwater vehicles ((AVUs; Dunlop et al. 2018). These instruments have yielded great insights into the deep scattering layer (e.g., Fornshell and Tesei 2013; Receveur et al. 2020; Cisewski et al. 2021).
5.5.3 Multibeam Echosounders
Multibeam echosounders embody the idea that multiple echoes are better than one. Developed in the 1960s by the US Navy and put into commercial use in the 1970s, multibeam echosounders use multiple sound sources and receivers to obtain seafloor depths at many points simultaneously. Think of a spray nozzle whose liquid emanates from a point and spreads like a fan. That’s the shape of the beam transmitted by multibeam echosounders. This fan-shaped sound bounces echoes off a strip of the seafloor, and the return echoes are captured by receivers on the hull of the ship. By capturing multiple strips per second and digitally stitching them together, a three-dimensional picture of the seafloor can be obtained. To map large areas, oceanographers use the mowing the lawn technique—a method where the instrument travels back and forth across the seafloor similar to mowing a lawn. A ship traveling at a moderate speed can map hundreds of square miles a day in this way.
Multibeam bathymetry—mapping seafloor depths using multibeam echosounders—builds on work that began as far back as 1870. Albert I, Prince of Monaco (1848–1922), a self-taught oceanographer, sought to produce the most comprehensive maps of the ocean the world had ever seen. Prince Albert organized a committee to compile all available data sources on the ocean’s depths. In May 1905 they published the first edition of the General Bathymetric Chart of the Oceans, or GEBCO. Still available in modern times—albeit in digital form—GEBCO remains the primary source of data on global seafloor bathymetry. (See also Weatherall et al. 2015).
Unfortunately, modern bathymetric maps of the deep ocean lack the spatial resolution to resolve features smaller than three miles (5 km) across. Imagine a stadium (such as Pasadena’s Rose Bowl) filled with people standing shoulder to shoulder. Drop an enormous sheet on top of the people, and you would see a surface with bumps and depressions according to the heights of the people. To an echosounder operating in the ocean far above, all those bumps and depressions would condense to one depth. None of the details would be visible. That’s the challenge of mapping the ocean.
To address this shortcoming, GEBCO recently launched the Seabed 2030 Project to produce the highest-resolution map of the seafloor possible by 2030. To obtain a more detailed picture, scientists will require hundreds of multibeam-equipped AUVs and ROVs that can get closer to the bottom but that travel at much slower speeds. As a compromise, different depths will be mapped at different resolutions. They admit “that the goal of seeing the world ocean completely mapped by 2030 is a difficult one,” but they hope their efforts will inspire others to join them (Mayer et al. 2018).
5.5.4 Side-Scan Sonar
When oceanographers require detailed images of the seafloor, they reach for their side-scan sonar. Transmitting beams of sound from both sides of the instrument, which is most often towed near the seafloor, side-scan sonar provides images of “near photographic quality.” The high resolution and contrast of side-scan sonar images, called sonographs, make this technology ideal for identifying human-made objects on the seafloor, such as ships, planes, mines, pipelines, and similar structures (e.g., Kenny et al. 2003; Blondell 2009; Savini 2011; Luchetti et al. 2018; Zhang et al. 2021). Side-scan sonar works like the medical ultrasound instruments used to generate sonographs of internal organs. Its success in finding shipwrecks spurred its application in oceanographic research, especially in marine geology and geophysics. Side-scan sonar has even proven useful for tracking marine organisms (e.g., Grothues et al. 2017; Guzman and Condit 2017).
While side-scan sonars produce highly detailed images, they’re two-dimensional. They lack information on depth or the vertical relief of an object. To overcome this limitation, oceanographers use multibeam echosounders alongside side-scan sonar or platforms that integrate both technologies (e.g., Shang et al. 2019). With computer software, they can overlay the multibeam bathymetry on top of the side-scan image and obtain a sonograph with bathymetry.
Synthetic aperture sonar (SAS)–such as the KATFISH™ SAS– represents a new-and-improved version of side-scan sonar. Synthetic aperture sonar systems produce a much higher resolution image by averaging multiple samples (i.e., multiple return echoes) and combining them (e.g., Hansen 2019). The result is a kind of ultra-high-definition sonograph. Smile, fishes!
5.6 Seismic Methods
We now turn our attention to sound used for observing features beneath the seafloor. Though not generally classified as echosounding, these techniques use sound waves to infer characteristics of the subseafloor, the region greater than 4.9 feet (1.5 m) beneath the seafloor surface (e.g., Walsh et al. 2016). These tools revolutionized exploration for petroleum. In modern times, these tools have helped us to understand plate tectonics and other geological and oceanographic processes. You’ll enjoy this section if you like explosives (or explosive sounds, at least).
5.6.1 Seismic Profiling Overview
Seismic profiling uses artificially produced seismic waves to map the three-dimensional structure of Earth’s crust. The technique originates in studies of earthquakes, a field of science known as seismology. When a segment of Earth’s crust under high stress ruptures, the crust moves, and an earthquake occurs. We feel an earthquake as a series of elastic vibrations called seismic waves, motions of the ground caused by the movement of energy through Earth’s crust. These ground motions can be detected using an instrument called a seismometer, a device for detecting and recording the three-dimensional movements of Earth’s crust during the passage of seismic waves.
In 1849 Irish engineer Robert Mallet (1810–1881), the “founder of seismology” (Dean 1991), struck upon the idea of generating artificial earthquakes to study how seismic waves travel through Earth’s crust. In a series of experiments in 1849 and 1850, he generated underground explosions using gunpowder. He then measured the speed of the resultant seismic waves. He discovered seismic waves travel faster in granite than sand (Mallet 1851). Mallet’s work established the field of artificial seismology, the use of human-generated seismic waves for studying the Earth’s interior. Subsequent work built on Mallet’s work using different explosives and improved methods for detecting seismic waves (Weatherby 1940). By the turn of twentieth century, seismologists had worked out the principles of seismic reflection profiling, the detection of the echoes of seismic waves that bounce off subsurface rock layers. It’s like echosounding, except the echoes come from beneath Earth’s surface (Hedström and Kollert 1949). By the turn of the century, with the Industrial Revolution in full swing, scientists could use seismic methods for petroleum and mineral exploration, a field of applied geophysics known as seismic prospecting.
Seismic profiling requires two key ingredients: a source to generate seismic waves and a means to detect the returning waves. Dynamite was once used to generate seismic waves on land and at sea. And while explosives remain in use on land, seismologists more commonly drop heavy weights or employ thumper trucks, vehicles with devices that pound the ground to create vibrations. Return echoes may be detected by geophones—microphones that listen for the sound vibrations caused by seismic waves on land. At sea, loud booms are produced through the sudden release of compressed air using a device called a seismic airgun. To detect the sounds, seismologists use hydrophones or ocean bottom seismometers, devices anchored on the seafloor that measure seismic waves. The hydrophones or ocean bottom seismometers detect the acoustic reflections—which arrive at different times depending on the depth and density of the layers—to obtain a visual image of the rock layers beneath the seafloor.
Seismic waves also undergo refraction—the bending of a wave front. Refraction in seismic waves occurs in response to the nature of the rock through which they travel; some rock layers permit waves to move faster, while others slow the wave down. Because seismic waves from deeper layers in Earth’s interior return to the surface at a greater distance from their source than refracted waves from shallower layers, the speed of return of seismic waves reveals something of the nature of the rocks in the interior. Based on these principles, seismic refraction profiling—the use of refracted seismic waves for determining shallow surface geologic structures—was developed.
5.6.2 Seismic Profiling of the Subseafloor
Both seismic reflection and seismic refraction profiling rely on the subsurface penetration and reflection of sound waves produced by a loud acoustic source, e.g., airguns. Where seismic reflection and seismic refraction differ is in the placement of detectors. Seismic reflection uses a string of airguns and hydrophones, called a streamer, towed behind a ship. Streamers may be several kilometers long and hold thousands of hydrophones. Seismic refraction uses seismometers and hydrophones at the ocean bottom to detect the refracted wave from surface-towed airguns. However, seismic refraction requires much greater distances to characterize the subsurface layers. So detectors are placed further from the source, perhaps as much as 19 to 25 miles away (30–40 km).
To obtain a profile, airguns are triggered at regular intervals (every 10–12 seconds) while the vessel is underway (at about 5 mph). The pulse of sound reflects off the seafloor (to 6 miles deep) and echoes back to the streamer. Sound waves are reflected and refracted by layers of sediments and rock beneath the seafloor. The wave arrival times and other characteristics can be combined using computer visualization tools to provide a seismic reflection (or refraction) profile, a two- or three-dimensional image of the subseafloor. These graphs reveal the thickness of different layers, buried subsurface features such as salt domes or seamounts, discontinuities and faults, and other features within the layers that make them valuable for oil and mineral exploration and scientific research. (See Crutchley and Kopp 2018 for a recent review of these methods.) As seismic methods have revealed, a whole other world exists beneath our feet.
5.6.3 Seismic Oceanography
When seismic reflection profiling became a common tool for mapping the geology of the subseafloor, geophysicists noticed reflections in the water column between the ship and the seafloor. They paid them no attention. After all, they were interested in what was beneath the seafloor. Why care about “nuisance” false echoes (Buffett and Carbonell 2011)? As sometimes happens in science, those “echoes” are important.
If you swim in a lake during summer, you know that the surface can be quite warm. But deeper—where you stick your feet—it can be downright freezing (at least it feels that way). Lakes and the ocean form layers. Because warm water is less dense than cold water, the warm layer floats on top of the cold layer. The interface between these layers is called a thermocline. It represents the boundary between different density layers in a lake or the ocean. As it turns out, those density layers reflect low-frequency sound. And those density layers are the echoes oceanographers saw in the water column during seismic reflection profiles.
In 1988, French oceanographers Joseph Gonella and Dominique Michon took an interest in those reflections. They were soon followed by American oceanographers Joseph Phillips and Donald Dean. While they took different approaches, both presented compelling evidence thermoclines reflected sound. Their work deserved attention. As Phillips and Dean (1991) put it, “The past reluctance to use the multichannel reflection method for investigating water mass structure appears to have been unfortunate.”
Nevertheless, in 2003, publishing in the prestigious journal Science, Holbrook et al. presented “acoustic images” of thermoclines, fronts, and water masses in the North Atlantic. Anywhere density differences occurred, their images showed a reflection. To verify the seismic observations, the scientists deployed expendable bathythermographs (also known as XBTs). These disposable temperature recorders transmit data through a wire to the ship. Onboard chart recorders or computers record the temperature of the water column as the XBT descends. The data from the two approaches matched, demonstrating the utility of seismic reflection profiling for imaging the water column.
Upon publication of the Holbrook paper, the oceanographic community moved quickly to “see” what was previously “unseen.” Several papers by other researchers followed (e.g., Nandi et al. 2004; Nakamura et al. 2006; Biescas et al. 2008; Ruddick et al. 2009; Papenberg et al. 2010; Holbrook et al. 2013; Gunn et al. 2020). Thus began the field of seismic oceanography, the application of seismic reflection profiling to studies of water column structure and dynamics.
Seismic methods promise to make large-scale structures such as fronts and eddies visible on an unprecedented scale. It’s like that heist film where the thieves blow smoke into the room to light up the laser motion sensors. As Gunn et al. (2020) put it, “Seismic reflection surveying has a hitherto unsurpassed ability to resolve thermohaline structures on spatial scales of tens of meters to hundreds of kilometers and on temporal scales of minutes to days.” That means oceanographers can now see the physical structure of the ocean as never before.
5.7 Marine Biotelemetry and Biologging
Oceanographers have also developed acoustic tools for observing individual organisms. These tools have brought insights into marine animal behavior not possible just a few decades ago (e.g., Block et al. 2016). Two types of animal tracking technologies have emerged in recent decades: (1) biotelemetry, the tracking of organisms using devices that continuously transmit signals to a receiver; and (2) biologging, the recording of data by devices attached to or embedded within an organism (e.g., Bauman 2019). While the methods vary, both prove helpful in oceanographic research and ocean conservation (e.g., Harcourt et al. 2019; Iverson et al. 2019; Williams et al. 2020).
5.7.1 Animal Tracking Tags
Acoustic tags, also known as pingers, transmit an acoustic signal that can be detected by a hydrophone. These tags, attached directly to the animal, require researchers to use a handheld receiver and follow the animal using a boat or to record the animal’s movements using an array of moored hydrophones and data loggers that store the signals for later analysis. Acoustic tags offer subsurface information where signals from satellite-based global positioning systems (GPS) cannot transmit. California State Long Beach marine biologist Chris Lowe (b. 1963) and his “Shark Lab” use acoustic tags to track great white shark behavior along the Southern California coastline (e.g., Anderson et al. 2022). Such efforts help scientists understand shark behavior, an important consideration for public safety on beaches (Lowe 2019). Acoustic telemetry has also been integrated with AUVs to track sharks (e.g., Lowe et al. 2018).
A few nonacoustic tags deserve mention. Argos satellite tags appeared in the early 1980s when the newly established Argos satellite system (not to be confused with the Argo floats) came online as a tool for environmental monitoring. The Argos system provides a data uplink for tags placed on air-breathing animals, such as turtles and whales. Data are transmitted when the animals come to the surface. Alternatively, pop-up satellite archival tags (PSATs) permit researchers to study the behavior of organisms that do not typically spend time at the ocean surface, such as tuna, marlin, and halibut. The PSAT releases from the animal after a fixed time and floats to the surface, where it can transmit its data. GPS tags store data on the device until the animal is recaptured and the data downloaded from the tag. These devices are small enough to attach to coconut crabs (Krieger et al. 2012), stingrays (Martins et al. 2019), and marine birds (e.g., Peschko et al. 2020).
5.7.2 Animal Oceanographers
Acoustic tags and electronic sensors have given rise to a new breed of oceanographer. The animal oceanographers, marine animals outfitted with electronic sensors, now routinely and boldly go where humans have rarely gone before. Animal oceanographers represent a new platform for observing the marine environment. After all, who knows the ocean better than the animals that inhabit it? Animals can freely swim in places where ships and instruments have limited access. They also seek out areas not obvious to human observers or sensors, such as highly productive fronts and upwelling regions. Consequently, biotelemetry and biologging have become an integral part of the Global Ocean Observing System (McMahon et al. 2021). Animal oceanographers have provided hundreds of thousands of temperature and salinity profiles, especially in the Southern Ocean, an challenging environment for humans (e.g., Treasure et al. 2017).
5.8 Passive Acoustic Monitoring
Long used in antisubmarine warfare and research on fishes and marine mammals, hydrophones have reemerged as a twenty-first-century tool for observing the ocean (e.g., Au and Lammers 2016). Their modernization and deployment by geophysicists to record earthquakes and undersea eruptions have caused a resurgence of interest in them for bioacoustics research. These new devices, called passive acoustic recorders, combine modern electronics with digital recording devices to record sounds for long periods of time (Au and Lammers 2016). Automated recording has generated new insights into animal behavior, ecology, and conservation above and below water (Browning et al. 2017).
5.8.1 Ocean-Bottom Hydrophones
Modern passive acoustic recorders come in various shapes and sizes (Sousa-Lima et al. 2013). Because they emerged as a modification of ocean-bottom seismometers, they are called ocean-bottom hydrophones, listening devices temporarily or permanently moored on the seafloor. Marine autonomous recording units developed by the Cornell Center for Conservation Bioacoustics were deployed in Massachusetts Bay in a 10-year study of Atlantic cod populations. This species produces sounds during spawning. The recordings provided valuable information on the seasonal behavior of the fishes and essential clues for their sustainable management (Caiger et al. 2020). Oceanographers working in the Gulf of Alaska used multiple ocean-bottom hydrophones developed at Scripps Institution of Oceanography—the high-frequency acoustic recording packages—to record and track the calls of fin whales, the second-largest whale in the ocean. The study revealed differences in the movements and behaviors of the whales. It demonstrated the usefulness of a multi-instrument approach for studying whale behavior (Wiggins and Hildebrand 2020).
5.8.2 Hydrophone on a Tag
Acoustic recorders can also be attached directly to animals. Digital acoustic recording tags (D-TAGs) incorporate hydrophones with other sensors to record an animal’s vocalizations and movements. They also record environmental information, such as temperature and depth (e.g., Burgess et al. 1998; Johnson and Tyack 2003). D-TAG studies of southern right whales on calving grounds off Brazil detected low-volume grunts between lactating females and their calves that had not been observed previously using ocean-bottom hydrophones. The study also suggested that right whales may increase their “chatter” during social interactions (Dombroski et al. 2020). Perhaps one day scientists will develop the technology to allow us to chat with whales on smartphones.
5.8.3 Hydrophone on a Glider
Hydrophones attached to gliders offer an opportunity to simultaneously collect environmental and sound data. These data are crucial for understanding the spawning behavior of commercially important fishes. In 2015 scientists deployed a hydrophone equipped glider on the reefs off St. Croix in the Virgin Islands to study ecologically important snapper and groupers during their spawning cycle.
5.9 Ocean Soundscapes
The 2010s witnessed a surge of interest in studies of soundscapes, the totality of sounds present in a particular region or habitat (e.g., Pijanowski et al. 2011). The field draws inspiration from studies of sounds in cities and natural environments (e.g., Southworth 1969; Schafer 1977). We now know that soundscapes reveal something about the behavior and social interactions of sound-makers, including us.
The study of soundscapes belongs to a new field of science known as soundscape ecology (e.g., Pijanowski et al. 2011). Soundscape ecologists recognize three primary sources of sound: (1) geophony, sounds emanating from geologic and physical processes; (2) biophony, sounds originating from biological sources (except humans); and (3) anthrophony, sounds produced by human vocalizations and human activities.
Acoustic researchers have long known about fish choruses, sustained and elevated sound-making by fishes for an hour or more (e.g., Cato 1978; McWilliam et al. 2017). Fish choruses resemble the dawn chorus of birds or the peeps of frogs in the evening. The availability of passive acoustic recorders has provided new opportunities for studying ocean soundscapes (Lindseth and Lobel 2018). Fish choruses have been observed in nearly every ocean (e.g., McCauley 2012; Staaterman et al. 2014; Buscaino et al. 2016; Heenehan et al. 2019).
Studies of coral reef biophony have revealed sophisticated patterns in the timing of sounds on coral reefs. These studies support what has come to be known as acoustic niches, partitioning of the timing, intensity, or spectrum of sounds in a community of organisms to avoid overlap in sound-making (Krause 1993). It’s the equivalent of everybody not talking at once. For example, observations using a passive acoustic recorder on a coral reef off Moorea (near Tahiti in French Polynesia) revealed distinct differences in sounds produced during the day versus the night. The study also revealed a sequence to the dominant sounds throughout the night, suggesting that animals “took turns” in their sound-making (Bertucci et al. 2020).
Ocean soundscapes exhibit complex and dynamic properties that provide invaluable insights into ocean ecosystems. They offer the potential for long-term monitoring of the biodiversity and conservation of ocean species (Lindseth and Lobel 2018). They also provide a much-needed baseline for establishing the impacts of human-generated sounds on marine organisms, the final topic in our chapter.
5.10 Ocean Noise
If you spend any time listening to the world around you—instead of blocking it out with earbuds—you will notice certain unpleasant sounds in your daily soundscape. Perhaps it’s a leaf blower or nightly fireworks from your local magic kingdom. Maybe it’s the din of traffic or an ambulance siren. Or maybe it’s the hum of a refrigerator or the relentless tick of a clock at night. All these sounds define what scientists call noise, any unwanted or undesired sound in the environment. Of course, one person’s “noise” may be another person’s “music.” Some people (like me) truly like the San Francisco punk band Flipper while others hate it (Fitzpatrick 2019). But in general, noise refers to sounds of a type and intensity that interfere with other acoustic sources or cause harm to them.
Numerous sources of ocean noise exist in the ocean: ships, offshore construction, sonar, seismic reflection studies, and more (e.g., Slabekoorn et al. 2010). Studies of ocean noise began during World War II to distinguish manmade sounds—especially enemy ships and submarines—from all other sounds (e.g., Knudsen et al. 1948). But as commercial shipping and recreational boating traffic grew, and science and industry adopted acoustic techniques for probing and observing the ocean, ocean noise also increased. Like the slowly boiling frog that doesn’t notice the pot getting hotter, ocean noise grew steadily without attracting much attention. That quickly changed in the 1990s.
5.10.1 The Ocean Acoustic Thermometry Experiment
I remember listening in rapt attention at the 1989 inaugural meeting of the Oceanography Society in Monterey Bay, California, as Scripps Institution of Oceanography physical oceanographer Walter Munk (1917–2019) described an idea to use low-frequency sound as a “thermometer” to observe ocean temperatures. The technique, known as ocean acoustic tomography, was first proposed by Munk and Massachusetts Institute of Technology physical oceanographer Carl Wunsch (Munk and Wunsch 1979). Because the speed of sound in the ocean increases as seawater temperatures increase, you can determine the degree to which the oceans are warming as long as you can accurately measure the sounds over long distances. By this time, scientific concern about human-caused global warming had grown considerably, and scientists were seeking independent ways to verify observations that Earth was warming. The idea seemed quite brilliant to me.
Initial tests in January 1991 using an intense sound source at Heard Island, a remote location in the southern Indian Ocean, demonstrated that the sound signal could be detected throughout the world ocean. The stage was set for a 10-year experiment, the Acoustic Thermometry of Ocean Climate (ATOC) project (Baggeroer and Munk 1992). Except the global experiment never happened.
An article in Science in April 1990 on ATOC caught the eye of marine mammal biologists (Cohen 1991), and a March 1994 syndicated LA Times article raised a public outcry. Scientists and the public began to voice concerns about the effects of the high-intensity sound on marine mammals. Because the animals were protected under the Marine Mammal Act of 1972, the National Research Council (NRC) responded by commissioning a study to review what is known about the effects of ocean noise and low-frequency sound on marine mammals (NRC 1994). The studies were inconclusive (NRC 2000, 2003), but the public backlash proved too damaging (e.g., Potter 1994). Regional deployments demonstrated the usefulness of acoustic tomography for observing ocean temperatures (e.g., Dushaw 2014). Still, the complete project as envisioned by Munk never came to pass.
The controversy over the ATOC experiment caught scientists by surprise. It also pitted environmentalists against environmentalists in a battle of concerns between global warming and marine mammal protection. It’s a cautionary tale for scientists to learn to better communicate with the public and a call for science educators to help the public better understand the nature of science. As Potter (1994) wrote nearly three decades ago: “The combination of increasing public concern with environmental issues, disenchantment over scientific and governmental integrity, and the widely accessible information highway combined to produce an explosive mixture which government, the public, and scientists need to learn how to handle more responsibly in the future.”
The story has a happy ending, however. In 2020, California Institute of Technology seismologist Wenbo Wu and colleagues published a modified method for ocean acoustic tomography using naturally occurring seismic waves, what they called seismic ocean thermometry (Wu et al. 2020). Comparing the speeds of ocean-traveling seismic waves across the equatorial Indian Ocean from 2005 to 2016, they observed temperature trends that compared favorably with those measured by Argo floats and estimated using computer models of ocean temperature. As they express at the end of their paper, “Global seismic ocean thermometry can thus substantially enhance our capability to monitor ocean warming.” While their methods and observations will require further confirmation by the scientific community, the promise of acoustic tomography—as first envisioned more than four decades ago—may finally be realized (e.g., Dushaw 2022).
5.10.2 Ocean Noise in the Twenty-First Century
Concerns over ATOC have also focused public attention on other acoustic technologies and ocean noise in general (NRC 2003). Seismic reflection studies using airguns drew the attention of marine mammal scientists as a possible cause for beaked whale strandings in the Gulf of California (Malakoff 2002). A federal judge ordered a halt to the work (LA Times 2002). That prompted an ocean conservation group to conduct and publish a study on the effects of seismic airgun testing on marine mammals with the provocative title, A Deaf Whale Is a Dead Whale (Heulsenbeck and Wood 2013). The report and headline made newspapers (e.g., USA Today 2013). They often resurface when stories about seismic reflection studies appear (e.g., New Internationalist 2019). Are such reports overly dramatic, focusing only on the negative issues to stir public emotion?
In a 2011 article in Oceanography, Boyd et al. ask, “Does the noise made by humans harm marine life?” As with many scientific questions, the devil is in the details. How do you characterize harmful ocean noise? How do you define harm? What types of marine life should be considered? These are not easy questions to answer.
As researchers in this field emphasize, a great deal of additional study is needed (Tyack et al. 2015; Williams et al. 2015; Erbe et al. 2019; deQuiros et al. 2019; Duarte et al. 2021). Human-generated noise can be defined in terms of its acoustic characteristics and variability in time and space (e.g., Lindgren and Wilewska-Bien 2016). Unfortunately, measurements of these variables are lacking (Erbe et al. 2019). Collectively, ocean ship noise along the US West Coast has increased since the 1960s. Current levels appear steady or decreasing. In other regions of the ocean, such as the Indian Ocean, ship traffic has increased dramatically (Andrew et al. 2011; Tournade 2014; Miksis-Olds and Nichols 2016; Ragland et al. 2022).
The lack of long-term comprehensive data on ocean noise in US waters prompted NOAA and the National Parks Service to establish in 2014 the Ocean Noise Reference Station Network, described as “an array of . . . calibrated autonomous passive acoustic recorders” (Haver et al. 2018). Efforts to address the global data gap have begun under the guidance of the International Quiet Ocean Experiment, a “program of research, observation, and modeling to better characterize ocean sound fields and to promote understanding of the effects of sound on marine life” (Tyack et al. 2015; Duarte et al. 2021).
Observing and characterizing harm to marine animals proves equally challenging. While there is evidence that military sonar may lead to beach strandings and deaths for some species, the evidence is not direct, and these occurrences are rare. Loss of hearing—temporarily or permanently—due to loud and sustained sound has been shown in captive animals. How these results apply to animals in the wild, who can move freely and generally avoid harmful sounds, is unknown. Effects on animal behavior, health, survival, and reproductive rates remain unclear (e.g. Weilgart 2017).
Nevertheless, new clues have emerged on the impacts of ocean noise on fishes and mammals. The behavior of Atlantic cod appears disrupted by seismic reflection surveys (van der Knapp et al. 2021), and Pacific salmon and herring alter their behavior in response to boat noise (van der Knapp et al. 2022). A study of vocalizations by humpback whales revealed that the whales sing louder when ocean noise levels are louder (e.g., Dunlop et al. 2014; Guazzo et al. 2020). This behavior of increasing sound level in response to increased background noise is known as the Lombard effect, named after French doctor Étienne Lombard (1869–1920). He, like you, noted that humans talk louder when their environment is louder (e.g., Zollinger and Brumm 2011). Other marine mammals, including minke whales and bottlenose dolphins, also exhibit the Lombard effect in response to increasing ocean noise (Kragh et al. 2019; Helbe et al. 2020; ).
As unsatisfactory as the answer “we don’t know yet” can be to a public accustomed to definitive answers, we can gain solace in the knowledge that research in this field is proliferating (Williams et al. 2015). While scientists acknowledge that “we are unlikely to resolve this challenge quickly or completely” (Tyack et al. 2015), the availability of new tools and the establishment of long-term acoustic monitoring networks provide promising avenues for a better understanding of ocean noise and its impacts. Public outreach and education—and transparent communication with public, commercial, and governmental stakeholders—will help establish the societal importance and public trust for the continued support of ocean acoustic science (Spence et al. 2022).
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