Beth Brost, Bernard Johnson, Diane Tulipani

A paper for MSCI 375, Biology of Marine Mammals, submitted October 5, 1998 

  Underwater noise pollution has only recently been recognized as a potential problem for the inhabitants of the oceans. The full impact it has on marine mammals is not completely known due to the difficulty in measuring their responses. The increase in man-made underwater noises could be a serious problem to their survival as it can interfere with their methods of communication and hunting strategy. Observed reactions and effects on specific cetaceans has been documented but not clearly understood. The following is a brief discussion about this dilemma.

 Why is sound important to marine mammals?

Marine mammals, like whales and dolphins, spend their entire lives in the ocean. It is important for these animals to be able to distinguish sounds, especially if they live in murky waters or in waters were there is low light penetration. Under these circumstances, marine mammals must be able to use sound to locate prey, to navigate, and to communicate.

Sound is measured in Hertz (Hz), which are cycles per second. It travels 4.5 times faster in water than it does in air (Waller & Geoffrey, 1996) and low frequency sounds travel farther underwater than high frequency sounds -- 100’s even 1,000’s of kilometers farther.

The exact method of sound production differs between the various marine mammals. Because odontocetes have been the most studied, we’ve chosen the bottlenosed dolphin as an example in describing how sound is produced and received by a marine mammal. For a bottlenosed dolphin, sound is produced in the nasal passages and compressed in the melon, which focuses the sound. Underwater, sound reaches the inner ear through the skull from all directions. It is believed that sound is received through oil-filled sinuses in the lower jaw and channeled to the inner ear. Because sound is coming through the skull from all directions, the inner ear is acoustically isolated from the skull by foam-filled air spaces. This adaptation helps diminish nondirectional sound coming through the skull. The hearing ranges of marine mammals differs from one species to the next. Odontocetes, for example, have a hearing range up to 150 kHz, mysticetes between 20 Hz to at least 3 kHz, pinnipeds range up to 70 kHz underwater and 30+ kHz on land, while humans have a hearing range between 20 Hz to 20 kHz.

Marine mammals produce sound for several reasons. One reason is to echolocate. Echolocation is the production of sounds that allow an animal to orient itself and to locate objects by means of the returning sound waves or echoes (Riedman, 1990). It is believed that odontocetes are the only marine mammal to use sound production for echolocation. Odontocetes generate intense, broadband pulses in a frequency range of 0.25 to 220 kHz (Evans, 1987). These pulses bounce off objects and produce echoes which can be used to build an acoustic picture of the object or their surroundings. The frequency of returning echoes and their loudness provide odontocetes with information, such as size, shape, density, distance, and movement of objects. Each species has different characteristics and ranges of sound frequencies perception. As an alternative to echolocation, it is believed that some odontocetes, like the killer whale, use passive listening (Barret-Lennard et al, 1994) which also provides cues for prey location without producing sound. Also, some dolphins may use non-visual external cues, such as swimming sounds, to locate prey.

Another reason for sound production is communication. Mysticetes, for example, use a lower frequency in the range of 20 Hz to 3 kHz. The advantage to using lower frequencies is that it travels farther underwater. In fact, in the SOFAR Channel, these sounds could reach up to 5600 km. Unfortunately, with a noisier ocean, some believe that range may not exceed 800 km. It is thought these sounds carry a great deal of information about the individual, such as its location, gender, and activity or emotional state.

 Sources of Underwater Noise

Marine mammals are now faced with an increasingly noisy underwater environment. There are many sources that can be divided into two broad categories: natural and man-made. Both types span the full range of hearing of marine mammals. As they have successfully adapted to their underwater environment, the impact of the naturally generated sounds are minimal. However, man-made noises are a serious problem.

There are many sources of natural, or ambient, underwater noises. Ambient noise is environmental background noise (Richardson et al., 1995). It is generated by such things as wind and waves (depending on sea state), the movement of sea ice and icebergs ( > 120 dB, 4-8 Hz), seismic noise from volcanic and tectonic activity ( < 500 Hz), and precipitation ( 100 - 500 Hz). Included in ambient noise are biological sources such as shrimp and fish (>12 Hz) (Dodds, 1998 ) and cetaceans (ex. fin whale 100 dB at 330 ft, blue whale 100 kHz) (Richardson et al., 1995). The biologically generated noises can range from nearly absent to dominant over both narrow and broad frequency ranges (Richardson et al., 1995).

Human activities on and in the water also generate much noise. For example, shipping and industrial sources are loud, and sometimes constant sources. Shipping traffic noise levels are about 200 dB with a frequency range of 20 - 300 Hz. Commercial fishing fleets using sidescan sonar or echo sounders to locate fish schools generate sounds ranging 180-230 dB and 5-500 Hz. Industrial sources of noise are oil rigs, dredges, and deep-sea drilling ships (115 dB, 250 Hz). Also, sounds from ice breaking ships ( 14 dB, 20 - 100 Hz) can be detected over 50 km away from the source. Recreation and watersports generate noise from the motors greater than 100 dB over a range of frequencies (12 Hz - 30 kHz).

The scientific community also generates it share of underwater noise. Some examples are from seismic blasts (150-200 dB, 1-1.5 kHz) for geological exploration of the ocean floor and sediment layers. A controversial source is the experiment being conducted by the Scripps Institute in conjunction with the US Navy. ATOC, the Acoustic Thermometry of Ocean Climate, is to measure global warming in the Pacific Ocean by measuring the velocity of sound waves generated at two specific sources near Hawaii and received 1000’s of miles away in the north Pacific (Svitil, 1995). The controversy is due to the fact that the sound to be generated is 195 dB at a frequency of 75 Hz. Some believe that this low frequency will interfere with the communications of the larger whales, specifically the mysticetes.

Another important source of underwater noise is that which is generated by the military, specifically the US Navy. The ship-based sonar produces sound waves at 200-230 dB, over a frequency range of 2-500 kHz. In addition to this standard type of sonar, the US Navy is developing and experimenting with a new low frequency active sonar (LFAS) to detect the newer, quieter class of submarines. This sound is generated at 230 dB and 75 Hz. Again, this low frequency is believed to cause problems for marine mammals. Also, the loudness level (230 dB) of the sound generated produces a pressure wave 1 billion times more intense than a 120 dB noise (ex. motorboat). How this may impact the physical conditions of marine mammals exposed to it is being strenuously debated by conservation groups and the US Navy.

 Responses by Marine Mammals to Underwater noise and what is being done.

The reactionary response of marine mammals to low frequency, high decibel noises varies from species to species. Unfortunately, the scientific data currently obtained is often limited to specific families. A great deal of information has been gathered on cetaceans from field observations and controlled experiments. The larger cetaceans appear to respond to loud noises in a more predictable manner which makes them ideal for study. Even within each individual family, various reactions have been documented, but no conclusions have been given for this.

As a general rule, whales will avoid sounds between 110 to 120 dB. Most of the whales studied attempted to avoid sound waves at a level of 115 dB. At higher frequencies, all species became frantic, their heart rate increased, and in some cases, vocalization ceased. The highest level a whale can endure before ear tissue damage occurs is still unknown.

In reaction to low frequency sonar, sperm whales became silent, stopped their activities and scattered when military sonar signals were broadcast (Watkins et al, 1989) and humpback whales showed avoidance behaviors when sonar was played back to them (Maybaum, 1989). There have even been reports of people being accidentally exposed to low frequency sonar while in the water. In March 1998, a woman, Chris Reid, was exposed to a 125 dB sound while scuba diving. Her experience coincided with a naval vessel which was conducting low frequency sonar tests 100 miles away. She complained of vibrating lungs and disorientation as a result (Declaration file in count, March 25, 1998).

There have also been reports of mass strandings around the Canary Islands which came as a direct result of LFAS. Consider the correlation between military testing of LFAS and whale strandings by scientists Pizarro, Vonk and Martin. Between the years of 1982 and 1989, the annual number of cetaceans that expired on the Canary Islands beaches was counted and graphed (Simmon & Lopez-Jurado, 1991). This evidence revealed that during the years 1985 and 1988, the highest number of strandings was recorded. This corresponds to the same time the military was in the vicinity testing LFAS in the nearby ocean.

There is no solid evidence that ATOC has been the cause of whale deaths, but there are several speculations to such. On November 3, 1995, a humpback whale beached north of San Francisco and, one week later, two dead humpbacks were found floating near the Farallon Islands near the vicinity of the ATOC source. Since there was no autopsy performed on the dead humpbacks, the exact cause of death was unknown. In any case, the National Marine fisheries Service ordered the ATOC program postponed while the deaths of the whales were further investigated. Although ATOC is presently being studied for it’s effects on marine mammals (Cetacean Society, 1996), there still remains a fear that it’s continued use may pose a risk to the inhabitants of the world’s oceans. Little is known about the potential damage ATOC may cause to marine mammals as well as other marine life.

In March 22, 1996, the ATOC experiment was picked up by the media. The Los Angeles Times front-page read "Plan to measure temperature may endanger marine mammals." That night, an NBC Nightly News television segment was broadcast about the same topic. Five days later, "Ocean tests delayed" was what the L.A. Times reported. The Internet has also played a big role in making people aware of the ATOC testing, perpetuating misinterpretations. In November 1996, the National Resources Defense Council ordered that the Navy release an Environmental Impact statement which would study effects of LFAS on marine mammals.

The oceans are already polluted by noise from shipping traffic and other industrial sources, recreational watersports, and scientific explorations that man is making it more difficult to protect what is already there. Little is truly understood as to how the different marine organisms, in particular marine mammals, are effected by the increased levels of noise in their environment. We first need to understand how marine mammals are affected by these disturbances in their world before we add to it.

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A note from Dr. Young:

This is a particularly difficult topic. It is nearly impossible to clearly document avoidance behaviors or physical injury and/or hearing loss of wild marine mammals over large areas of the ocean. In addition, we have no good frame of reference for how loud a sound is underwater, since our ears are not adapted to hear well underwater. Even the common unit of the decibel (dB) is not easily comparable between air and water. Whereas 120 dB is the pain threshold for humans in air, a 120 dB sound in water (which technically should be tied to a reference pressure) may not be of a comparable intensity to a marine mammal. In short, we have very little idea just what effect these sounds have on marine mammals. Things like the LFAS signals may have a profound and negative effect or they may have very little effect. At this point, we really don't know, and that may be the scariest part.

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