The natural world has its own soundtrack, so much so that a significant body of research is dedicated to the study of its acoustics, often falling under the labels of acoustic ecology, soundscape ecology, and bioacoustics.
Researcher Bernie Krause, founder of Wild Sanctuary Inc., in Glen Ellen, California, has pioneered much of the work that shapes how we view soundscape ecology. He coined the term “biophony” in reference to the collection of organism-produced vocalizations in any given habitat. Over the years Krause and his team have recorded and collected the melodies of wildlife worldwide and archived the results.
Krause makes the harrowing observation that 40 percent of the soundscapes he has visited and documented are blemished by a component of human-made, or anthropogenic, noise. So what impact does this human-made sound have on the marine environment?
We humans are a noisy lot. We produce a cacophonic mess of hums, clicks, bangs, and whines originating from our transportation, construction, and navigation. Our auditory influence can be felt worldwide, even within the ocean depths. The International Ocean Noise Coalition, an organization comprised of 150 non-governmental organizations worldwide, indicates in certain regions the level of oceanic noise has doubled on a 10-year basis for the last six decades.
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| A large, car-carrying ferry is one of many large vessels creating noise in the ocean that disturbs marine life. Photo credit: Nancy Griffin |
Make no mistake, the sea is a naturally clamorous place. Water movement such as waves and tides, and meteorological events such as rainfall are responsible for a great deal of it. Marine creatures seem wise to the fact that water is an excellent medium for sound propagation and exploit this fact, vocalizing for purposes of mating, social bonding, feeding, and navigation. Within this congregation of cries and calls, different types of living things stake out their own niche, utilizing unique frequencies to avoid muddling the messages they are trying to communicate.
At times, underwater production of biological noise can even impact our own use of acoustic technologies. Schools of Alpheidae, commonly referred to as pistol or snapping shrimp, have been known to actually interrupt sonar communication.
The more common scenario, however, is human noise interrupting the communication pipeline and daily lives of marine organisms. While there are still many questions, the majority of what we do know about sonic effects on marine organisms is in relation to fish and marine mammals, the subjects of a large proportion of acoustic impact studies.
The work of multiple researchers has identified some possible adverse effects of noise on fish, which include changes in their natural distribution, temporary to permanent hearing loss, damage to sensory organs and masking of other biologically important sounds fish need to hear, including those produced by predators, prey and other members of their species. However, these negative consequences may vary greatly with factors such as the strength and constancy or duration of sound exposure, as well as the species, age and life stage of the fish.
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| Whales use the ocean to communicate with each other over hundreds of miles. Noise from human activities can disturb communications between marine animals. Photo credit: Nancy Griffin |
Most scientific work on this issue has focused on cetaceans like whales, dolphins, and porpoises. A wealth of studies exploring the topic includes many originating from institutions within the Gulf of Maine In recent years, Woods Hole Oceanographic Institution (WHOI) in Woods Hole, Massachusetts, has diversified marine mammal acoustic research to include production and use of sound by marine mammals, and also how the mammals are impacted by the anthropogenic component of it.
Research suggests impacts range from abandonment of important feeding grounds and altered foraging behavior, to severe tissue damage and beach strandings. Opponents argue these relationships are correlative at best, meaning it’s difficult to prove real connections exist, rather than coincidental associations.
One of the big hindrances in understanding the complete repercussions of noise pollution is our lack of information concerning how most marine organisms relate to sound. Do they hear like we hear? What does “hearing” mean in an aquatic environment? Humans have a hard time relating to sound perception underwater as our senses are woefully inadequate for the setting.
We do know a diversity of structures exist for sensing auditory vibrations and that not all creatures hear or detect these vibrations in the same frequency range. A prime example originates from recent research at Florida Atlantic University as described by Science Daily, suggesting many manatees suffer from ship strikes because they don’t hear oncoming boats. This ultimately means we cannot assume sound affects all marine creatures great and small in the same manner.
We also cannot assume all anthropogenic sound is equal in its intensity and impact. As described in a report completed by the Stellwagen Marine Sanctuary, the human component of underwater noise in general tends to be derived from “a wide variety of activities including vessels of all types, construction, military events, offshore oil exploration, scientific undertakings, the use of acoustic harassment devices (AHDs), and dredging operations.”
Whether it’s the repeated ricocheting blast of a seismic air gun or the low throb of a cargo vessel, the sounds from these sources are characteristically different in regards to their loudness and duration.
Many of the operations reported in the Stellwagen report are commonplace within the Gulf of Maine. The Gulf is home to multiple maritime ports, not the least of which includes the Port of Boston, a major avenue for commercial and recreational shipping traffic which is likely the most ubiquitous source of human-produced racket in the area. That traffic is largely made up of incoming cargo ships and vessels containing shipments of natural gas and petroleum products, but Boston Harbor is also fast becoming a hub for the high-end cruise market.
The Gulf is rife with rich fishing grounds, and fishing fleets add to a more localized, although slightly less sonorous, flux of noise. Dredging, the underwater removal of sediment for purposes of maintaining navigational channels, has also become a commonplace activity and is one more discordant addition to local anthropogenic noise levels.
It is bewildering that we’re not really sure what the real results of noise pollution in the Gulf of Maine currently are or will be as the sources of human-noisemaking increase. Sound waves’ behavior in water can at times be considered fickle and unpredictable (see accompanying sidebar). We still have significant strides to make in our understanding of the collective marine soundscape ecology and the role of sound in the experiences of ocean life.
We know we’re worried, but we’re not completely sure why. However, we can be encouraged that awareness of noise pollution is increasing and the issue is on the radar of politicians and the scientific community. With some sustained effort, we’ll keep the natural world singing.
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Sound Underwater
Sound behavior is readily influenced by the qualities of the water mass it’s moving through, so predicting how introduced noise will ultimately behave is a dicey business. The speed of sound is the primary difference between the nature of sound in the ocean versus land.
Noise travels at a rate of approximately four to five times faster through water than through air. Certain characteristics of water such as temperature, salinity, and depth/pressure serve to distort the measurement. For instance, sounds travels slower in freshwater than it does in saline marine waters. In the sea, whenever sound waves meet conditions that vary from those they originated in, the wave tends to bend, causing another complication.
One mysterious location in the ocean, dubbed the Sound Fixing and Ranging (or SOFAR) Channel, where a seemingly magic combination of conditions occur, allows sound to propagate vast distances with limited interruption. Savvy marine creatures such as humpback whales use this channel to enhance their communication efforts.
The usual measurement of sound’s intensity—what we tend to equate with “loudness”— the decibel, must be interpreted differently for sounds reported in terrestrial versus aquatic environments. A decibel is a unit of measurement describing the ratio between a sound’s intensity relative to a reference intensity.
While din produced on land and underwater is described in both cases using decibels, the reference intensity for each is different. Cornell University, which hosts a well-renowned bioacoustics program, uses the apt comparison of the measurement of degrees and temperature on their website: “Imagine being told that it’s 30 degrees outside. What would you conclude? If it were 30 degrees Celsius, you might choose to wear sandals and shorts. If it were 30 degrees Fahrenheit, you had better bundle up because it’s freezing! This confusion can arise because Fahrenheit and Celsius are very different scales of temperature measurement which both employ the same word: ‘degrees.’ ”
The word “decibel” functions much the same way as “degree” does in this analogy. For example, as described on the Resonance Publications website, http://www.resonancepub.com/unwateracou.htm, a supertanker measured at 190 dB in the ocean would be grossly equivalent to a terrestrial noise level of 127 dB. Ultimately both measurements are considered abrasively ear-piercing but we still need to view them within the correct context.
Online Resources:
BBC Wildlife Weird Nature Youtube clip, Pistol Shrimp
Discover Sounds in the Sea: Audio Gallery
Voices in the Sea – Recordings of the calls of numerous species of cetaceans
World Forum for Acoustic Ecology
Soundscape: The Journal of Acoustic Ecology
Soundscape: Ocean Acoustics – Underwater Listening
Fish Ecology: Fish and other underwater sounds
Fish Ecology: Studies on Soniferous Fishes
Noise Pollution Clearinghouse – a source for news articles and other publications covering multiple perspectives on the issue of noise pollution
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