The topic of underwater sound is complex and requires a foundation in multiple sciences to fully comprehend. It becomes even more complicated when animals are exposed to anthropogenic sounds in the marine environment. Scientists are just beginning to explore the questions that need to be answered. The scientific process is at work, and it will take time to get these answers, but we are making progress with new discoveries occurring all the time.
This quiz focuses on the science of underwater sound - what is known and what is not known. Your challenge is to determine whether each statement listed below is a fact (something that has been verified to be accurate through the scientific process) or a myth (something that is believed to be false). Once you decide, click on the blue "Answer" box and you will get the accurate response along with links back to places in the DOSITS website where you can get more information on the topic.
Good luck!
The greatest uncertainty in understanding the effects of underwater sound on marine animals is knowing how sound propagates.
MYTH
Determining whether a sound will affect a marine animal requires knowing both how the sound will spread or propagate and how well the animal can hear the sound. The greater uncertainty is in the animal data. Scientists know a great deal about the physics of sound in water and comparatively little about how or what marine animals hear.
References:
How do you determine if a sound affects a marine animal?
Sound rapidly gets weaker as it moves away from a source.
FACT
If we think about sound as a rock thrown into a pond and look at the pond from the side, you will notice that the wave gets smaller as it moves away from the place where the rock fell (the source of the waves). The wave gets smaller because it spreads out and because some of the sound energy is absorbed by seawater.
Sounds from World War II are still circling the globe, trapped in the sound channel.
MYTH
The sound (SOFAR) channel was discovered in World War II. Sounds travel further in the SOFAR Channel than in other parts of the ocean, but all sounds in water scatter and vanish, just as in air.
Background sounds heard by porpoises are dominated by human noise.
MYTH
Porpoises and other odontocetes hear best at high frequencies (10,000 to 50,000+ Hz), whereas background or ambient noise is primarily due to humans (noise generated by distant shipping) in the frequency range of 20-500 Hz.
Background sounds heard by the large whales are dominated by human noise.
FACT
The large whales include all species of baleen whales, such as blue, fin, and humpback whales. There are no direct measurements of the hearing abilities of baleen whales. Anatomical evidence and vocalizations strongly suggest that they are adapted to hear low frequencies. The background or ambient noise is primarily due to humans (noise generated by distant shipping) in the same frequency range (20-500 Hz) that the large whales are believed to hear best.
Scientists understand how important the effects of anthropogenic sounds are on marine mammals.
MYTH
Researchers suggest that increased background noise and specific sound sources might impact marine animals in several ways. The potential impacts include sounds that:
1. cause marine animals to alter their behavior
2. prevent marine animals from hearing important sounds (masking)
3. cause hearing loss (temporary or permanent) or tissue damage in marine animals
It is also not clear how important these impacts are to the well being of the animals and their populations.
Most marine mammal strandings are due to underwater sound.
MYTH
Approximately 40,000 stranded marine mammals were reported in the United States alone by the National Marine Fisheries Service stranding network over the decade 1990-2000, with an average of 3,600 strandings per year (National Marine Fisheries Service, 2000). In three well-documented cases worldwide, there is sufficient information about the sonar operations, the times and locations of the strandings, and the injuries to the animals to associate the strandings with sonar use. In these three cases, 32 animals are known to have died.
Marine mammals must be able to hear a sound in order for it to affect their behavior.
FACT
Hearing is the universal alerting sense in all vertebrates. Sound is so important because animals are able to hear events all around them, no matter where their attention is focused. Many species of blind amphibians, reptiles, fishes and mammals are known, but no naturally profoundly deaf vertebrate species have been discovered. Although hearing is important to all animals, the special qualities of the undersea world emphasize the use of sound.
Marine mammals use sound to communicate, navigate, and locate prey.
FACT
People and marine animals use sound in the sea to accomplish many tasks. Since light travels relatively short distances in the ocean, sound is often used by animals for such basic activities as finding food or a mate, navigating, and communicating.
Decibel levels in air are the same as decibel levels in water.
MYTH
The decibel is a relative unit of measure, not an absolute one. Confusion arises because relative intensities in water are referenced to 1 microPascal (µPa) whereas sound waves in air are referenced to 20 microPascals (µPa). Therefore, relative sound intensities given in dB in water are not directly comparable to relative sound intensities given in dB in air.
Sound moves about 1500 meters per second in seawater. That's approximately 15 football fields end-to-end in one second!! Pretty impressive! Sound moves much more slowly in air, at about 340 meters per second, only 3 football fields a second.
Sound can be used to measure ocean temperature, currents and waves.
FACT
People routinely use sound in the sea for many applications. Because sound travels or propagates differently with different temperatures, currents, and waves, the change in sound propagation can be used to collect information about the oceanŐs characteristics.
Fish species produce many sounds, including grunts, croaks, clicks, and snaps.
FACT
Fishes produce a variety of sounds using different mechanisms and for different reasons. The three main ways fishes produce sounds are by striking or rubbing together skeletal components, using sonic muscles that are located on or near their swim bladder, and by quickly changing speed and direction while swimming. The majority of sounds produced by fishes are low frequency, typically less than 1000 Hz.
Stranding events involving multiple beaked whales have coincided closely in time and space with military activities using sonar.
FACT
Investigating the cause of a stranding is very difficult, because scientists must try to reconstruct what happened after the event, have little or no history on the individual animals, and, as a result, can generally draw firm conclusions in only about half of all strandings. However, in three well-documented cases, there is sufficient information about the sonar operations, the times and locations of the strandings, and the injuries to the animals to associate the strandings with sonar use. These events occurred in Greece (1996), the Bahamas (2000), and the Canary Islands (2002).
Anthropogenic sound is the leading cause of marine mammal deaths.
MYTH
Marine mammals face threats from many different human activities, including fishing, habitat destruction, ship strikes, whaling, and sound production. Of these threats, fisheries bycatch kills the most marine mammals. Globally, it is estimated that more than 650,000 marine mammals are killed annually by being accidentally caught in fishing nets (Read et al., 2006)
Hearing loss due to noise does not occur if the frequency of the sound to which the animal is exposed is outside the range that the animal can hear.
FACT
Hearing loss from sound exposure depends on the sensitivity of the animal to a sound and the interaction of three characteristics of the sound: the frequency of the sound, the intensity of the sound, and the duration or how long the animal is exposed to that sound. Hearing loss does not occur if the frequency of the sound to which the animal is exposed is outside the range that the animal can hear.
The only way to reduce the effects of anthropogenic sound on marine life is to eliminate the sound source.
MYTH
A variety of approaches have been used to reduce the effects of anthropogenic sound, although the extent to which these measures are effective has not been determined:
Avoiding marine mammal habitats.
Detecting animals and modifying the sound-producing activities.
Scientists agree that we don't know enough about how underwater sound may be affecting marine life.
FACT
Researchers suggest that increased background noise and specific sound sources might impact marine animals in several ways. The effects vary depending upon the intensity and frequency of the sound, and other variables. It is also not clear how important these impacts are to the well being of the animals and their populations.
Low-frequency sound levels in the ocean have been increasing since the introduction of steam-powered shipping.
FACT
In the frequency range of 20-500 Hz, ambient noise is primarily due to noise generated by distant shipping. Noise generated by shipping has increased as the number of ships on the high seas has increased (Andrew et al., 2002).
Scientists are objective. Their feelings and political views rarely or never influence their research or conclusions.
MYTH
Science is a process for asking questions about the natural world and testing the answers. When too little information is available, scientists note observations for which there is no present explanation, because they might later be useful in testing hypotheses. The curious scientist resists leaping from observations to cause and effect relationships without employing the Scientific Method to validate any hypotheses that might be posed.
One way to arrive at "scientific truth" is to conduct an opinion poll of scientists.
MYTH
The Scientific Method is an orderly process for asking questions about the natural world and testing the answers. Hypotheses that have been consistently validated through observations or experimentation can eventually be advanced to the status of theory. A theory is a thoroughly substantiated explanation of some aspect of the observable world. Theories come as close to objective truth as possible.
