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LatestPhoto
The crew of the Odyssey found many whales very close to major shipping lanes in Sri Lanka.
Photo: Chris Johnson

May 6, 2003
How Ships' Traffic Noise affects Whales In a Shipping Channel
  Real Audio Report
  28k


Log Transcript

This is Peter Madsen speaking to you from the Odyssey in Sri Lanka.

The Odyssey crew is currently working 50 miles southwest of Dondra Head near the southern tip of the island. Over the past three days we have sighted between forty and fifty sperm whales, even though we are sailing in one of the busiest shipping lanes in the entire Indian Ocean. The majority of ships travelling around the southwest coast of Sri Lanka are moving between the Suez Canal and Eastern India and from there on to elsewhere in Asia or to Australia.

Yesterday, while crossing this busy shipping channel, we detected the clicks of a new group of sperm whales. They were barely audible over the noise of numerous tankers. We had not expected to hear any animals in this region, and were simply crossing the shipping lane in an effort to reach a quieter area where we could listen for whales.

We tracked the whales through the night until they fell silent at 3.30am. But at 7.30am we sighted their blows in the distance, illuminated by the rising sun against dark grey storm clouds to the west. As we approached, it became evident from the number of blows plus the 'coda' clicks we could now hear on the acoustic array (our underwater microphones), that this relatively large group was socializing at the surface.

It was surprising that a group of adult females and subadults, were feeding and socializing in such a major area of heavy shipping traffic. We watched one cluster of 10 whales slowly travel across the path of an enormous container ship that was bearing down on it. As the ship cut through their wake it barely cleared the whales wake.

This left all of us aboard the Odyssey discussing the effects on whales of such loud, manmade, underwater sounds.

Whether travelling through air or water, every sound is a travelling wave of pressure fluctuations. If the distance between the pressure events in the sound is short, the frequency (or pitch) of the sound is said to be high, and if the distance between the pressure events in the sound is long, the pitch or frequency of the sound is said to be low. Pitch or frequency is measured in units called Hertz, which describes the number of sound waves (i.e. fluctuating pressure waves) per second. The unit kHz thus describes how many thousand sound waves occur each second. Thus, 10,000 waves per second has a frequency of ten thousand Hertz, (i.e. 10 kHz).

Unlike light, heat, odors, or any other sensory stimuli sound propagates well in water. For this reason, many marine organisms use sound for communication, orientation and finding prey.

LatestPhoto
A group of sperm whales.

Listen to the click sounds produced by Sperm Whale recorded from the Odyssey:
  Real Audio: > 28k   > 64k

Listen to the same click sounds masked by ship's noise at a distance of 1000 meters from the Odyssey:
  Real Audio: > 28k   > 64k
Photo: Chris Johnson

The use of sound by animals in the sea has reached its epitome in whales and dolphins. The large baleen whales produce powerful low frequency communication sounds that may travel for hundreds or even thousands of kilometres. The toothed whales produce two sound types, tonal whistles that are used for communication and short duration, ultrasound clicks used for echolocation. Production of ultrasound echolocation pulses by odontocetes has led to theco-evolution of ears adapted to detect ultrasound. Most dolphins hear best at frequencies between 30 and 100 kHz, well above the 20 kHz upper limit for human hearing. The trouble with such high frequencies is that such sounds are lost to attenuation (i.e. their acoustic energy is converted to heat energy) after only a relatively few meters of travel. However, low frequency sounds travel relatively enormous distances before being lost to attenuation. Baleen whales appear to take advantage of very low frequency sounds to communicate over long distances. The largest baleen whales make sounds of less than 30 Hz, well below the frequency range adult humans can hear-they are so-called infrasounds. So most cetaceans, though they also may hear well within our range of hearing also hear sounds of lower or higher frequencies than we do. This means that it is a mistake to superimpose the auditory expectations we humans have onto what we expect marine mammals to hear.

Like most mammals, cetaceans have an acute sense of hearing, meaning that detection of faint sounds must be limited by ambient noise in the sea rather than by the whales' hearing thresholds.

Noise is defined as any sound that is not the signal of current interest to any listening animal or machine. Cetaceans are subjected to noise from a wide range of natural sources, such as waves, surf, other animals, volcanic activity, etc., and they have evolved an auditory system that can deal with all such sounds. However, during the last 50 years, increased human activity in marine habitats has led to a three-fold elevation of ocean noise levels at frequencies below 1 kHz.

The man-made increase in ocean noise levels is mainly due to heavy shipping activity and can be heard throughout the world's oceans 24 hours a day. Ship noise is to some degree generated by the engines of the ship but the main noise source from large ships is due to a phenomenon called cavitation. A propeller moves a ship through the water by creating a lower pressure on its leading face so that the propeller actually sucks the ship forward. Cavitation occurs when water moves so fast across a propeller blade that the pressure on the propeller face falls to a level at which it begins to boil (because of the fact that the lower the pressure, the lower the boiling point of water). The bubbles that start to form as the water boils soon collapse, producing short broadband clicks at high sound source levels. Summed together they constitute a kind of rushing roar.

Because of the broadband noise from cavitating propellers, the noise from any motorized vessel may have severe acoustic effects on marine wildlife. Usually, the impact of man-made noise is assigned to different levels by whether it causes changes of behavior, 'masking' of other signals, or causes temporary (or permanent) hearing trauma.

'Masking' is a physical phenomenon that occurs when an otherwise detectable signal is drowned out by ambient noise. Those of us who have tried yelling to the person next to us at a train station or in a schoolyard have experienced the effort needed to overcome masking noise. The greatest masking occurs when there is an overlap between the frequencies in the signal of interest and the frequencies in the noise. To choose an example relevant to whales: the maximum distance at which an echolocating toothed whale may detect its prey, or the maximum distance at which a receptive female might hear a calling male will be reduced by masking noises such as ships' traffic noise. The more masking noise, the greater the reduction of the detection range. The closer that the frequency of the masking noise is to matching the noise of interest, the greater the effect that that masking noise will have.

The mammalian auditory system consists of a set of band pass filters, each covering a frequency band of approximately 23% of the center frequency of that band: for example: the band width of a mammalian auditory filter with a center frequency at 500 Hz would be (0.23 x 500Hz) 115 Hz. This implies that the delectability of a tone of sound within the frequency borders of a filter is not affected by noise outside the frequency borders of the filter. On the other hand if the frequency content of the noise falls within the borders of the filter, the signal is masked and the delectability of the signal drops.

When we have a problem detecting sperm whale clicks because of the underwater noise produced by ships, sperm whales must certainly also have at least some problem detecting the echoes from their echolocation clicks, as well as the clicks from other whales with which they are communicating. In fact, all functions that involve detection of any sound are impeded to a large extent when any animal is exposed to the wideband noise from ships. The loud and ever present shipping activity south of Dondra head here in Sri Lankan waters will inevitably mean that whales and dolphins have a harder time finding food and communicating in the area.

So why would the whales stay in such an area? They probably stay here because food is abundant. However, because of ships' noise the area is likely to have a smaller carrying capacity (fewer animals per area) than it had before heavy shipping arrived. Of course, the whales can also be expected to be more vulnerable to being struck by ships since the noise from other ships masks the noise from ships that are on a collision course with the whale. Under some circumstances the masking may be enough to mean that a whale will start to move out of the way too late.

LatestPhoto
Dr. Peter Teglberg Madsen - R/V Odyssey Chief Scientist.
Photo: Chris Johnson

There are ways to protect whales that congregate in areas frequented by heavy shipping traffic, and if it becomes evident that a major shipping channel is in conflict with a feeding or breeding ground that whales have chosen there are precedents for moving shipping lanes. This was done recently on the east coast of the United States where a shipping lane was moved 4 miles so as to protect a group of highly endangered northern right whales that linger in the area.

Secondly, ships don't have to emit so much noise. A noisy propeller from a ship, actually signifies a low efficiency of the propulsion system since the energy that is dissipated while creating cavitation does is not used for propulsion. The technology to make propellers quieter exist. It is used in fast, but acoustically stealthy military submarines. The problem is to get such information released for commercial use, and to persuade the shipbuilders and buyers to invest in quieter propellers for their improved efficiency.

For example: a reduction by half of the noise level from a large cargo ship would reduce by eight times the volume of ocean within which that ship was masking the sounds that whales near it were trying to hear. (The engineering is actually available to reduce the problems of masking by shipping. The problem is that most of the focus on noise pollution in the sea has been on much smaller projects with far more localized effects, an example being the potential effect of a new, experimental, military sonar on marine mammals. The reason for such emphasis seems to be that such problems have a much higher media profile. It would seem prudent to adopt a larger perspective on noise pollution. One way to start would be by facing squarely the worldwide problems that are caused by heavy commercial shipping, along with the degree to which various sources of underwater noise pollute the seas. It might thereby become apparent that the biggest source by far of underwater noise pollution is commercial shipping, and that very little, if anything, is being done to alleviate that problem.

How refreshing it would be to see a country such as Sri Lanka, with a maritime history as old as ships, take the lead in finding and promoting solutions to this global problem. It seems especially apt since Sri Lanka hosts more whale species within its waters than any other comparably sized area we have investigated, and Sri Lankan waters offer, in our experience, the best conditions we have encountered in which to learn about the lives of whales.

Links:

  • Roger Payne discusses the topic of noise pollution in depth: Part 1 & Part 2.
  • What did the crew report on one year ago in Australia? Two years ago in Papua New Guinea?
    Three years ago in the Galapagos Islands? -> Real Video: > 56k > 200k

    Written by Peter Teglberg Madsen & Roger Payne

 
 
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