Noise and Cetaceans
The unique characteristics of underwater noise were noticed as early as 1490 by Leonardo Da Vinci, who wrote:
“If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you.”
Of course, Da Vinci was listening to the quiet sounds of sailing ship hulls slipping through the water, not the noise from propellers and steam or diesel engines. Since the mid to late 1800s, when steam engines with propellers (or screws – based on Da Vinci’s designs from as early as 1452) began to replace sailing ships, the ocean has grown progressively louder; and now includes the noise from numerous ships, pile driving and ship loading; not to mention the extremely loud sounds produced by military sonar and the explosions of seismic exploration in the search of oil and gas. In most locations of the world you would now find it impossible to hear the noise of a passing sailing ship above the background noise.
Sound is as important to cetaceans as light is to humans. In most oceans visibility underwater is limited to a few 10s of metres and is regularly much less. In response to the challenges of using light to navigate and locate food in the ocean, cetaceans have adapted to take advantage of the physics of sound underwater. Sound travels ~4.4 times faster in sea water than in air and, all things being equal, travels ~100 times further underwater than in air. As in air, low frequency sounds travel much further underwater than high frequencies (think of the distant sound of the bass drum heard will before the rest of the marching band at the parade). If conditions are right, underwater sounds can travel thousands of kilometres across entire ocean basins – low frequency blue whale calls can travel in “deep water sound channels” that refract waves up and down allowing them to travel thousands of kilometers.
The same characteristics of underwater sound that whales have employed to their advantage can also work to their detriment. Many divers are well aware of how far sound travels underwater; and that humans produce a lot of underwater noise. Propellers produce some of the loudest continuous sounds in the ocean: Large ships can produce sounds levels over 170 dB* and even small fast traveling pleasure boats produce noise of 145 to 160 dB. Seismic surveys and navy sonar produce some of the loudest sounds in the ocean at over 240 dB. To put this in perspective, research has shown that killer whales react strongly to a received level of 135 dB (re 1 µPa) – the pain threshold. Much quieter noises will impair the ability of killer whale to detect other whales calls and to locate their prey. Obviously, as sound moves away from the source it slowly gets quieter as the sound energy is spread out over a larger area, as bubbles, plankton, land, etc. absorb sound energy and as sound waves are refracted and reflected. Using the sonar equations we can calculate how far these sounds may travel and remain above the killer whale threshold.
Propeller noise from a fast power boat with a 160 dB source level will be heard as a 135 dB noise at a distance of about 20 metres from the boat, but will still be at ~70 dB 14 km from the boat – enough to mask other whales calls and interfere with the ability of whales to locate prey. If multiple vessels are present their noise will add to a louder noise, although this addition is complex: two vessels producing 140 dB sounds side by side will appear to produce 146 dB noise together.
There is strong evidence that seismic and navy sonar have caused trauma and death in some species of cetaceans. A 240 dB seismic blast will be heard as a 135 dB sound (the pain threshold) at a distance of ~28 km from the source.
*note: all source levels are reference to 1 µPa @ 1 metre. The decibel or dB is a scale used to compare sounds of different intensity. Underwater sounds intensity is compared to a change in sound pressure of 1 micro Pascal (1 µPa) at a distance of 1 metre from the source. In air, a higher standard reference sound pressure of 20 micro Pascals is used – thus sounds of the same energy underwater as in air are recorded as 26 dB higher underwater than in air.
MASS STRANDING EVENTS ASSOCIATED WITH SONAR
1996 event off the west coast of Greece
- 12 or so animals, all beaked whales
- 2 day period (12 -13 May) over 35 km of coastline
- Shallow Water Acoustic Classification (SWAC) experiment, SACLANTCEN (D’Amico et. al., 1998)
2000 Bahamas Islands Event
- 16 cetaceans, both beaked and minke whales
- 36 hour period (15 -16 March) over 240 km of coastline
- U.S. Navy ASW exercise involving hull-mounted sonar systems on 5 ships
2002 Canary Islands Event
- 14 or so animals, all beaked whales
- Most believed stranded on morning of 24 September, on the SE and NE
- sides of two islands
- Neo Tapon exercise involving 11 NATO countries to secure the strategic Strait of Gibraltar
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