I was recently speaking with a group of students visiting from Mexico when one asked, "Why do whales beach themselves?" The answer isn't an easy one!
Sonar comes from the contraction of the phrase, ‘sound navigation and ranging’. In more technical terms active sonar is the use of sound sent out into the water and then reflected to determine the location of an object. Passive sonar makes use of listening for sounds and triangulating their source. The development of sonar was for military purposes but a co-researcher of mine uses passive sonar (does not emit sound) for location, counting, and identification of whales.
Does sonar cause whale strandings?
Two thousand years ago Aristotle wrote: “It is not known why they sometimes run aground on the seashore; for it is asserted that this happens rather frequently when the fancy takes them and without any apparent reason'.”
Clearly there have been reasons for whale strandings before the advent of military sonar. This does not prove that military sonar is not responsible for some of the strandings today.
When active sonar is used an intense burst of sound is released underwater. These sweep the ocean like a floodlight, revealing objects in their path as echoes return to the source.
French F70 frigates are fitted with VDS (Variable Depth Sonar) type DUBV43 or DUBV43C towed sonar
These bursts of sound can reach 240 decibels (billions of times more powerful than the level that causes hearing damage in humans). During testing off the California coast, noise from one of the Navy's low-frequency sonar systems was detected across the full width of the northern Pacific Ocean.
How Sonar Harms Whales
By the Navy's own estimates, even after 500 kilometres, these sound bursts can retain an intensity of 140 decibels -- a hundred times more intense than the level known to affect the behavior of large whales.
Many of these beached whales have suffered physical trauma, including bleeding around the brain, ears and other tissues.
A 1986 West Australian stranding of False Killer Whales
These injuries are similar those resulting from underwater explosions or barotraumas (injury from pressure). I have seen these injuries and provided forensic evidence in legal cases dealing with underwater explosives. They found that many more animals were affected, injured, or chased from the area. Scientists are concerned about the cumulative effect of these bursts of sound on marine animals.
“The Navy’s most widely used sonar systems operate in the mid-frequency range. Evidence of the danger caused by these systems surfaced dramatically in 2000, when whales of four different species stranded themselves on beaches in the Bahamas. Although the Navy initially denied responsibility, the government's investigation established that mid-frequency sonar caused the strandings.” according to the Natural Resources Defense Council. Similar mass strandings have occurred in the Canary Islands, Greece, Madeira, the U.S. Virgin Islands, Hawaii and other locations.
Can we build less dangerous sonar?
It is possible to build sonar systems that use frequencies or power levels that will not harm whales. The problem is that we are ethically unable to carry out research that would deliberately harm whales so we can measure the effects. We can not have controlled exposure experiments because at least 20% of the test animals would have to be stranded, injured, or die to meet statistical analysis requirements.
A Supreme Court decision in the U.S.A. (2000) stopped the U.S. Navy from testing powerful sonar systems in most of the world's oceans after a federal judge ruled that it could "irreparably harm" whales, dolphins and fish. This decision does not relate to other Navies.
The other issue has to do with military competition and security. Supporters of more powerful sonar will claim that we are putting whale safety ahead of national safety if we limit what only some countries can test.
Humpback whales are among the marine mammals effected by sonar.