Whale strandings: are we to blame?
Yesterday a 13m sperm whale died after becoming stranded on a beach in Kent.
The last few years have seen a few incidents like this around the UK. A 10m long sperm whale washed up in Lincolnshire in 2004, a bottle-nosed whale became stranded in the Thames in 2006, and a humpback whale was found dead in the Thames in 2009. It's very difficult to tell the real cause of incidences like this but the million dollar question is, are we to blame?
Juvenile whales may simply get lost and find themselves in shallow waters or those lacking in sufficient food. Whales that have collided with ships may become injured or get infections and so end up on our shores. Illness too can affect an animal's ability to navigate, but it has also been suggested that shipping noise might significantly disturb whales' sonar navigation.
There is some evidence that human use of sonar can lead to beaching. Active military sonar has been implicated in mass strandings of whales, such as the 17 beaked whales in the Bahamas in 2000. The loud sonar in this case actually caused severe injury to the whales' ears. In the Canary Islands in 2002, there was evidence that panicked whales died of decompression sickness, caused when nitrogen builds up in the blood stream as the animals ascend too quickly.
Both the US Navy and our Royal Navy use sonar in a similar way to whales as a means of discovering the locations and identities of objects in the sea. It seems that certain frequencies can cause physical harm to whale anatomies when emitted at excessive volumes, but even at low volumes, the sounds may simply panic the whales.
In 2008 scientists discovered that the navy sonar significantly resembled the echolocation sounds used by hunting killer whales. Killer whales, true to their names are lethal hunters that prey on large and small whales alike. In their hunting pods they have no trouble overtaking and killing a grey whale calf, even one that's being protected by its mother (as the clip below shows). Navy sonar may therefore cause prey species of the killer whales to panic.
Public concern in the US prompted an order for Department of Defense to limit its use of certain frequencies of active sonar during peacetime. The European Parliament requested that EU countries refrain from using their equivalent sonar until its environmental impact has been established.
In 2008 26 dolphins were found dead in various river passages in Cornwall. When post-mortem examinations revealed no signs of illness or poisoning, wildlife experts considered the effects of reported sonar use by the Royal Navy in the days leading up to the strandings.
A study co-authored by the Whale and Dolphin Conservation Society found a strong link between naval mid-frequency sonar and whale and dolphin strandings, but the evidence is still circumstantial.
It now seems likely that the Pegwell Bay sperm whale was a juvenile on migration that found itself in waters too poor to sustain its diet. But with a 25 percent increase in cetacean strandings over the last 20 years, you have to wonder whether the sea and its creatures are trying to tell us something.
Sam Dixon for The Curious Owl, a sideways look at British nature.
Update 15 March 2011: A new study suggests that beaked whales are disturbed by naval sonar.