Why Do Whales Strand Themselves? Mass Strandings Explained

Single strandings: sick, injured or dead whales

Most stranded whales and dolphins come ashore alone. Many are already dead and simply drift onto the beach with wind and currents. Others are alive but weakened by disease, parasites, old age, starvation, injuries from ship strikes or entanglement in fishing gear. A whale that can no longer swim well ends up in shallow water and is pushed ashore by waves.

Once on land, a whale is in serious trouble. Without the support of water, its own weight presses on its lungs and organs, its body overheats, and its skin dries and burns in the sun. A rising tide can cover the blowhole of an animal lying at the wrong angle. Without help, most live single strandings end in death, and even with help many of these animals are too ill to survive.

Why do whole groups strand together?

Mass strandings, in which two or more whales other than a mother and calf come ashore together, are rarer and harder to explain. Only about ten species do this regularly, and all of them are toothed whales. They include long-finned and short-finned pilot whales, false killer whales, melon-headed whales, the sperm whale and several dolphins.

These species share two traits. They normally live in deep water far from shore, so shallow coasts are unfamiliar to them, and they live in tight-knit family groups. If one member gets into trouble, perhaps a sick leader or a whale caught by the tide, the others may follow it into the shallows rather than leave it. This is also why refloated pilot whales often swim straight back to companions still on the beach.

Baleen whales almost never mass strand. A rare exception came in 2015, when more than 300 dead whales, probably sei whales, were found in remote fjords in southern Chile, possibly killed by a toxic algal bloom.

Whale traps: shallow, gently sloping coasts

Some places see mass strandings again and again. Farewell Spit at the top of New Zealand’s South Island, where the tide can retreat several kilometers across sand flats, has been called a whale trap; in February 2017, several hundred long-finned pilot whales stranded there. In September 2020, more than 450 long-finned pilot whales stranded in Macquarie Harbour, Tasmania, Australia’s largest recorded stranding, and most of them died. Cape Cod Bay in Massachusetts is a hotspot for dolphin strandings.

These sites tend to have shallow, gently sloping seabeds and large tidal ranges. One idea is that echolocation works poorly there. Sound bouncing off a very gradual slope may return such a weak echo that whales do not “see” the shore, and stirred-up sand may make things worse. A falling tide can then leave a whole group stranded within minutes.

Sperm whales in the North Sea

Europe has its own stranding trap. In January and February 2016, 29 sperm whales, mostly young males, stranded on the coasts of Germany, the Netherlands, the United Kingdom and France. The North Sea is shallow and has no deep water for squid-hunting whales, so males that enter it from the north, perhaps following prey, can become lost and starve or run aground.

A 2017 study suggested that solar storms, which disturb the Earth’s magnetic field, might have confused the whales’ sense of direction. This is a hypothesis rather than a proven cause, and similar events have been recorded in the North Sea for centuries.

Naval sonar and other loud noise

The clearest human link is with powerful military sonar. In March 2000, 17 whales and dolphins, most of them beaked whales, stranded in the Bahamas after a US Navy exercise using mid-frequency sonar, and the investigation identified the sonar as the most likely cause. In September 2002, 14 beaked whales stranded in the Canary Islands during a naval exercise. Examinations of those whales found gas-bubble lesions, a sign of something like decompression sickness.

Beaked whales are extreme deep divers, as described in How deep can whales dive? One theory is that sonar frightens them into surfacing too fast. After Spain banned naval sonar exercises around the Canary Islands in 2004, no further mass strandings of beaked whales were reported there. Seismic surveys for oil and gas are also suspected in some cases, though the evidence is weaker.

Do whales ever strand on purpose?

There is no evidence that whales strand themselves to die. A few animals, though, beach themselves deliberately to hunt. At Península Valdés in Argentina and the Crozet Islands in the Indian Ocean, orcas rush onto beaches to grab seals and sea lion pups and then wriggle back into the sea on the next wave. In the salt marshes of South Carolina and Georgia, bottlenose dolphins push schools of fish onto mud banks, slide out after them and then slip back into the water. These are controlled, skilled hunting methods, not accidents.

How do stranding networks respond?

Many countries have trained networks that respond to stranded whales. In the United States, NOAA coordinates the Marine Mammal Health and Stranding Response Program with local partners, and in the UK, British Divers Marine Life Rescue answers live strandings while the Cetacean Strandings Investigation Programme examines dead animals.

Trained teams assess each animal, look after live whales until the tide returns and refloat those that are fit enough, often releasing a group together. Animals that are too sick to recover may be humanely put down by a veterinarian. Examinations of dead whales help scientists track disease, pollution, ship strikes and the effects of noise, which is how many of the causes above were discovered.