Cetacean stranding, commonly known as beaching, is a phenomenon in which whales and dolphins strand themselves on land, usually on a beach. Beached whales often die due to dehydration, collapsing under their own weight, or drowning when high tide covers the blowhole. Several explanations for why cetaceans strand themselves have been proposed, but none have so far been universally accepted as a definitive reason for the behavior.
Although a range of cetacean species regularly strand in groups, including beaked whales, pygmy sperm whales, false killer whales and common and bottlenose dolphins, pilot whales are the most common in New Zealand. These whales sometimes strand in pods of more than loo and, of the half a dozen sites around the coastline involving mass whale strandings, Golden Bay beachings are the most regular and involve the largest numbers. Other locations include Ninety Mile Beach, the coastline near Whangarei, the Mahia Peninsula, the east coast of the Coromandel Peninsula and the Chatham Islands.
A number of very young whales are sometimes seen swimming with the pod, suggesting that perhaps the whales come into the bay to calve.
The whales are sometimes first sighted swimming towards the bay by local boats and aircraft that provide reports to onshore watchers. More often than not, however, the first report of the whales is after they have beached.
It is still not really known why whales strand, although there is no shortage of theories. Whales are guided by their own sonar and the sounds they emit bounce back to them by way of resonance along bones in their lower jaws that act as a kind of vibrating tuning fork: a positive casting bounces off the shoreline while a negative one indicates the open sea. A poor sonar response from a gently sloping shoreline (as in Golden Bay, New Zealand) is one widely touted explanation as to why whales get into difficulty.
Another explanation for mass strandings is the "assisting sick companion" theory. As in any wild population of animals, an individual whale may be sick or dying. Rather than drown, it will beach itself. Certainly most single strandings around the coast seem to be of sick whales. When a whale starts to peel off from the pod, pilot whales, being highly social animals, will respond to the distress signals and try to assist.
We know that some whales certainly are sick, they are infested with parasites or are old, but that's no reason for 150 whales to come ashore. When one of them strands it calls the others in, but it's not necessarily a sick one that strands. Most of the whales that end up on New Zealand's Golden Bay beaches are seemingly healthy.
It is thought that predators such as orcas, which work in teams to attack other whales, can cause panic or confusion among pilot whales. During a mass pilot whale stranding in Golden Bay, New Zealand, in 1993, orcas were spotted swimming outside the spit; possibly the pilot whales preferred to take their chance in the shallow waters rather than face their aggressive fellow cetaceans.
Other theories about the causes of whale stranding include bad weather or sea conditions, or the presence of strong offshore winds. Interestingly, a high proportion of Golden Bay strandings have occurred during unusual weather patterns. And off-shore winds can sometimes cause waves to move away from the shore (rather than the normal on-shore direction). Possibly this causes confusion among the whales.
One theory which has minority support among scientists assumes that whales navigate the oceans by following global magnetic lines and patterns. Sometimes these can be distorted by magnetic anomalies in some areas. Golden Bay whale watcher Buzz Davis says he was surprised when studying a magnetic map of Golden Bay, to find that the whole bay gave out a negative casting. He likened it to a "whiteout" experienced by humans.
There is probably no single cause of stranding. In reality, it is probably a combination of many interconnected factors that, on the day, cause a stranding to occur.