The term "mass stranding" refers to events in which groups of distressed cetaceans (whales, dolphins, porpoises) come ashore alive. They can involve anywhere from a few to several hundred animals. Mass strandings regularly occur in several parts of the world (primarily Australia, New Zealand and Cape Cod), yet so far we have no universally accepted, comprehensive explanation for this syndrome.

In many cases, the animals show no obvious signs of health problems other than those resulting from coming ashore. Once a cetacean comes ashore, a cascade of physiological changes occurs, often resulting in shock and death. Because the species typically involved are extremely social, the bonds that hold groups together are perhaps strong enough to supercede the survival instincts of individual animals. Although we don't know what specifically might set off a mass stranding event, we know that once animals start coming ashore, it's extremely difficult to stop the process from continuing and escalating. Affected animals will relentlessly follow one another ashore, as if crippled by widespread panic, even when there is clear access to open water.
Gregarious offshore species such as Atlantic white-sided dolphins (Lagenorhynchus acutus) and long-finned pilot whales (Globicephala melaena) are particularly known to mass strand in New England, mainly on Cape Cod.

Mass Stranding Trends

For these data, strandings refers to a mass stranding involving three or more animals.

Species Strandings Since 1973 Typical Month Number of Animals Involved
Pilot Whales 15 December 29
White-sided Dolphins 25 March 12
Common Dolphins 7 January 7
Risso's Dolphins 2 November 4
Bottlenose Dolphins 1 December 4

Recent Mass Strandings

A staggering 97 dolphins died during a stranding event in January and February of 1998. Carcasses were recovered over a four-week period from over 25 miles of shoreline between Dennis and Wellfleet, on Cape Cod, Massachusetts. Eighty of these were Atlantic white-sided dolphins (Lagenorhynchus acutus) and 16 were common dolphins (Delphinus delphis). Unfortunately, none were healthy enough to be relocated or saved for rehabilitation. A team of biologists, technicians and veterinarians, including many volunteers, responded to the event, providing supportive care, collecting blood samples, and later conducting necropsies and preparing tissue samples.While the exact cause of this stranding is unknown, and may never be known, we believe that a combination of factors, including the new moon (producing unusually extreme tides), and a powerful coastal storm may all have contributed to the event. Another noteworthy aspect of this mass stranding was that it involved two species of dolphins rather than just one. About a year later, in March of 1999, a similar stranding event occurred, again during stormy weather with high winds and unusually extreme tides. This time a total of 54 white-sided dolphins stranded on the Cape Cod coast, from Wellfleet to Eastham. Fortunately, three of these animals were found to be in good physical condition and were transported to a beach in Provincetown, where they were released into open water.

Pilot Whales

Although mass strandings typically occur during winter months and at times of severe weather, they can occur at any time of year and under any conditions. During the summer of 2000, we responded to a mass stranding of 11 pilot whales on Nantucket Island, on July 4. The weather was clear and calm and the animals showed no obvious underlying health problems.This was the first pilot whale mass stranding to occur in this region since 1992.