On average more than 300 dolphins and whales strand on New Zealand beaches each year.
This natural phenomenon has occurred for thousands of years. But why do dolphins and whales strand?
There’s no simple answer. However, a combination of anything from disease to extreme weather, could be the cause.
A stranded dolphin or whale might be old or sick. It could be infested with parasites, be poisoned by natural toxins or have had problems with birthing.
When chasing prey near shallow sloping beaches they may accidently beach themselves. They can also be trapped by receding tides or swept off course by strong currents. Some scientists believe that abnormalities in the Earth’s magnetic field may cause them to beach.
Boat noise, such as sonar, can disrupt a whale's or dolphin's ability to communicate, feed and navigate. Ships can run into them, and fishing nets can trap them. And industrial waste and litter pollute the sea, including plastics that dolphins and whales can swallow.
Rapid changes in water pressure created by underwater earthquakes, volcanic eruptions or seismic testing may damage the sinuses and middle ears of whales, preventing them from diving and feeding. This may then lead to illness and navigational error causing them to strand.
New Zealand’s long, contorted coastlines undoubtedly have a role to play. Long fingers of land jut out to open sea and in many places deep water comes close to shore. Shallow beaches with big tidal ranges provide areas where whales can swim at high tide but get caught at low tide. Golden Bay in Nelson is a classic whale trap. This curving bay with shallow-sloping beaches and a large tidal range has witnessed hundreds of strandings.
Whatever the reason for the initial stranding, social cohesion may result in mass beaching. Their strong social bonds may prevent them deserting a helpless member. Going to their aid, these animals may then also become stranded themselves. The largest mass stranding on record dates back to 1918, when 1,000 pilot whales stranded on the Chatham Islands.
Just as social cohesion in whales can cause mass strandings, the most effective human response is collective, and many whales are saved each year with the help of volunteers. To find out how you can help click here
What sorts of whales strand?
Because of New Zealand’s extensive coastline and range of waters, many different species strand – from coastal dolphins to oceanic and deep-water whales. Close to half the world’s species of whales are seen around our coasts. This includes eight species of baleen whale and 30 species of toothed whales, including 10 dolphins and one porpoise.
Most strandings are of solitary animals, most commonly the common dolphin and the pygmy sperm whale. Though occasionally various species of the lesser known beaked whales strand alone on New Zealand shores.
Humpback whales migrate past New Zealand each year, heading north after feeding all summer in the Antarctic or south as summer kicks in. In winter they breed in the coastal waters of the Pacific Islands. On such long journeys, these animals may occasionally strand.
New Zealand’s most frequent mass-strander is the long-finned pilot whale. These animals travel in groups of 10 to 60, and sometimes come together in thousands. Socially cohesive, when one individual strands, others in the pod stay with it and usually strand as well.
When do they strand?
Single strandings occur throughout the year, although mass strandings normally take place during the summer months.