Almost 400 whales have died following New Zealand's largest ever mass beaching. Around 70% of the 416 pilot whales have already died, with volunteers working to try to rescue the remaining.

The Department of Conservation said the whales swam ashore at Farewell Spit on the north of South Island. Spokesman Herb Christophers told AFP that because there were so many carcasses, the 500 volunteers were struggling to get those left alive into the water.

Pilot whales
10 February 2017: Hundreds of stranded pilot whales lie dead on the shore, one of New Zealand's largest recorded mass whale strandings, in Golden Bay New ZealandRoss Wearing/Reuters

"The dead ones that are floating around out there are obstructing their course out to sea," he said.

Over 100 whales were re-floated at high tide, but most had swum back to shore – whales are known to swim back to beaches after being rescued to try to re-join their pods. While volunteers continue with the rescue effort, the outlook for the remaining whales is poor.

Andrew Lamason, regional manager of the Department of Conservation, told Radio New Zealand: "With that number dead, you have to assume that the rest are in reasonably poor nick as well. So we're sort of preparing ourselves for a pretty traumatic period ahead."

This is New Zealand's third biggest mass whale strand. The largest took place in 1918 when 1,000 whales washed up at Chatham Islands. In 1985, 450 were beached in Auckland.

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Volunteers try to keep alive some of the hundreds of stranded pilot whales in Golden Bay, at the top of New Zealand's south islandRoss Wearing/Reuters
Pilot whales
Volunteers attend to some of the hundreds of stranded pilot whales which are still alive in Golden Bay, New ZealandAnthony Phelps/Reuters

Why do whales get stranded?

The cause of mass strandings is still not well understood, with many factors playing a role. When single individuals wash up, it is normally because they are sick or injured. However, for larger numbers, the reasons are less clear.

Harmful algal blooms can result in whole pods being beached, while disease events are also considered a common cause. The use of high-powered sonar in naval exercise has too been implicated in the past.

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A woman pours water over one of the pilot whales that was washed up on the shore in Golden Bay, New ZealandAnthony Phelps/Reuters
Pilot whales
People attempt to keep alive some of the hundreds of stranded pilot whales in Golden Bay, New ZealandRoss Wearing/Reuters

But sometimes mass strandings occur for no apparent reason. David Lusseau, a professor of behavioural biology at the University of Aberdeen, said this is often seen in pilot whales. "This has puzzled biologists for centuries: why would a healthy animal put itself in such danger if there is no reason to? Is it possible that they are deliberately harming themselves? Even attempting suicide?" he said.

"Pilot whales live in matrilineal societies. They spend their lives in schools that are composed of extended families centred around the females – mothers and their daughters are the family focal points.

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People walk between some of the stranded pilot whales in Golden Bay, New ZealandAnthony Phelps/Reuters
Pilot whales
A pilot whale lies on a sandbank marked with an 'X' to indicate that it has died in Golden Bay, at the top of New Zealand's South IslandAnthony Phelps/Reuters

"Based on this social structure, the common assumption has long been that healthy animals strand themselves as an altruistic gesture, that they do so to continue caring for distressed family members. Recent work, however, casts doubt on this analysis, with genetic tests showing that animals stranded close to each other during mass events may not be related after all."

Pilot whales
Still image taken from a video showing the pilot whales on the beach in Golden Bay, New ZealandReuters

He added that while this social caring hypothesis is generally favoured, in truth we still do not really know what causes mass whale strandings. "These individuals strand to stay in contact with their sick or injured companions, whether relatives or otherwise. What we don't fully understand are the mechanisms behind this. These can range from simple 'hardwired' instincts to complex behaviours, which may allow them to reflect on the needs of other members of their group, even act altruistically. The truth probably lies somewhere in between."

Pilot whales
Still image taken from a video showing stranded pilot whales on the beach in Golden Bay, New ZealandReuters