Half of the whales, from two different pods, survived the stranding, but with no prospect of being re-floated the Department of Conservation (DOC) said that they had made the “heart-breaking” decision to euthanize them.
The two pods were stranded at Mason Bay on the remote southern Stewart Island, approximately 2 km apart, on Nov. 24.
They were discovered by a hiker in the sparsely-populated island in the south pacific.
According to DOC Rakiura Operations Manager Ren Leppens, half of the whales had already died by the time they were found.
They decided to euthanize the remainder because of the condition of the whales, and the remote, difficult-to-access location.
Leppens said in a statement, “Sadly, the likelihood of being able to successfully re-float the remaining whales was extremely low.
“The remote location, lack of nearby personnel and the whales’ deteriorating condition meant the most humane thing to do was to euthanize.
“However, it’s always a heart-breaking decision to make.”
The island, some 650 square miles in area, is home to around 400 people.
The beach was the site of three previous mass strandings. In 1998 more than 300 whales beached themselves at the spot.
New Zealand has more recorded strandings than almost any other part of the world, typically the long-finned pilot whales like those stranded on the beach as at Mason Bay.
Other locations with a history of mass strandings that form natural “whale traps,” such as Farewell Spit on the North Island, can be monitored for signs of whale activity. But with so few people on the island, Mason Bay cannot be monitored.
Although marine mammal strandings are “relatively common” in New Zealand, with around 80-85 incidents a year, they are mostly of single animals, according to the DOC.
Mass strandings are much rarer.
Pilot whales are one of the most populous whale species. They get their name from the way they travel in pods: One male leads the way as a “pilot” with the rest of the pod of up to 200 whales following.
They show very strong herd behavior.
The tight social cohesion and “pilot” behavior could be an important factor in stranding, and then re-stranding after being rescued and refloated, according to some researchers.
However, the reason for strandings of whales and dolphins is not fully understood said the DOC. Possible reasons include: Sickness, navigational error, geographical features, a rapidly falling tide, being chased by a predator, or extreme weather.
Some conservation campaigners have suggested high-intensity sound waves from oil and gas exploration, and the use of naval ship sonar could also be interfering in the animals’ navigation systems.
However, conservation officials note that these should not be confused with natural whale traps, such as the one at Farewell Spit, where whale strandings have been noted for centuries.
The largest recorded stranding of pilot whales was back in 1918, when over 1,000 were trapped on the shores of the remote Chatham Islands, which lie 500 miles due east of New Zealand in the south pacific ocean.