Residents reported yet another slew of pilot whales stranded on two remote beaches in the South Pacific near New Zealand. This time, almost 500 beached – with no survivors.

250 whales were stranded on Chatham Island on Friday Oct. 7, and about 240 were later found on the nearby Pitt Island on the morning of Oct. 10.

Pilot whales are the most affected of the species that are beached, which includes other whales, dolphins and porpoises, as well as sea turtles. It’s a common phenomenon, but the large numbers observed by conservationists over the weekend constituted two irregular “super pods”, groupings that can be hundreds more than the usual 30-50 animal stranding.

“Having such a high number of whales in one location is unusual, but it’s certainly not unheard of,” Daren Grover, general manager of Project Jonah, a nonprofit that rescues marine mammals, said.

Grover said most whales had already been dead when his team arrived at Chatham, with very few survivors left. Beached whales and other stranded marine mammals eventually die of dehydration or by drowning when tide water enters their blowholes.

Underpinning the hurried rescue operations was that waters around the island chain, that already lies 280 miles east of New Zealand’s South Island’s shores, were swarming with sharks.

After assessing the situation, in efforts to reduce risk for humans and whales , the New Zealand Department of Conservation decided to  euthanize the few remaining survivors.

“This decision is never taken lightly, but in cases like this it is the kindest option,” Dave Lundquist, New Zealand’s technical marine advisor, told CNN on the euthanizations.

The carcasses were left on the beach to decompose naturally.

The frequency of strandings can’t be divorced from the sheer, massive loss of life that occurs, and government officials and conservationist organizations have been awash with work.

200 pilot whales were stranded in Tasmania as they lost their way through the Tasman Sea – at the very same beach where Australia’s biggest ever whale stranding event took place two years ago. The same beach where 380 out of 470 beached whales perished.

Scientists have not yet come to a consensus on exactly why these strandings happen. They do know that beaches like those on Tasmania’s west coast, with deep water and rapidly moving tides, can increase the probability of mass strandings.

Large marine mammals like pilot whales and dolphins navigate by detecting magnetic fields, as it’s been observed their migratory routes run parallel to Earth’s magnetic field lines.

Explanations for global strandings have included the cetaceans’ possible confusion by geomagnetic anomalies and their social tendencies that might lead them to follow friends who stray into dangerously shallow territory.

Albeit indirectly, researchers predict climate change’s ravages on global ocean current passage ways may also increase strandings as species shift distributions to new areas.