A rescue operation is underway to save any surviving pilot whales among the 500 or so stranded along the coast of the Australian island of Tasmania on Wednesday.
"Our focus is on those animals that are still alive," Nic Deka, regional manager for Tasmania's Parks and Wildlife Service, told a press conference. "The mortality has increased, but there are a significant number that are alive so we will continue to work with those.”
An estimated 270 whales were first found trapped on a sandbar on Monday. At least a third of them have already died while 25 were successfully refloated by rescuers.
Another 200 were discovered Wednesday in a bay just under six miles from where the first pod was discovered, Deka said. Overall, at least 380 whales have died and 50 have been rescued.
Crews were being sent out by boat to assess the situation, Deka added, but most rescuers would remain focused on the first pod.
The cause of the mass stranding has yet to be determined.
However, Nicola Hodgins, policy manager for the British charity Whale and Dolphin Conservation, told NBC News that pilot whales are incredibly social creatures and that If one gets sick or veers in the wrong direction, others will follow, putting the entire pod at risk.
"They will not allow an animal to be sent off to die on its own," she said. "It's just probably one of the strongest social bondings of any species."
Diseases, toxins from algae and extreme weather can all cause the whales to swim off course, Hodgins said. Human-caused aquatic noise — from shipping, oil and gas exploration and sonar — can also disrupt whales' hearing and echolocation.
Australia and New Zealand are both hotspots for whale strandings, Hodgins said. While it's unusual to find so many in one place, it's not unprecedented: as many as a thousand whales have been stranded together.
Once beached, the whales have a matter of days before their organs, no longer suspended by the water, are damaged, Hodgins explained.
"They end up being suffocated by the weight of their own bodies, which is why it's an incredibly painful way to go," she said.
Download the NBC News app for breaking news and politics
Calves that lose their mothers are unlikely to survive long term, Hodgins said. And if a significant number of older matriarchs are lost, it could impact the pod's knowledge of migratory routes and behaviors.
"It's just heartbreaking, absolutely heartbreaking to see these animals are struggling," she said.
Because of the strong social bonds, the survivors may also be determined to return to the rest of the stranded pod, only to get stuck again, Hodgins said. As of Wednesday, Australian officials said that had yet to happen, with the rescued group staying safely away in deeper water.
Rescuers remained optimistic, Australian wildlife biologist Kris Carlyon said at a press conference.
“I think we have a really good chance of getting more off the sandbar and out through the gates. We are still very hopeful," Carlyon said.
The Associated Press contributed to this story.