Rescuers and volunteers were scrambling on Friday to save nearly 200 pilot whales that were stranded on New Zealand's South Island. At least two dozen of the whales were reported dead, and more than 80 people descended on the beach near the shallow waters of Farewell Spit to begin the difficult task of moving survivors back into the water.

The area on the South Island's northwest corner is an especially treacherous one for pilot whales because the shallow waters there can be confusing to navigate, New Zealand's Department of Conservation reported. Large strandings of pilot whales happen occasionally on New Zealand during the region's summer.

Trevor Spradlin, a marine mammal biologist with the U.S. National Oceanic and Atmospheric Administration, has said that pilot whales are particularly at risk for group stranding because they are highly social animals that live in close-knit family groups, or pods.

"Their need for group cohesion is very strong, so these animals stay together," Spradlin said. He added that scientists believe that a pod typically gets stranded after one animal gets stuck—often an older or sick individual that was weakened by disease, hunger, a boat strike, or pollution.

"The rest get caught up in tide fluctuations and they get lost, because they focus on being with the sick individual," said Spradlin.

He added that such strong group cohesion also can be an asset to survival, because if the group can be freed, weaker individuals can receive protection and support from the stronger survivors. (In one rare case, a dolphin was even seen guiding stranded whales out to sea.)

Not Much Time to Rescue Them

Related to dolphins, pilot whales live around the world and are nearly as large as orcas. They are mostly dark gray, brown, or black but may have some lighter patches. The animals can reach a length of up to 21 feet (6.5 meters) and a weight of 7,050 pounds (3,200 kilograms).

When it comes to rescuing stranded whales, the techniques often depend on the remoteness of the site and the availability of equipment such as heavy movers, slings, or cranes.

"Time is usually of the essence," said Spradlin. "Cetaceans are designed to live in water, so when they are stranded on a beach their internal organs aren't used to having that weight on them and they start getting crushed."

To buy time, volunteers are trained to keep stranded marine mammals cool and wet as long as possible. (See how stranded pilot whales were rescued in the Everglades.)

If the animals can't be pushed back to sea, often they are euthanized to avoid a slow, painful death. Some groups use explosive charges to kill the animal as quickly as possible. Others use exsanguination, which involves cutting a major artery where the tail meets the body, and the animal bleeds out. Some use drugs, in an effort to kill the animals as humanely as possible.