Ten short-finned pilot whales are dead after a pod of 51 of the animals became stranded in shallow water in a remote region of the Everglades National Park in Florida.
The outlook for the remaining 41 whales is grim, according to Linda Friar, a spokeswoman for the national park, who said the difficult marine terrain could make it impossible for them to get out.
She told Nature World News the area where the whales are stranded, around Highland Beach on the Gulf of Mexico side of the park, is "several football fields long" and surrounded by shallow shoreline.
It is not exactly clear how long the whales have been there, but a team of 25 rescuers from marine mammal stranding network organisations arrived on the scene on Wednesday.
The beach's very remote location, more than an hour away by boat from the nearest boat ramp, is hindering rescue efforts.
The rescuers arrived on the scene to find six of the whales already dead, and four were in such poor condition they had to be euthanised.
The other 41 whales were freely swimming and not showing signs of injury. But officials have said the remote location of the beach makes it hard for rescuers to bring in the heavy equipment needed to save them, while the sand bars and channels in the terrain between the deep ocean water and the shallow beach water means it is doubtful they will be able to swim out on their own - even in high tide.
"This area of the park is probably the most challenging for something like this. When the tide goes out, there's hundreds of yards of very shallow shoals," Ms Friar continued.
"We don't' know how long they've been out of home range. The outlook does not ultimately look good for the remaining live whales."
The National Oceanic and Atmospheric Administration, or NOAA, said biologists will perform necropsies on the dead whales in an attempt to determine why the became stranded.
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