At least 20 pilot whales were found dead on Thursday off Florida's southwest coast, officials said.
The whales were among a group of 23 that swam inland over the weekend near Naples, Florida, said Blair Mase, an official with the National Oceanic and Atmospheric Administration (NOAA).
The deaths bring the total number to 28 whales that have died in waters near Florida's coast in January, puzzling scientists struggling to understand why they continue to come ashore.
Eight died earlier this week, four naturally and four after being euthanized, when a group including one pregnant female swam into shallow waters near Fort Myers, Florida.
Scientists were able to identify the group of 23 using pictures of their dorsal fins, Mase said during a conference call with reporters. Each fin is unique in the same way no two human fingerprints are the same, she said.
"Historically kind of come in spurts," Mase said. "This is unusual and something we're looking into and monitoring."
In early December, 51 pilot whales swam into shallow water in the Florida Keys, triggering a frantic rescue effort. At least 22 died.
Earlier this week Mase told Reuters tests showed the whales that died in December were not infected with the morbilivirus virus, a measles-like infection that was responsible for the deaths of more than 1,000 bottlenose dolphins last year.
Mase said the eight whales that died earlier this week "looked emaciated" and when examined had empty stomachs.
Jill Richardson, a professor at the University of Miami's Rosenstiel School of Marine and Atmospheric Science, said the whales' social structure could be playing a role.
Pilot whales are a social, deep-water species that live in pods of 20 to 90 whales and typically will not leave ailing or dead members behind. The animals forage on squid, octopus and fish and cannot live long in shallow water.
"An animal may be ill, unable to navigate properly and find themselves out of habitat and the rest of the pod will follow them in," Richardson said.
The bonds are so strong that dead whales have to be cleared from beaches before others swimming in shallow waters can be guided back out to sea.
"They'll keep coming back," she said.
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