(CNN) -- The struggle between life and death is not going so well for dozens of pilot whales stranded in shallow waters in Florida.
Rescuers have been trying to save about 30 whales by herding them into deeper waters.
Of the 51 whales originally stranded, 11 have died and five gone missing in the shallow waters on the edge of Everglades National Park, NOAA Fisheries officials said.
Scientists spotted 20 whales Friday afternoon about five nautical miles off shore.
The pod was moving in a southward direction and inshore, the National Oceanic and Atmospheric Administration said.
"The whales were reportedly swimming more slowly and in a more disorganized fashion, which may suggest exhaustion, dehydration or malnutrition," marine scientists said.
NOAA Fisheries is keeping crews on standby for necropsies and to rescue any live stranded whales.
Pilot whales are protected under the Marine Mammal Protection Act, but they are not endangered.
Mass strandings by pilot whales are not uncommon in Florida.
In 2012, nearly two dozen pilot whales stranded and beached themselves. In 2011, 23 pilot whales stranded and beached themselves.
CNN's Mariano Castillo and Marlena Baldacci contributed to this report.