In this photo provided by the Florida Keys News Bureau, Stacey Anderson, right, of the Marine Mammal Conservancy, right, and an unidentified MMC volunteer, left, care for a pilot whale Friday, May 6, 2011, at Cudjoe Key, Fla. The whale is one of about 16 that stranded Thursday, May 5, off the lower Florida Keys. Six other whales are being cared for in the temporary sea pen, two died and responders are endeavoring to secure the others in the sea pen. (AP Photo/Florida Keys News Bureau, Bob Care)

(AP) -- Rescuers workers treated and secured several pilot whales Friday that have been stranded off the lower Florida Keys.

Rescuers gathered eight of the stranded whales in a temporary sea pen, where veterinarians examined them and performed medical tests. One whale's condition deteriorated Friday, and the animal had to be euthanized. The whales were constantly supported in the water by volunteers standing alongside them.

Marine mammal experts deemed the whales in the sea pen were in stable, but guarded condition.

"These guys are really weak now," said Robert Lingenfelser, head of stranding operations for the Marine Mammal Conservancy. "They haven't eaten in a while, so one of our jobs is to get them re-hydrated and ready for the next step of eating."

The were found stranded in multiple areas in shallow waters and near Cudjoe Key.

Responders confirmed that 13 whales were dead. Necropsies are planned.

"We're still in the process of rescuing the live marine mammals, trying to recover the dead ones so that we get good tissue samples for the scientists to investigate what's going on here," Lingenfelser said. "Right now our operations are geared toward recovery of all the animals out there, and they're spread out all over the place."

A few additional whales were spotted in Atlantic Ocean waters Friday, according to Karrie Carnes, communications manager for the Florida Keys National Marine Sanctuary. Teams of officials were recovering living whales and relocating them to the sea pen.

Officials said they were awaiting the arrival of additional personnel, transport vehicles and resources offered by other organizations.

They hope to transfer surviving pilot to rehabilitation facilities sometime within the next few days. Alternatively, Carnes said, any healthy enough whale may be taken to deeper water for a release attempt as early as Saturday.