Story Highlights• Aquarium conducting necropsy on Ralph
• Other whale sharks appear to be fine
• Ralph stopped swimming and received treatment before dying hours later
• Aquarium euthanized ill beluga whale Gasper earlier this month
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ATLANTA, Georgia (CNN) -- Scientists worked Friday to determine what killed Ralph, one of the four giant whale sharks at the Georgia Aquarium.
The 22-foot-long fish stopped swimming Thursday afternoon and divers whisked it from the bottom of its football field-sized tank to an exam area. Teams worked to save him for about eight hours before he died at 9:30 p.m.
"When we found him sitting on the bottom we immediately knew that he was in trouble," said Jeff Swanagan, the aquarium's executive director. (Watch Ralph before he died and how the staff responded )
Swanagan said a necropsy, or animal autopsy, was under way to try to determine the cause of death. Aquarium employees are also monitoring the other animals.
He said whale sharks Norton, Alice and Trixie all seem to be swimming normally. (Watch how the aquarium will learn from Ralph )
"We're watching it very carefully, but there's nothing indicating any other problems in the exhibit or anywhere else in the aquarium and no problems with any of the water chemistries," he said.
Ralph was the second popular animal to die at the aquarium in a nine-day period.
On January 2, Gasper the beluga whale was euthanized. He had suffered from a string of chronic illness even before he came to the aquarium.
Swanagan said the deaths were unrelated and that the animals lived in separate tanks with different water supplies.
"To have two big charismatic animals [die] creates a linkage that's probably not there," he said. "I've been doing this since 1979 and I've had cases where I've had two days in a row of births and two days in a row of deaths. Nature just sometimes doesn't give you a schedule of tragedies that's convenient."
Ralph had a physical exam in November with about 50 people taking part -- including veterinarians, biologists, divers and photographers.
Swanagan said the staff was concerned about some unusual swimming behavior a couple of months ago, but that had improved. Ralph's appetite also had been on and off. (Full story)
"There was nothing we could see that would define that this animal was as sick as he turned out to be," he said. Swanagan said there was no indication that stress from the exam could have contributed to Ralph's death but said that was one of the things they would look into.
Saved from the dinner table
Ralph and Norton have been the biggest stars at the aquarium since it opened in November 2005. They were joined by females Alice and Trixie in June.
The aquarium got the whale sharks by negotiating with Taiwan, which catches the animals for food. It is the only aquarium outside of Asia to showcase whale sharks.
The World Conservation Union lists the whale shark as a vulnerable species. Heavy fishing of the whale shark in several areas of Asia is believed to be one reason for its population decline.
Scientists hope to breed the sharks, but Swanagan said that's years down the road.
"The animals are all pre-teenagers and it would be many years before they would be ready to breed," he said. "Our friends in Okinawa, Japan, have had three whale sharks for 10 years in an exhibit that's two-and-a-half times smaller than ours and their animals are not yet breeding."
Whale sharks are the world's largest fish and can grow up to 66 feet in length. They are gentle animals that feed on plankton and small fish that they filter into their wide, flat mouths.
Swanagan said the Georgia Aquarium has been working with whale shark researchers for four years, but there are still a lot of things that they don't know.
He said some leading marine scientists worked for years without seeing a whale shark in the wild.
"No one knows, comprehensively, the life cycle of these animals," he said.
Swanagan said he's glad that more than 4 million visitors got to see Ralph.
"People have such an aversion to sharks, so to have these sort of friendly sharks, like whale sharks be ambassadors to get people to want to learn more about sharks is a very important reason to have them," he said.
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