Georgia Aquarium officials say it may be months before they know what caused Wednesday's death of another whale shark, the second this year at the only facility outside Asia that displays the huge, rare fish.

Norton's death also came only a few weeks after two new whale sharks arrived at the aquarium from Taiwan.

At the time, Taiwanese fishery officials had said they were satisfied the aquarium provided the quality care the young whale sharks — named Yushan, which means "Jade Mountain" and Taroko, named after a national park in Taiwan — would need.

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Aquarium officials planned a necropsy Wednesday afternoon for Norton. His death came five months after the death of Ralph, another whale shark that was among the aquarium's first stars after it opened in 2005.

"We were caught by surprise because he had responded to therapy," Ray Davis, the aquarium's senior vice president of zoological operations, said of Norton.

Davis said the remaining four whale sharks — Yushan, Taroko and the female whale sharks Alice and Trixie — were doing fine.

In the last few months, Norton had stopped eating and showed erratic swimming behavior. Husbandry staff noticed a decline in Norton's swimming behavior on Tuesday and blood work confirmed a decline in his health, officials said.

Norton had been placed on a 24-hour watch.

Early Wednesday morning, the whale shark stopped swimming and settled to the bottom of the aquarium's centerpiece Ocean Voyager tank, aquarium officials said.

Divers brought him to a stretcher and "after every option had been exhausted to improve Norton's health, the team made the decision to humanely euthanize him," the aquarium said.

Before Ralph died in January, he had stomach problems that led to an inflammation of a membrane in his abdomen, according to aquarium officials. The aquarium theorized that his death may have involved a chemical used in the tank to treat parasites.

Aquarium officials said the tank's treatment routine — which has since been changed — likely contributed to Ralph's loss of appetite, but they say it's not clear that it had anything to do with the fatal peritonitis.

"Our husbandry and veterinary team are investigating multiple theories for any links between the deaths of the two animals," the aquarium said in a letter to its members.

Many attending the aquarium Wednesday had just learned of Norton's death, including Christine Obijeski of Alpharetta, Ga., who brought her 3-year-old daughter, Kristen, and her 4-year-old nephew, Richard Poelvoorde.

"I told them Norton had died and they asked me why," Obijeski said. "They said he might have been sad because Ralph wasn't here."

Georgia Aquarium scientists say the 6-million-gallon tank should be roomy enough for the massive sharks — which can grow up to 40 feet long.