Another whale shark died Wednesday at the Georgia Aquarium, the second this year at the only facility outside Asia to display the huge, rare fish.
Aquarium officials said identifying the cause of Norton's death could take months. A necropsy was planned.
Just two weeks ago, two young male sharks arrived from Taiwan, and fishery officials there said at that time they were satisfied the Georgia aquarium provided quality care.
In January, Ralph, a whale shark who was one of the aquarium's first stars when it opened in 2005, died from peritonitis, an infection in his abdomen.
Aquarium officials said Wednesday that Norton had stopped eating in recent months and swam erratically. Early Wednesday, he settled to the bottom of the aquarium's centerpiece Ocean Voyager tank.
He was euthanized after his health didn't improve. Norton's death would be investigated for any possible link to Ralph's death, aquarium officials said.
Ralph had stomach problems that inflamed a membrane in his abdomen, but the aquarium has also said a chemical used in cleaning the sharks' tank may have contributed to Ralph's loss of appetite. The tank-cleaning routine since has been changed.
Ray Davis, the aquarium's senior vice president of zoological operations, said the remaining four whale sharks were doing fine.
Many visitors Wednesday had just learned of Norton's death, including Christine Obijeski, 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."
The new arrivals, Yushan and Taroko, and female whale sharks Alice and Trixie share a 6-million-gallon tank. The rare species can grow up to 40 feet long, but the aquarium says the tank is big enough for them.