Gasper, one of the Georgia Aquarium’s five prized beluga whales, was euthanized Tuesday morning after months of declining health, officials said. The 17-year-old whale had been ill before arriving at the aquarium in October 2005, officials said. He and his tankmate Niko had been moved to Atlanta from an amusement park in Mexico City, where Gasper had developed skin lesions while living in a tank under a roller coaster. His immune system was weak from chronic illness, including the bone disease osteomyelitis, and medical tests showed his health had deteriorated further over the past few weeks, the aquarium said in a statement Tuesday.