VANCOUVER - A beluga calf born to much fanfare last summer at the Vancouver Aquarium has died of a "bizarre" condition that has baffled the facility's veterinarian and his colleagues around the world.

Aquarium staff said Nala was thriving until about two weeks ago when she stopped nursing as usual. Blood samples showed she had an infection, which was treated with antibiotics.

She was soon back to normal and frolicking with her mom before taking a turn for the worse on Sunday, aquarium veterinarian Martin Haulena said Tuesday. She died just after 10 p.m. on Monday.

Haulena said staff discovered that Nala, who was born last June to Aurora, the aquarium's 20-year-old female beluga, had a pocket beneath her larynx, or voice box, which contained two stones and a penny.

But he said the pocket isn't supposed to be there and no one knows if it developed when debris became trapped in the space or whether the beluga was born with the sac.

The stones may have been dropped in the pool by birds and it's not uncommon for the whales to play with them before they end up in their stomachs, Haulena said.

"But to cause a problem right in that larynx is really, really weird," he told reporters at the aquarium.

As for the penny, he said it may have been dropped in the pool by a visitor despite signs in the area asking visitors to keep objects away.

Nala's death was hastened by an accumulation of fluid in her lungs and airway as she struggled to breathe, Haulena said.

"I'm still wracking my brain trying to figure out what we could have done about it," Haulena said.

"I'm working very, very closely with the pathologist and I'm still working very, very closely with my colleagues around the world to figure out if anyone's ever seen anything like this," he said.

There wasn't much anyone could have done to save Nala, Haulena said.

"I think we're one of the best, if not the best, animal care teams in the world," he said.

"We had a great healthy calf who was under constant scrutiny, constant monitoring. When things went awry we were on top of it. We came up with a situation that no one's ever seen before."

When it became obvious that Nala's death was inevitable on Monday night, staff rushed to be by her side. Many didn't make it there on time.

"It's very, very heartbreaking," Haulena said.

It's not the first time a whale has died at the facility. Aurora gave birth to a male calf named Tuvaq in the summer of 2002 but he died three years later.

There are now five beluga whales remaining at the Vancouver aquarium, where Canada's first whale was born in captivity in 1977.

Peter Hamilton, of the group Lifeforce, reiterated his call for the aquarium to stop keeping whales in captivity.

"The physical and psychological needs of these intelligent, social dolphins cannot be provide for in aquarium prisons," he said. "It's life in a toilet bowl for them."

But Brian Sheehan, curator of marine mammals at the aquarium, said the survival rate of calves in captivity is 40 to 50 per cent in the first year, according to research in Florida. He said survival rates in the wild would be about the same because belugas deal with a harsh environment and predators.

Sheehan said Nala's mom, Aurora, has been doing "contact calls" since her death.

"She's obviously impacted greatly by what's going on with the loss of Nala so we're doing everything that we can for her. Whenever she wants to interact with us, we're there."

On Tuesday, the aquarium was packed with kids on field trips and nine-year-old Emily Ponti said she was sad to hear Nala had died.

"She was so cute," she said.