The zoo conducted an exhaustive investigation into the unexplained deaths of 41 rays. (Courtesy www.lifeincalgary.ca)

A lack of oxygen likely killed 41 cownose stingrays last spring, and Calgary Zoo officials insist that, though the terminology is different, that's what they've said all along.

"The answer is 99.9 per cent sure: we believe the dissolved oxygen was too low," Clement Lanthier, president of the Calgary Zoo, told the Calgary Sun on Monday.

The zoo had just opened the $250,000 exhibit — which allowed visitors to touch the creatures — when 41 of the 43 rays suddenly died in May.

'I think we need to be very frank here. Our main expertise is not in fish here at the Calgary Zoo.'— Clement Lanthier, Calgary Zoo president

On Tuesday, zoo officials explained that a series of unrelated events contributed to the deaths.

One of the four pumps in the exhibit went down on an unseasonably warm day, said Cathy Gaviller, the zoo's director of conservation, education and research.

"As the temperature climbs, the ability of the water to hold oxygen is reduced," she said. "We'd also recently added an additional number of rays, so they would've been putting somewhat more pressure on that.

"So I think a number of events by chance all happened at the same time to cause this dip in the oxygen levels."

When staff noticed the first signs of distress, they rushed to move the rays to another tank — disturbing the water and mixing in enough oxygen to fool subsequent tests, which showed oxygen levels were fine.

The Calgary Zoo reopened the stingray exhibit in December. (CBC)

"When you jump in the water and splash around trying to net the fish, that forces oxygen into the water," said Gaviller.

Lanthier's admission is the first time the zoo has confirmed what happened to the stingrays. Amid intense public interest and media scrutiny last year, zoo officials had said an exhaustive investigation could not find a conclusive cause of death.

But zoo officials on Tuesday said they never tried to hide anything, pointing to a statement released on Dec. 11, 2008, which read in part: "A number of potential causes were eliminated through the course of the investigation, however, several possibilities remain including unknown toxins or a possible deficiency of dissolved oxygen in the water, but a conclusive cause may never be known."

Gaviller added on Tuesday, "I think that's the same thing we're saying. I think it's just been a different person wording it slightly different, making it sound different. There's nothing really new."

Stingray exhibit reopened

The zoo reopened the stingray exhibit in December, but abandoned the hands-on component. One of the 10 new rays delivered for the new display died of a parasite last week.

"I think we need to be very frank here: our main expertise is not in fish here at the Calgary Zoo," Lanthier said at a news conference on Tuesday morning, adding that the zoo will continue to work on improvements in that area.

Zoocheck Canada, an animal rights group based in Calgary, is calling for an independent investigation into the Calgary Zoo.

The group's spokeswoman, Julie Woodyer, said the zoo has been deteriorating for the last decade and an independent operational review needs to be done.

Gaviller rejected that idea.

"I'm not sure what Zoocheck's credentials are in terms of knowing how to look after a stingray tank. I think we do a good job of looking after our animals.... It's easy to sit back and criticize."

With files from the Canadian Press