The health of the remaining nine stingrays has improved. (Colin Hall/CBC)

One of the Calgary Zoo's remaining stingrays died Tuesday as officials continued their investigation into the sudden deaths of three-quarters of the ray exhibit over the weekend.

The latest ray death brought the total to 35. The remaining eight rays were being held in an oxygenated tank under constant medical supervision. Officials said their health was improving, but it was too early to say when and if they would be put back on display.

Zoo spokeswoman Laurie Herron told the Canadian Press that two of the rays have started to eat again.

On Sunday, after a public visit, the ray keeper noticed some of the cownose rays were swimming erratically and seemed distressed. By Monday, 34 had died.

Officials said that initial tests on the water and food suggest that they were fine.

Cathy Gaviller, the zoo's director of conservation, research and education, said the zoo will "investigate every possibility.

"We don't want to speculate right now on all the myriad of possibilities," she said. "What we do know is that our own in-house water quality parameters were all testing normally and were all in the normal range."

Most speculation has revolved around people touching the tank, as the exhibit recently started to allow the public to touch and feed the rays.

Toxin in tank suspected

However, the zoo's Sandie Black told CBC News that "the touching is not a big concern at this point."

"You can imagine there might be a myriad of potential toxins that could have been introduced into the tank. This was a 10,000-gallon (37,854-litre) tank, so in terms of lotion on hands, perfumes, those are really quite low on our list of concerns," she said. "In many similar exhibits, they have not shown to be a problem."

'It's going to take us several weeks before we have any answer, and unfortunately, it may be a situation where we never get an answer.'—Doug Whiteside, zoo veterinarian

Doug Whiteside, a zoo veterinarian, has speculated someone may have put a toxin, such as a herbicide or a pesticide, into the tank.

"Obviously, it's something that we don't want to think about, because it would be a tragedy if that was the case," said Whiteside. "We want to try to rule out some other things first, before we start to explore that avenue."

Whiteside said it could take weeks to learn the cause of death.

"It's going to take us several weeks before we have any answer, and unfortunately, it may be a situation where we never get an answer."

The zoo said the exhibit, which opened in mid-February, will remain closed until further notice. The neighbouring elephant exhibit will also be closed.

Animal deaths 'inevitable'

R.J. Bailot, with animal rights group Zoocheck Canada, said he was not surprised by the animal's deaths.

"If people are going to be able to see these animals and put their hands in their water, it's inevitable that these animals are going to die. It's a mockery of wildlife," he said.

Bailot also said these types of exhibits are not educational because it provides people with "a false image of wildlife.

"Tourists are going to now think they can interact with stingrays," he added.

He also criticized the fact that the stingrays had their tail barbs removed to prevent them from harming people, which he compared to "declawing a grizzly bear, simply for entertainment purposes for humans and children."

The zoo has lost several animals over the past two years, including four gorillas and six-year-old Hazina the hippo.