CALGARY - An independent investigation is needed to look into the operations of the Calgary Zoo following a spate of wildlife deaths at the facility, an Ontario-based animal protection group said Tuesday.

The call from Zoocheck Canada comes as the president of the Calgary Zoo admitted human error is almost certainly to blame for the deaths of 41 stingrays last spring.

"We need to be very frank here and our main expertise is not in fish at the Calgary Zoo," president Clement Lanthier admitted at a news conference.

Lanthier calculated he was "99 per cent sure" a lack of dissolved oxygen in the water tank killed 41 of 43 cownose rays last May.

"This is something we want to expand. I think we have a responsibility to connect Calgarians - not only to the land animals but what's going on in the ocean."

The stingray exhibit was just reopened to the public in December. Zoo officials said at the time it was impossible to say exactly what had caused last May's deaths.

Cathy Gaviller, director of conservation, education and research at the zoo, still believes that.

"The honest answer is we can't say for sure because we can't prove it," she added. "It's not like we're 99.9 per cent sure and that's a scientific measurement of our certainty levels - it's a figure of speech."

"We can't prove this is the case, but we strongly believe dissolved oxygen was the cause of the mortalities."

There have been a number of other deaths at the Calgary Zoo, including four lowland gorillas, a baby elephant and a hippo that had been transported from the Denver Zoo.

Julie Woodyer from Zoocheck Canada said operations at the Calgary Zoo have been in a downward spiral and there needs to be someone outside the industry to investigate all of its operations.

"I personally think the person at the top is responsible for training his staff, for making sure that he knows how to care for the animals that he's bringing into the facility," Woodyer said from Toronto.

"To me, this just goes to the irresponsibility of this particular facility," she said.

"The Calgary Zoo had a very good reputation for many, many years and, probably in about the last decade, has slipped gradually and continues to get worse as time goes on."

Zoocheck wrote a letter to the Calgary Zoo in November 2008 asking the facility seek an independent investigation into the zoo's operation.

"Presumably if the zoo felt it had no issues and thought it was doing an exemplary job as they are constantly claiming they are, they would welcome such an investigation," said Woodyer.

Gaviller rejected the idea of an independent investigation.

"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 and sometimes I think, what is Zoocheck doing for conservation," she said.

"It's easy to sit back and criticize."