Officials at the Calgary Zoo remained baffled on Tuesday as they tried to puzzle out just why 34 of their stingrays suddenly died.

The juvenile cownose stingrays, about the size of a dinner plate, were kept in a "petting" tank where visitors were able to touch them, a common feature in a number of North American zoos and aquariums, according to the zoo officials in the Western Canadian city.

On Sunday, within a few hours of opening, the normally voracious fish lost their appetites, became discolored and started dying, with 34 of the 43 rays quickly succumbing to an ailment or cause that has yet to be diagnosed.

"It was a very acute onset of something, a very sudden change," said Cathy Gaviller, director of conservation, research and education for the Calgary Zoo. "It was high mortality in a very short period of time."

The zoo, as yet, has no real idea of what happened to the rays, which are common in the waters off North America's east coast.

Water tests show there were no problems with the tank or its filtration system, though the zoo has also sent off samples for independent testing...