THE CANADIAN PRESS

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CALGARY - The Calgary Zoo is reopening an exhibit of cownose stingrays even though an investigation into the deaths of 41 rays seven months ago could not determine what caused their demise.

"We have planned the repopulation of the exhibit gradually and carefully," Cathy Gaviller, director of conservation at the zoo, said in a news release Thursday.

"We wanted to take our time and do everything very methodically in a step-by-step manner to enable us to monitor conditions closely."

The first shipment of five rays arrived Dec. 4 and the next rays are expected later this month.

Gaviller said the zoo has put in additional safeguards to reduce the risk of death, including installation of increased oxygenation equipment in the pool and video monitoring in the exhibit area.

As well, the public will not be allowed to touch or feel the rays as they were before.

In May, the rays in the "touch-tank" - where people crowding along the sides of the aquarium were allowed to reach in and touch the animals - started to die.

At first, it was thought that hand creams, sunscreens or other pollutants from people's hands may have caused the deaths.

But the problem spread so fast - 26 died the first day - veterinarians ruled that out.

The Calgary police were called in and they said the cause was an "undetermined source or substance."

Gaviller said extensive testing was done and disease, bacteria and water quality was ruled out. Several possibilities remain, including unknown toxins or possible deficiency of dissolved oxygen.

"Although we were hopeful a conclusive answer would be found quickly, we realized from the start that it was like trying to find a needle in a haystack."

In July, half the stingray population at the Brookfield Zoo in Chicago was killed when an apparent malfunction in the pool's heating system warmed the water too much for the highly temperature-sensitive animals.

The Calgary incident also caused the Toronto Zoo to temporarily suspend the touch-tank aspect of its popular stingray exhibit.