Zoo officials still don't know what killed 41 stingrays. (CBC)

The Calgary Zoo is planning to reopen its stingray exhibit seven months after the unexplained death of 41 rays.

The first shipment of five stingrays has already arrived at the zoo, and another is expected to arrive later this month.

As a safeguard, the zoo has installed additional oxygenation equipment in the pool, as well as security video monitors. People won't be allowed to feed or touch the rays, for now.

The exhibit is reopening even though zoo officials haven't found an answer to the deaths in May, which came just three months after the exhibit opened.

"We have taken time to make this decision because we wanted to review all the test results and determine the best course of action for the future," said Cathy Gaviller, the zoo's conservation director in a statement on Thursday.

"The zoo remains committed to our original objectives for this exhibit — to teach and engage visitors so they will learn how to make responsible choices that will have a positive impact on the ocean environment and its sea life."

An "exhaustive investigation" failed to turn up a reason for the stingray deaths in May, said zoo officials. An independent laboratory tested for bacteria, disease and hundreds of possible toxins in the pool and didn't find anything.

"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."

Michael Alvarez-Toye, a spokesman with the Calgary Animal Rights Coalition, said the decision to restock the tank when the zoo doesn't know what caused the deaths is irresponsible.

"It's almost like they've got an empty shelf on display and they want to fill it in time for Christmas," he said.