The mystery over what triggered most of the Calgary Zoo's popular stingrays to die off in May will likely remain unsolved.

As the two surviving cownosed rays continue their ceaseless orbit around their spacious $250,000 tank, the zoo admits long-awaited toxicology tests on water and tissue samples failed to explain what killed the other 41 fish.

"Chances are we won't ever know for sure what happened," zoo conservation director Cathy Gaviller said yesterday.