54 stingrays killed in Chicago zoo accident

A zoo in Chicago is mourning the loss of all 54 of its stingrays after they suffocated in their tank on Saturday.

In a period of roughly 20 minutes, ozone and oxygen levels inside the Stingray Bay habitat enclosure at Brookfield Zoo became unstable, killing the fish, the Chicago Tribune reports.

An alarm that monitors the water in the stingray tank went off, alerting staff, who rushed to try and get the situation back under control.

An air bubbler that releases pure oxygen was put into the holding pool, followed by the submersion of air pumps to create a fountain effect. A large oxygen pump was also put in the tank as the situation became desperate.

But although the tank’s levels were restored in under 30 minutes most of the 54 fish died over the next hour and a half.

The zoo had four southern stingrays and 50 cownose rays.

"We are devastated by the tragic loss of these animals," Bill Zeigler, senior vice president of animal programs, said.

"Our staff did everything possible to try and save the animals, but the situation could not be reversed."

The zoo is investigating what went wrong and the habitat will remain closed to the public for the remainder of the summer.