The entire stingray population at the Brookfield Zoo's Stingray Bay habitat has died due to a drop in the oxygen level inside the exhibit Friday.
Four southern stingrays and 50 cownose rays died as a result of the malfunction, according to zoo officials.
The oxygen level dropped dangerously low on Friday, putting the animals at risk. Veterinary staff provided medical treatment to the stingrays and the oxygen levels were immediately readjusted, but none of the stingrays recovered, zoo officials said.
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"We are devastated by the tragic loss of these animals," said Bill Zeigler, senior vice president of animal programs for the Chicago Zoological Society, which operates the zoo. "Our staff did everything possible to try and save the animals, but the situation could not be reversed."
Zoo officials have not yet identified the cause of the habitat malfunction. As they investigate, the Chicago Zoological Society has closed the summer-long temporary exhibit for the remainder of the season.
The stingray exhibit has operated since 2007.
A similar incident happened at the Brookfield Zoo in 2008, according to the Chicago Tribune. That year, 16 stingrays died when temperature controls malfunctioned in a 16,000-gallon saltwater tank.
The water temperature had risen to 10 degrees warmer than its usual level, causing 16 of the 33 stingrays to die. Eight sharks and three horseshoe crabs were also housed inside the tank but did not perish.