Washington's National Zoo is searching for answers in the puzzling deaths of 11 freshwater stingrays and two arowana fish on Monday.
Yesterday staff performed necropsies on the stingrays and failed to determine a definite cause of death. However, shortly after finding the dead rays, officials tested the water in the tank and found it had low levels of dissolved oxygen. Lack of dissolved oxygen is the most common cause of fish kills.
Workers have returned the oxygen levels in the tank to normal and the remaining seven rays and fish are doing well, officials say.
Zoo spokeswoman Pamela Baker-Masson said the night before the deaths were discovered all protocols were followed by workers.
Officials don't believe human error resulted in the animals' deaths.
