Lack of Oxygen in Water Suspected After Most of National Zoo's Stingrays Die

Eleven freshwater stingrays like this one died this past weekend at the National Zoo when oxygen levels in their tank got too low.
Eleven freshwater stingrays like this one died this past weekend at the National Zoo when oxygen levels in their tank got too low. (Smithsonian Institution National Zoological Park)
By Petula Dvorak
Washington Post Staff Writer
Thursday, May 28, 2009

Most of the National Zoo's stingrays died over the holiday weekend, probably the victims of water problems in the Amazonia exhibit's aquarium, according to zoo officials.

Eleven of the zoo's 18 freshwater stingrays and two arowana fish were found dead about 7 a.m. Monday in the 55,000-gallon tank designed to replicate a flooded Amazon forest, officials said.

The exhibit was open to the public yesterday, while zookeepers monitored the health of the remaining stingrays, said zoo spokeswoman Pamela Baker-Masson.

Zookeepers tested the water as soon as the deaths were discovered and began supplementing the tank with reservoir water after they found low levels of dissolved oxygen, officials said.

Insufficient dissolved oxygen in water is the most common cause of fish kills, in both captivity and nature.

The stingrays range from one to two feet in diameter and are usually gray, with reddish-black spots speckled across their tops. They are a beloved exhibit at the zoo, where children can look through the clear walls of the tank to watch them glide through water and skim across the tank floor.

The Calgary Zoo in Canada had a huge loss -- 41 stingrays -- last year. For months, speculation swirled on whether it was sabotage, poison or neglect. Early this year, officials there announced that a low level of dissolved oxygen in the tank had killed the large population.

The National Zoo staff performed necropsies on the dead creatures yesterday and did not find a definite cause of death, which makes it more likely that the oxygen levels were to blame, officials said.

By 10:15 a.m. Monday, the oxygen levels were back to normal, officials said. The seven surviving stingrays, three discus fish, the long, skinny boulengerella fish and a large school of guppies that remain in the Amazonia tank are all doing well, officials said.

All the protocols and checks were properly followed Sunday night, so zoo officials do not believe human error caused the deaths, Baker-Masson said.

The Amazonia stingrays have been at the zoo for the past 15 years and have seen their offspring successfully sent to other zoos across the country, "and that's really quite a feat in our little zoological world," Baker-Masson said.

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