Eleven stingrays and two arowanas (fish) were found dead in their pool at the National Zoo’s Amazonia exhibit Monday, May 25, shortly after 7 a.m. Preliminary tests of the water from the pool showed low levels of dissolved oxygen, indicating the deaths may be due to mechanical difficulties rather than biological issues. Necropsy reports by the Zoo’s pathology department did not indicate a definite cause of death in either of the species. No other pools in Amazonia were compromised.

Recent maintenance and cleaning of the pool showed no signs of problems Sunday and nothing unusual was observed by staff during standard animal-care rounds through Sunday evening. After intense inspection Monday, staff confirmed that all filtration equipment for the pool was operating well within normal parameters.

Upon finding the first dead arowana, staff immediately pulled samples of the pool’s water for subsequent testing and began supplementary aeration with very rapid water changes with water from Amazonia’s 28,000-gallon reservoir pool. By 10:15 a.m. Monday, the dissolved oxygen in the pool was restored to an acceptable level. Surviving in the pool are seven stingrays, three discus fish, boulengerella and a large school of guppies.

Amazonia opened to the public in 1992. Its 55,000-gallon aquarium was created to display a variety of Amazon River fish. Over the past 15 years, stingrays at the Zoo have been healthy, reproducing animals and have supplied numerous offspring to other accredited zoos and aquariums across the country. Within the past year, the stingrays birthed a number of ray pups from two different litters, which include one male and three females that remain at the Zoo in a separate pool.

Arowanas, also known as aruanas or arawanas, are freshwater fish with bony heads and an elongated body covered by large, heavy scales.

Until an exact determination of death is found, staff at Amazonia will maintain supplementary aeration of the pool in question and perform continued dissolved-oxygen meter readings and water-quality testing. Keepers continue to extensively monitor the surviving animals in the pool, including the respiration rates of the stingrays.