HINES, Ore. – ODFW biologists confirmed that a natural mixing event, or "turnover," likely caused the death of stocked rainbow trout recently observed at Burns Gravel Pond on Hwy 78.

 

The pond is located two miles east of Burns, Ore., on Hwy 78 on property owned by ODFW. The pond's two basins are connected when water levels are high in the spring but then become isolated as water levels drop during the summer.

 

Turnover events can happen in fall when warmer oxygen-rich waters on the surface start to mix with colder oxygen-poor water at the bottom. Oxygen levels then drop, exacerbated by organic matter such as phytoplankton and vegetation (e.g. algae blooms), and kill fish.

 

Biologists recorded very low dissolved oxygen levels at the surface and bottom (2.5 mg/L and 0.01 mg/L respectively). Rainbow trout need minimum 5 mg/L of oxygen to live and will die when oxygen falls below that level.

 

ODFW biologists counted nearly 794 dead rainbow trout in the larger pond (3.8 acres) and two in the smaller pond (2.4 acres). Three green sunfish were also found dead in the main pond. The trout ranged in size from seven to 15 inches and were likely trout stocked earlier this spring/early summer. A few fish surfaced during the survey, suggesting a small number survived the low oxygen event.

 

ODFW will stock more legal sized (8 to 10 inch) rainbow trout once oxygen levels resolve and the pond can support the fish.