Burns Pond fish die-off caused by blue-green algae; ODFW plans to re-stock trout this fall

SALEM, Ore. – ODFW biologists confirmed that a blue-green algae 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.

Cyanobacteria and algae blooms can occur when high levels of nutrients like nitrogen and phosphorous occur and water temperatures warm. Algae blooms often result in low dissolved oxygen levels, especially in deeper water. As surface temperatures cooled last week, water in the pond likely mixed creating low dissolved oxygen throughout the entire water column in the pond resulting in the fish kill. For more information about cyanotoxins visit the Oregon Health Authority's website.

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 1,609 dead rainbow trout Sept. 4 and 951 additional rainbow trout on Sept. 9 in the larger pond (3.8 acres). Also, 24 largemouth bass and 54 green sunfish were 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. A similar event occurred last year around the same time with fewer dead fish observed. ODFW will stock more legal sized (8 to 10 inch) rainbow trout once oxygen levels resolve and the pond can support the fish.