Toxic algae and low oxygen trigger mass fish die-off at Burns Gravel Pond
By Mindy McKerrall on Friday, September 11th, 2026 in Eastern/Southeast Oregon News Eastern/Southeast Oregon Top Stories
HARNEY COUNTY-State wildlife officials have confirmed that a toxic blue-green algae bloom paired with severely depleted oxygen levels led to a substantial fish kill at Burns Gravel Pond, claiming over 2,500 stocked rainbow trout alongside dozens of warm-water species.
The die-off unfolded in two waves. Oregon Department of Fish and Wildlife (ODFW) biologists tallied 1,609 dead rainbow trout during an initial survey on Sept. 4, followed by an additional 951 dead trout on Sept. 9. The event also took a toll on the 3.8-acre pond’s resident warm-water species, killing 24 largemouth bass and 54 green sunfish.
Located two miles east of Burns along Highway 78, the ODFW-owned property consists of two basins. While the basins connect during high-water spring conditions, summer evaporation isolates them into standing bodies of water—creating ideal conditions for cyanobacteria (blue-green algae) to flourish when high nutrient levels of nitrogen and phosphorus combine with warm water temperatures.
The primary culprit behind the mortality was not just the algae itself, but a sudden shift in water column dynamics as summer temperatures began to drop.
Algae blooms typically cause low dissolved oxygen levels, particularly in deeper water. When surface water cooled last week, the pond underwent a rapid turn-over, mixing oxygen-depleted bottom water throughout the entire water column.
The lost trout ranged in size from 7 to 15 inches, representing stock introduced earlier in the spring and early summer seasons. Biologists noted that a small number of fish were observed surfacing during recent surveys, indicating that a fraction of the population managed to survive the low-oxygen squeeze.
A similar turnover and oxygen depletion event occurred at Burns Gravel Pond around the same time last year, though with fewer losses.
ODFW plans to re-stock the pond with legal-sized (8- to 10-inch) rainbow trout later this fall once water quality recovers and dissolved oxygen returns to safe levels for aquatic life.
Anglers and visitors seeking more information on blue-green algae, cyanotoxins, and health precautions can visit the Oregon Health Authority’s Cyanobacteria Advisories page.
