PHOENIX – The Arizona Game and Fish Department recently investigated a fish die-off of thousands of threadfin shad at Apache Lake in the Tonto National Forest.

AZGFD collected water samples on Tuesday, Feb. 28 and determined the die-off was caused by a golden alga bloom largely restricted to the riverine portion of Apache below Roosevelt Lake.

Golden alga is not harmful to humans, and therefore Apache Lake remains safe for water activities, including fishing.

“Golden alga thrives on elevated salinity associated with drought conditions,” said Marc Dahlberg, Water Quality Manager at AZGFD. “Therefore, recent heavy rains will hopefully knock down salinity levels and help control golden alga blooms.”

At this point, AZGFD believes the fish kill has been restricted to threadfin shad and a few gizzard shad, both of which are highly sensitive to the toxin produced by golden alga.

The last such fish die-off in this region occurred in 2007, also in upper Apache Lake. However, there were significant fish kills caused by golden alga in Saguaro, Canyon and Apache during the early-to-mid 2000s that affected all fish species.

Fisheries biologists for the Department will continue monitoring water quality and fish health on all Salt River-chain lakes.