Wildlife officials say golden algae killed thousands of fish in a 20-mile stretch of the Salt River. 

Golden algae were discovered in Texas in the 1980s; they were first found in Arizona in 2005. The algae produce a toxin that attacks the gills of fish and causes them to suffocate.

The Arizona Game and Fish Department says it began receiving reports of dead fish Thursday. A team of biologists was sent to the area of the Salt River upstream from Roosevelt Lake Friday to gather water samples and examine the carcasses.

Game and Fish doesn’t believe the golden algae will extend into Roosevelt Lake. The agency will continue to monitor waterways along the Salt River, including Apache, Canyon and Saguaro Lakes.