Thursday, December 7, 2023

Seeing dead fish? It could be caused by golden algae



Judi Pierce has been the public information officer for the Brazos River Authority for 20 years. Her office is in Waco.


If you’ve visited Lake Granbury recently and wondered why you’re seeing dead or dying fish, it could be a golden algae bloom.


Yes, just like flowers, microscopic algae living in surface water can bloom. But unlike flowers, when in bloom, golden algae can release a toxin that can be deadly to fish and other aquatic species, such as clams and mussels – causing fish kills.

One of several species of algae that live in the lake year-round, Prymnesium parvum or golden algae, is a microscopic organism found worldwide in surface waters, especially water with higher salinity levels. It usually remains dormant; however, when a bloom occurs, it could cause the death of several or even thousands of fish.

Golden algae were first identified in 1985 as the cause of a fish kill in the Pecos River. Since then, it’s been attributed as the cause of fish kills in the Colorado, Canadian, Wichita, Red and Brazos river systems.

Golden algae blooms often occur during colder months in Texas, usually after a major temperature shift, though fish kills caused by the algae have been noted during summer months as well. The algae releases a toxin that affects fish’s gills during a bloom. Smaller fish, such as shad, usually succumb first with larger game fish dying as the bloom continues or appears with higher toxicity levels. Water is often discolored, appearing brown, much like the color of tea. Often a foamy substance will also appear. 

According to the Texas Parks and Wildlife Department (TPWD), no risks to human health is associated with golden algae; however, people are warned not to consume dead or dying fish. Likewise, creatures that consume dead or dying fish, such as pelicans or other animals, are unaffected. 

According to the TPWD, over the past 30 years, golden algal blooms have killed an estimated 34 million fish throughout the state.

Lake Granbury has experienced numerous fish kills since 2001, including the largest in 2003 when an estimated 5 million fish, from shad to large game fish, were lost.

The Brazos River Authority was one of numerous organizations that worked with the TPWD’s Golden Alga Task Force to research and attempt to find a means of controlling blooms. The group, which included an international network of stakeholders, reported that despite their efforts, the factors that cause blooms and toxin formation are still not well understood.

Though the group had some success in treating blooms in ponds, there is still no feasible means of controlling golden algae in large reservoirs, such as Lake Granbury, or in river systems.

What does this mean for Lake Granbury? TPWD tests for golden algae toxicity regularly in Lakes Granbury, Possum Kingdom Lake and Lake Whitney. Though recent testing found no blooms on PK and Whitney, toxic levels were found in several areas on Lake Granbury.

Unfortunately, until some form of control is found, fish kills caused by golden algae will continue with some years seeing no activity and others seeing numerous kills. 



The Brazos River Authority has established a reporting system for harmful algal blooms where you can report a fish kill that can then be researched to determine if golden algae is the cause. 

If you are near Lake Granbury and notice dead or dying fish on the water’s surface, grab your phone and take photos of what you see. Then go to this link ( and upload your photos, noting what you see. Or, if you are not able to take photos, simply describe the number and type of dead fish. With your help, the BRA will track the bloom’s progress and hopefully gain insight that will help work toward a method of control.