Golden Algae Bloom


With drought conditions increasing and lake levels declining at most reservoirs in the Brazos River basin, many residents near Lake Granbury wondered if the hundreds of dead or dying fish turning up on the reservoir were somehow related to the lack of rainfall. During the months of February and March, the Brazos River Authority received more than 80 reports of fish kills in areas throughout the reservoir.

Unfortunately, the fish kills were not entirely unexpected. From November through December, Brazos River Authority’s aquatic scientists collect monthly samples to be analyzed for Golden algae (Prymnesium parvum) by Texas Parks and Wildlife. In early February, the BRA received notice from the TPWD that water samples contained high levels of Golden algae on Lake Granbury.

Tiffany Malzahn, Environmental and Compliance manager for the BRA, notified managers that fish kills could be imminent at Lake Granbury. “Lakes Whitney and Possum Kingdom reports are fine with mixed algal community,” Malzahn said. “At Lake Granbury, p. parvum is dominant. I would say a fish kill is imminent at Lake Granbury, but we’ve already gotten reports of dead fish at Sky Harbor this morning.”

A golden algae bloom was already in progress.

Interestingly, reports of fish kills are often preceded by reports of an increase in the number of pelicans reported on or near the reservoir, which was the case in February.

What are golden algae?


Golden algae is a naturally occurring microscopic alga that typically occurs in brackish water. Like other types of algae, golden algae lives in most surface water worldwide. It is not until the alga “blooms” that it becomes toxic. Golden algae blooms can produce toxins that are lethal to fish and bivalves (mussels and clams). According to the TPWD, no risks to human health are associated with golden algae; however, people are warned not to consume dead or dying fish. Likewise, creatures that do consume dead or dying fish, such as pelicans or other animals, are unaffected.

Though the BRA had an hours-long warning of algal toxicity on this occasion, usually, a golden algae bloom occurs without notice. Most often, reports of dead or dying fish are made by those living near the lake or by fishermen who frequently report brownish or tea-colored water in the area.

Golden algae were first identified in Texas 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 release 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. Often a foamy substance will also appear in the water. According to the TPWD, over the past 30 years, golden algal blooms have killed an estimated 34 million fish throughout the state. Though the Brazos River basin has seen a few smaller blooms over the past several years, long-time residents of Lake Granbury might remember the blooms occurring in 2003 and 2005, where the lake’s fish populations were nearly decimated with more than 5.5 million fish, from shad to large game fish, lost.


Following these large fish kills, 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. 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.

Tracking Blooms

In 2022, the Brazos River Authority’s Environmental Services Department introduced a program to track fish kills caused by harmful algal blooms. The program, which is accessible to the public, allows citizens to report fish kills that can then be researched to determine if golden algae are the cause. People can report the time, date, location, and types and the number of fish observed through the free program, which also allows for uploading mobile phone photos. The BRA then provides tracking information to the Texas Parks and Wildlife Department to aid in their research.

By the end of March, the golden algae bloom had slowly receded. With warmer months ahead, additional blooms are not anticipated.

To view the HAB reporting program or to make a fish kill report go to https://arcg.is/14COnK