Golden Alga (Prymnesium parvum)

Golden alga, or Prymnesium parvum, is one type of microscopic alga that is frequently present in the waters of the Brazos basin in low concentrations. Golden alga is unique from other alga found in the basin in that occasional blooms can be toxic to fish, mussels and clams.

Though golden alga can occur in all types of water bodies, harmful events typically occur in brackish or salty waters. It is the only species of algae in the Brazos River basin to have caused documented toxic events.

The toxins produced by golden alga are not known to be harmful to humans, livestock or wildlife. It characteristically appears as brownish or tea-colored water. The alga population tends to expand during colder months.

In 2003 and 2005, the freshwater organism nearly devastated some fish populations in Possum Kingdom Lake, Lake Granbury, and Lake Whitney in the Brazos basin and many other Texas lakes outside of the basin.

While golden alga is frequently present in the waters of the basin in low concentrations, it is most likely to bloom, causing fish kills, during cold weather events. A golden alga fish kill may last for days, weeks or months, and may affect whole water bodies or isolated portions of water bodies.

Given the high frequency of wintertime golden algal fish kills, BRA aquatic scientists partner with the Texas Parks and Wildlife Department to take monthly samples during the fall and winter for early detection of potential golden alga issues on Possum Kingdom Lake and Lake Granbury.

Indicators of toxic golden algal blooms include:

• Water that appears to be tea-colored
• Impacts to small fish occur first, with larger fish impacted as the duration of the toxic event continues, expands in area, or increases in intensity. All species of fish can be affected.
• Fish may be gulping for air at the water’s surface because the toxin attacks their gills and the gills no longer function appropriately.
• Fish swimming in irregular patterns or manner often appear to be intoxicated.
• Hemorrhaging is visible in the fish. It is first noticeable in the gills, head and base of the fins, then moves into the tissue giving the fish a pink to red appearance.

When there is a report of dead fish in the Brazos River basins’ reservoirs or rivers, the BRA’s environmental team will assist the Texas Parks and Wildlife Department’s Kills and Spills Team in documenting the event and investigating the cause.

No matter the cause, there are good standard practices to use any time dead fish are found in your area. Good practices include not swimming near dead or dying fish and not eating dead or dying fish. Eat only fish that appear healthy at the time they are caught.

Prompt notification is key to successfully determining the cause of a fish kill.

If you see dead or dying fish on a Brazos River Authority reservoir, a golden algae bloom may be the cause. Please help us track the progress of golden algae blooms by providing information on what you've seen through our Harmful Algal Bloom tracking system.

The HAB system will allow you to provide the date, time, and place of the possible bloom. Providing information on an estimated number of dead or dying fish as well as the types of fish you see will help us to understand and track the progression of the bloom or find the cause of the fish kill.
You may find the HEB reporting system here: https://arcg.is/14COnK
or by scanning this code: