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Golden Alga in Texas

Archived News Release

Feb. 1, 2001

Fish Kills at Possum Kingdom and Granbury Under Investigation

Austin, TEXAS — Texas Parks and Wildlife biologists suspect that recent fish kills on two North Texas reservoirs are the result of toxic algal blooms. The bio-toxic golden alga Prymnesium parvum has been identified as the source of the fish kill on Possum Kingdom Reservoir and preliminary reports show that a similar bloom has developed in Lake Granbury.

Texas Parks and Wildlife Kills and Spills Team biologists, game wardens, and Brazos River Authority rangers are investigating both fish kills.

Prymnesium parvum is a plant that exists in small quantities year round throughout the Brazos River watershed from Possum Kingdom to Granbury, according to TPW. Large concentrations, or blooms, cause water discoloration ranging from yellow to coppery-brown and are believed to be toxic to fish.

" We don't know what triggers the bloom or what has allowed it to bloom in particular areas of the lake and not others," said Jack Ralph, TPW Program Leader for the Inland Kills and Spills Team. "What we'd like to get across is that it's always there in some small concentration, but what triggers a large bloom of it nobody knows. We do know that increased salinity favors a bloom of this alga and we are trying to determine what other factors give it a competitive advantage to dominate a water body. We want to minimize those factors, if possible."

As part of the investigation into the fish kills, TPW biologists plan to conduct an overflight of Possum Kingdom, Lake Granbury and the portion of the Brazos River that connects the two on Friday. There have been no reports of water discoloration or dead fish in the Brazos River itself, added Ralph.

The Possum Kingdom Reservoir bloom, first reported Jan. 11, remains active and has expanded to cover nearly half of the 19,000-acre lake. The bloom is estimated at five to eight-square miles and covers the shallow, in-flow portion of Possum Kingdom, according to Joan Glass a Region 2 kills and spills team biologist for TPW.

On Lake Granbury, dead fish were first observed on Jan. 26 and TPW estimates that 1,000 to 2,000 fish have died.

In both cases, rough fish such as freshwater drum and gizzard shad comprise the majority of the fish killed. On Possum Kingdom, game fish such as stripers, white bass, black bass, catfish and crappie and sunfish have also been affected. TPW biologists estimate that 176,000 fish have died as a result of the bloom.

" We'd like to reassure anglers that this is not a species-specific or selective bass-killing virus," said Phil Durocher, Inland Fisheries Director for TPW.

Prymnesium parvum is a brackish-water alga that was identified for the first time in Texas during a fish kill on the Pecos River in the 1980s. While lethal to gill-breathing organisms, fish and bi-valves, the algal toxin is not known to harm other species or humans. " Prymnesium parvum is not known to be a human health problem," said Kirk Wiles of the Texas Department of Health. "But, as is always the case, people shouldn't pick up dead fish and take them home to eat. That's just common sense." " Normal algal blooms last two weeks to a month," stated Glass. "This is a natural algae plant and we have no way of knowing what it will do. We will continue to monitor the bloom while it remains active and we will continue additional sampling throughout the spring."

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Aditional Information:

Would you like to know more?
The Biology of Golden Alga summarizes what we know about the alga and its toxins.

Where does golden alga fit compared to other single-celled organisms?
The Golden Alga Family Tree gives examples of and information about golden alga and other protists.

What does golden alga look like?
TPWD Golden Alga Images has photos of fish kills, golden algal cells, and short videos of live golden alga. These images may be used for noncommercial/educational purposes as long as TPWD is given credit and other site policies are followed.

Golden Alga Information Card: TPWD has collaborated with the Texas Commission on Environmental Quality and other entities to produce a golden alga information card. Download a PDF from the TCEQ website or request a free hard copy from TPWD at

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