Archived News Release
April 23, 2001
Toxic Alga Fish Kill Still Active, TPW Assessing Damages
AUSTIN, Texas — Preparations for fish stockings on Possum Kingdom Reservoir are moving forward, prompted by fish kills here and on two other Texas reservoirs along the Brazos River this year that were caused by a naturally occurring toxic algal bloom.
Texas Parks and Wildlife officials confirmed that plans to stock fish in Possum Kingdom in early June are on track. According to Mark Howell, TPW district fisheries biologist in Wichita Falls, "We stocked adult threadfin shad in the lake last week and are scheduled to stock striped bass and Florida bass fingerlings in June. PK is high on our statewide fish stocking priority list."
Howell said it is too early to draw any conclusions about the health of the fishery at Possum Kingdom, and more monitoring is planned. "We know a lot of fish found refuge from the toxic alga up in the river, particularly the striped bass, and those fish are moving back down into the lake, and people are catching them," he said. "But, the fish still haven't redistributed in the reservoir, and at this point it's difficult to say for certain what the damage has been to the resource."
TPW plans to conduct gill net surveys in May to assess the striper and catfish populations on Possum Kingdom and will evaluate crappie and largemouth bass during fall electrofishing and trap net surveys. "I would expect to see a decrease in numbers for all species, but how much we can't be sure," said Howell.
While the occurrence of Prymnesium parvum, a species of golden alga, has dissipated at Possum Kingdom and Lake Granbury, the alga is believed to be responsible for the recent deaths of at least 3,000 fish on Lake Whitney, according to preliminary estimates by TPW's Kills and Spills Team.
"The alga was discovered several weeks ago in this reservoir and immediately below the dam, but only a few incidental dead fish were reported," said Jack Ralph, Inland Kills and Spills Team leader. "Apparently the algal density was too low or the water conditions at that time were not conducive to fully activating the toxin. The situation appears to have changed."
According to Ralph, now that the alga resides in this portion of the Brazos River it could bloom again anywhere in the area. "We don't know what triggers the bloom or what has allowed it to bloom in particular areas and not others, so a kill cannot be predicted," said Ralph. "What we'd like to get across is that it's always there in some small concentration, but what triggers a large bloom 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."
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. It exists in small quantities year-round throughout the Brazos River watershed from Throckmorton to the Lake Whitney area, according to TPW studies. Large concentrations (blooms) cause water discoloration ranging from yellow to coppery-brown and can often be toxic to fish.
Rough fish such as freshwater drum and gizzard shad comprise the majority of the fish killed so far. On Possum Kingdom, game fish such as striped bass, white bass, black bass, catfish, crappie and sunfish have also been affected.
"The pond culture industry in Israel has found aggressive treatment with ammonia can reduce a golden alga toxic bloom, allowing restocking of the culture ponds," said Dave Sager, TPW freshwater conservation branch chief. While the treatment might be effective for farm ponds and hatcheries, it would not be cost-effective or practical for large water bodies. "In addition to the cost and problems with treating such large reservoirs, the concentrations of ammonia required would have adverse impacts on other organisms in the aquatic ecosystems," Sager noted. "The cure could be as bad as the problem."
While the golden alga does not pose a threat to the overall health of the fisheries in the Brazos River watershed, the bloom has resulted in considerable losses to the resource. TPW estimates about 450,000 fish have died from the toxin at a value of nearly a half-million dollars.
TPW fisheries biologists are assessing the affected reservoirs to determine the necessary level of fish restocking.
While lethal to gill-breathing organisms — fish and bivalves — 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."
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 firstname.lastname@example.org.