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Golden Alga Frequently Asked Questions

What is it and how does it kill fish?

Algae are primitive plants that are usually aquatic and lack true stems, roots, and leaves. The golden alga (Prymnesium parvum) is a microscopic, flagellated organism that normally is suspended in the water column. We do not know if this alga is native and not identified before 1985 or if it is an invasive species accidentally introduced to North America. When this alga gains a competitive edge over other species and blooms (a bloom is an explosive increase in the population of one or several species), it may produce toxins that result in fish kills. Several toxins may be released that affect gill-breathing organisms (mainly clams and fish but also crayfish and the gill-breathing stage of amphibians). Unlike toxic red tide blooms on the coast, golden alga toxins have no apparent lethal effect on other organisms. Cattle and other animals have been observed drinking from rivers during ongoing golden alga fish kills with no apparent effects. Texas Department of State Health Services officials have stated that the golden alga is not known to be a human health problem, but people should not pick up dead, or dying, fish for consumption.

The majority of golden alga fish kills occur during the winter months when the water is cold. This is the time of year not favorable for the normal green algae that populate our inland waters and may give the golden alga a competitive edge. The alga also prefers more saline waters than normal freshwater conditions, which may also contribute to bloom initiation. Additionally, the toxins of golden alga are influenced by cations in the water such as calcium, potassium, and magnesium. The presence of these cations, higher pH levels, and more saline waters may be the reason that fish kills have mostly been confined to waters west of Interstate 35.

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Where does it occur?

In Texas the golden alga has caused fish kills in five major river systems including the Canadian River Basin (Lake Meredith stilling basin), the Red River Basin (Lake Baylor, Lake Childress, Diversion Lake, Dundee State Fish Hatchery, Lake Kemp, Lake Pauline Lake Texoma, Lake Wichita, Plum Lake, the Red River, and the Wichita River), Brazos River Basin (Paint and California Creeks, Buffalo Springs Lake, Lubbock City Lakes, Possum Kingdom Reservoir, Lake Granbury, Lake Sweetwater, Lake Whitney, and the Brazos River), Colorado River Basin (E.V. Spence Reservoir, Lake Colorado City, Moss Creek Lake, Wadley-Barrow Pond in Midland, and the river between E.V. Spence and O.H. Ivie Reservoirs), and the Rio Grande Basin (Lake Balmorrhea, Red Bluff Reservoir, headwaters of Lake Amistad, and the Pecos River). In addition, golden alga has been identified in other lakes and rivers in Texas but without associated fish kills.

Other states have also been impacted by this alga. States that have reported golden alga include: Alabama, Arizona, Arkansas, California, Florida, Hawaii, Louisiana, Maine, Mississippi, New Mexico, North Carolina, Oklahoma, South Carolina, Texas, Washington and Wyoming. View the map of states where Golden Alga has been identified.

(Are you having problems with Prymnesium parvum, but your state isn't listed? Please send us details and help us keep our site up to date.)

For more details on the bloom history see Historic Blooms in Texas

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Can fish escape a toxic golden alga bloom?

Yes, they can escape the bloom if there is a nearby toxin-free area. The early effects of the toxin are reversible if the fish can swim to a toxin-free area. In a reservoir (or lake) with many coves or creeks and different water depths, all of the fish may not be killed during a bloom, although there may be individual coves where significant mortality occurs as the fish become cut off from the main water by the bloom. In a shallow hatchery pond, however, all of the fish present may be killed if a bloom is not promptly treated. Note: Although aquaculture ponds can be treated successfully, these treatments cannot be effectively used in the larger and more complex reservoirs and rivers.

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What are the ecological and economic impacts?

From August 1981 through August 2003, an estimated 17.8 million fish were killed in Texas due to toxic golden alga. These fish kills occurred in 20 reservoirs and several rivers in five river basins. Despite the fact that toxic golden alga can affect all species and sizes of fish, most of the fish killed have been forage fish, primarily threadfin and gizzard shad. These fish grow quickly and lay many eggs for rapid reproduction. Because of this, a reservoir can recover, even from a massive fish kill, if the waterbody has ample time between fish kills. Forage fish do form the basis of the food chain, though, so on-going fish kills may harm larger game fish in two ways, by the lethal toxins of golden alga and by reduced food supply. In larger, on-going fish kills, local people that rely on the waterbody for professional or recreational incomes may be affected by the fish losses, as fishermen and campers may choose other places to go. Most fish kills, however, do not affect the entire lake the entire time. Different parts of the lake may continue to support excellent fishing. Travelers are encouraged to contact local area parks to determine the effects of the golden alga bloom and fish kill at that location on that day.

Fish kills due to toxic golden alga in Texas seem to be increasing in duration and location. In February 2005, the largest one day total of dead fish occurred at Lake Whitney in the Brazos River Basin, where an estimated 4.9 million fish were killed. Over four million fish were also killed in the golden alga bloom on Lake Granbury, also in the Brazos River Basin, during Winter and Spring 2005. In winter and spring 2004, over 1.5 million fish were killed in Lake Texoma in the Red River Basin. In each of these fish kills, most of the fish killed were threadfin and gizzard shad, although other types of fish were killed, including gar, carp, buffalo, catfish, largemouth bass, white bass, striped bass, warmouth, bluegill, crappie, drum, and sunfishes.

More on the Impacts of Blooms and on Historic Blooms in Texas.

Our Texas state fish hatcheries have also experienced significant impacts from toxic golden alga. In 2001 at the Dundee State Fish Hatchery in the Red River Basin, over 5 million fish were killed in hatchery ponds before golden alga was found. The Dundee fishery is a "put, grow and take" operation for reservoirs across the state, growing striped bass that do not spawn naturally in Texas. An entire year's production of striped bass was lost. This fishery in Texas generates approximately $150 million in angler expenditures annually, so the loss of an entire year's production at the hatchery was significant although the fishery was able to recover. Possum Kingdom State Fish Hatchery in the Brazos River Basin was located on an affected reservoir at the time but did not experience a fish kill because it was closed for renovations. Our Texas state fish hatcheries have developed a management plan (The Management of Prymnesium parvum at Texas State Fish Hatcheries) to help decrease and avoid future losses.

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What do we know, or more appropriately, do not know about the alga?

There is limited information known about many aspects of golden alga. In addition to on-going work within Texas hatcheries and monitoring of fish kills by our Inland Fisheries biologists, Texas Parks and Wildlife Department is working with researchers, other agency officials, and interested parties within and outside of Texas to better understand and potentially control harmful golden alga (Prymnesium parvum) in Texas. A literature review of golden alga was completed in 2001, and in January 2002 TPWD coordinated a multi-agency Report to the Texas Legislature on Golden Alga in Texas. In October 2003, TPWD hosted a golden alga workshop to bring together researchers from all over the United States and abroad to determine what methods might be used to manage the Golden Alga blooms occurring across Texas. This website for Harmful Algal Blooms in Texas was launched in January 2004 to help keep the public informed. Additionally, since 2003, TPWD has coordinated research funding from several sources for golden alga projects in targeted areas of research. Research projects have covered intense monitoring of a waterbody experiencing regular fish kills due to golden alga (Lake Whitney in the Brazos River Basin), a statewide survey to determine where golden alga is present in Texas, laboratory and reservoir studies to determine the alga’s environmental needs, genetic studies, a historic assessment of past fish kills, an economic impact study of the fish kills at Possum Kingdom Reservoir in the Brazos River Basin, and potential control mechanisms. Some of these research projects are on-going; more information on them can be found under Golden Alga Research and Management .

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How do you test for golden alga toxicity?

TPWD Inland Fisheries personnel use a bioassay (diagnostic test using living organisms) adapted from an Israeli protocol. This bioassay can detect sub-lethal concentrations of the toxin(s) in terms of ichthyotoxic units (ITUs, a measurement of the harmful effects upon fish). Briefly, the test uses various dilutions of test water (e.g. from a bloom or suspected bloom), a chemical solution known as a "cofactor", and test fish. The cofactor works to enhance the toxic effect in order to detect sub-lethal levels of the P. parvum ichthyotoxin.

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Does golden alga occur throughout the water column?

Golden alga can occur throughout the water column depending on the depth of the water. The cells need light for photosynthesis, so they may be limited by how deep into the water the sunlight reaches. However, some parts of the golden alga life cycle can occur in the dark, and resting cysts of golden alga sink to the bottom of the water column after formation.

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What is a resting cyst?

A resting cyst is a non-active life stage that many types of algae, including golden alga, undergo. The cyst may form during unfavorable conditions (ex: low nutrients or predation) and may be able to become an active swimming cell again when conditions improve. Very little is known about the resting cysts of golden alga.

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Are the resting cysts harmful to fish?

Most are not. Most resting cysts are dormant and do not produce toxins. Once an alga emerges from the cyst to an active stage, though, the ability to produce toxins may recur. Resting cysts of golden alga have not been tested for their ability to produce toxins.

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Will warm water cause a golden alga bloom to decrease?

Warmer waters may allow the algal community to change, which can decrease a golden alga bloom. However, if golden alga becomes numerous in the phytoplankton (algal) community, it can last all year despite rising temperatures. Fish kills due to golden alga have occurred in some Texas lakes and rivers throughout the summer months. Worldwide scientific literature states that P. parvum (golden alga) is not toxic at 30 degrees C (86 degrees F) or above, and Texas hatcheries have been able to decrease golden alga management strategies during the heat of the Texas summer.

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Will increased water flow stop a bloom or lead to a diminished bloom?

We do not know. Bloom situations are complex and involve changing water flows, salinities, nutrient concentrations, light intensity, and temperatures, all of which may increase or decrease a golden alga bloom. Increased water flow may dilute salinities to levels that do not support a bloom. Rainfall can wash more nutrients into the waterbody, which can allow green algae populations to increase and diminish a bloom. Alternatively, increased nutrients can also allow the bloom to increase in duration and size. Increased turbidity (decreased clarity of the water) may decrease a bloom by preventing sunlight from reaching the alga (called "shading out"). Additionally, some compounds such as special clays may also decrease some types of algal blooms. One of our research projects studied the potential effects on different types of clay on golden alga blooms.

Removal of Prymnesium parvum through clay and chemical flocculation

media download(PDF 458.3 KB)

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How do you measure the amount of golden alga in the water?

Our biologists collect water samples and conduct cell counts to estimate how many golden alga cells are in the water. They put a small amount of the water sample on a specialized grided slide underneath a microscope and then count the number of golden alga cells inside the grid. (Each cell is quite small.) The area of the grid is known, which allows the counts to be expanded into estimates of the number of golden alga cells in one milliliter (1 mL = 0.001 liters or 0.000264 gallons; there are 5 mL in one teaspoon). Cell counts do not always correlate with toxicity, although increased cells counts were found prior to the fish kills in Texas due to golden alga. Golden alga can produce enough toxin to cause a fish kill when cell concentrations are as low as 10,000 cells/milliliter, but fish losses typically do not occur until algal density is at 20,000 cells/milliliter or more. Because of the lack of correlation between cell counts and toxicity with golden alga, we have chosen to not report cell counts on our website. Instead, we report when golden alga is increasing and when it becomes dominant in the algal community. Both of these are indications that toxicity may increase and may lead to a fish kill.

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What do percentages of golden alga in the phytoplankton (algal) community tell us?

As the percentage of golden alga increases in the community, there is an increased tendency for a bloom to cause a fish kill. When the percentage of P. parvum is greater than 50% of the algal community (termed “dominant” in our reports), a golden alga bloom is possible, and a fish kill may result. However, depending upon a variety of factors including water quality conditions, a fish kill may not result even when golden alga dominates the phytoplankton community.

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What are we doing about golden alga now?

The Texas Parks and Wildlife Department is coordinating efforts to monitor fish kills caused by toxic golden alga, to research golden alga for better understanding and management, and to manage the recreational fisheries of Texas with river authorities, state agencies, and university researchers. TPWD Law Enforcement, Coastal Fisheries, and Inland Fisheries staff members have been involved in these efforts. Studies at two state fish hatcheries have resulted in control methods for golden alga at the pond level, however these methods are time and labor intensive and are not economically or ecologically feasible for application to natural systems.

TPWD is ready to speed recovery of affected waterbodies through the use of several resource management strategies, including restocking of the fishery. Use of this strategy remains problematic, however, as long as the affected waterbody continues to experience periodic fish kills caused by the alga. Putting more fish into waters where they would soon die is not practical. Coordinated research and management efforts about the alga and its biology remain significant and important.

To this end, TPWD has a Golden Alga Task Force that is working with researchers, other agency officials, and interested parties within and outside of Texas on understanding and working to control harmful golden alga (Prymnesium parvum) in Texas. In 2003, the Texas Legislature authorized the use of $600,000 per year for two years (total 1.2 million) for research on golden alga in targeted areas. In 2005, additional funding for continued golden alga research was authorized ($225,000) and supplemented by matching federal aid funds (total $450,000). The money to fulfill these authorizations is raised through user-based fees, specifically the licenses that fishermen pay to fish in Texas. Targeted areas include the development of management tools, approaches and technologies to help aquatic managers detect, combat, and manage golden alga in Texas. These projects are currently underway and are described on our Research and Management Page. Additionally, TPWD is committed to working with other states as they confirm the presence of golden alga. By sharing information and techniques, efficient use of resources is maintained. The solutions to the problems of toxic golden alga (like other harmful algae), will not be solved easily or quickly, although progress and successes will occur. Guidelines for Golden Alga Prymnesium parvum Management Options for Ponds and Small Reservoir (Public Waters) in Texas media download(PDF 422.9 KB) have been developed and are now available to provide information and options presently available for controlling golden alga toxic events in ponds and small reservoirs in public waters. The information is provided to help cities, water utilities, river authorities, and others evaluate options for addressing toxic events.

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What is the Golden Alga Task Force?

This is a group of Texas Parks and Wildlife Department (TPWD) personnel organized to manage the departmental response to golden alga issues and problems. Members of the task force include fish hatchery, fisheries, and aquatic biologists from the Inland and Coastal Fisheries divisions with additional personnel as needed; Dr. David Sager coordinates the task force. The task force coordinates and interacts with other agencies and universities to address golden alga issues and is responsible for administering federal grant and legislatively-authorized research funds to address golden alga problems in Texas. The mission of the TPWD Golden Alga Task Force is to protect aquatic ecosystems by understanding toxic golden algal blooms and to avoid, minimize, and mitigate their impacts through coordination and collaboration with stakeholders. The task force formulated and revised the GATF Vision and Objectives in October 2006; objectives were revised in the areas of research, management, and coordination. Current and previous research projects are described on our Research and Management page.

What have we learned from similar situations with other harmful algae?

Unfortunately, the golden alga situation is analogous to red tides in coastal waters. Red tides are caused by different toxic algae but also result in widespread fish kills. Extensive research has been conducted on red tide algae for over 30 years that has increased knowledge about the algae but has not yet resulted in viable treatments to control bloom outbreaks. Many algal blooms have been found to be associated with overloading of nutrients to the water. Initially, scientists suspected that toxic algal blooms also resulted from too many added nutrients in the water, but at this time there is no strong evidence that toxic red tide or golden alga blooms are caused by nutrient pollution from humans. The ultimate goal of Texas Parks and Wildlife Department (TPWD) is to learn enough about golden alga to be able to effectively manage it and its impacts. Until viable management options are determined, though, TPWD's emphasis remains on research and information about harmful algal blooms. TPWD is a regular distributor of information on red tide outbreaks and golden alga blooms in Texas waters, with updates posted on our website and recorded on our 1-800 information phone number (1-800-792-1112, press 4 for fishing and then 9 for harmful alga blooms).

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Who do we contact for additional information?

Texas Parks and Wildlife Department

To report a fish kill, please call one of the 24-Hour Communications Centers (512-389-4848 (Austin) or 281-842-8100 (Houston)), one of the Kills and Spills Team biologists, or your local Game Warden.

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How can I help?

The best action you can take is to be sure that fish kills are reported to TPWD. (See Who Do We Contact For Additional Information, above).

Additionally, helping to decrease the spread of all nuisance plants and animals will benefit Texas. Golden alga appears to be widespread in Texas, occurring in several major river basins that have never experienced a known fish kill due to toxic golden alga. We do not know if golden alga was transported into the state at one time or has always been here (and changes have allowed it to bloom and become toxic). Possible transportation methods for golden alga are unconfirmed but may be varied, including migratory waterfowl traveling between waterbodies, atmospheric deposition, and transfer between waterbodies on boats and trailers and in bait water. Nevertheless, there are several common sense solutions to decrease the spread of nuisance plants and animals; these solutions may also help to decrease the spread of golden alga. For more info, please see Ways to Help Prevent the Spread of Nuisance Plants and Animals.

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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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