Reprinted with permission from article published in the Chihuahuan Desert Discovery , Winter 1991, by Gordon W. Linum, Jack Ralph, and Joan Glass, TPWD
Relatively unknown in the western hemisphere prior to 1986, a microscopic, toxin-producing yellow-green alga has since been responsible for an estimated 2.5 million dead fish in Texas. The alga responsible was first identified in Texas during the investigation of an extensive fish kill in the Pecos River. Since then, fish kills in the Pecos and two additional Texas river basins, the Colorado and Brazos, have been attributed to its presence.
Israel, Denmark, Great Britain, South Africa, and Scotland have experienced problems with this organism for years. The Resource Protection Division of the Texas Parks and Wildlife Department in cooperation with the Texas Water Com-mission are working together to document its distribution and impact in the state.
Prymnesium parvum is a small (3000 to the inch) member of the yellow-green algae family. Algae are primitive, usually aquatic plants that lack true stems, roots, and leaves. Some of the larger, more common algae species can be seen floating on the surfaces of ponds, and are often mistakenly referred to as “scum”.
To propel themselves through the water, P. parvum employs two hairlike structures called flagella, each about 1.5 times their body length. An interesting characteristic of this organism, and one which separates it from other algae, is the shorter but similar appearing structure called a haptomena, which is located between the flagella. It is thought that the haptomena is used as an anchor and possibly directing food into its body. This is unusual for an algae since most derive their energy from sunlight through photosynthesis. Prymnesium not only utilize sunlight as an energy source, but are capable of consuming other material.
Prymnesium are brackish water algae but have been shown to survive in waters with salinities ranging from essentially freshwater to 3.5 times that of seawater. They appear to live best in waters which vary greatly in salinity. Few other organisms can tolerate such a broad range of conditions.
Presently, the only indicators of P. parvum occurring in a water body are the yellow color associated with a dense population and the conspicuous volumes of foam prevalent below areas of turbulence (riffles, dams, waterfalls, etc.). Standard water quality parameters do not give an indication to its presence.
When stream conditions appear normal and no fish are stressed or dying, Prymnesium are nearly imperceptible. During such a period, Texas Parks and Wildlife conducted a survey of the Pecos River and no toxic algae were found in the water samples. This does not mean that they were completely absent, but indicates that their numbers were very low.
Other researchers have shown that Prymnesium are able to remain dormant in sediments until conditions are right for large increases in numbers to occur. Such an explosion in algae proportions is termed a bloom. Like red tide, which is caused by the bloom of another group of algae, the precise conditions for an explosion in population growth is not clearly understood.
The mixture of toxins collectively called prymnesin are actively secreted by the algae. This is a major difference between P. parvum and the organisms responsible for red tide along the coast which release toxin upon their death. Prymnesin affects gill-breathing organisms including fish, tadpoles and clams. Millions of dead clam bodies were observed floating down the Pecos River during the first fish kill.
Under normal conditions, gills regulate the exchange of dissolved ions and gases between water and fish. When the toxins enter the gills, the ability to regulate this exchange is lost and the fish die from physiological imbalances. Prymnesin also increases a fish’s susceptibility to other toxins that may be in the water, and concentrations which are normally nontoxic may become harmful.
Fish exposed to prymnesin will actively seek refuge such as incoming tributaries and may even leap onto shore. They scrape their sides against the stream bottom in an attempt to relieve the physical irritation. Later, the fish become lethargic and are often washed downstream. Usually, the mouth, gills, and fins will have a bloody appearance.
Prymnesin toxicity is influenced by environmental factors and diminishes with an increase in salinity. For this reason, even though the organisms able to live in coastal waters, it’s toxins have never created a problem in that environment. Ultra-violet light and visible light also inactivate the toxin, al does a pH value of less than six (acidic) or greater than nine (basic). Certain bacteria found in nature also break down the toxin.
Unlike red tide, Prymnesium parvum causes no ad-verse health effects to
people, terrestrial wildlife, or live-stock. Livestock drinking from streams
harboring blooming algal populations exhibit no distress, nor do scavengers
feeding on dead fish.
A more serious concern is the loss of natural resources, angling opportunities, and tourism. The Colorado River, upstream of Lake Buchanan in central Texas provides excellent white bass fishing, and the Pecos River immediately below Red Bluff Reservoir in west Texas supports a fair to good fishery for hybrid stripers. The lower Pecos River produces channel and flathead catfish. Backed up water in the Pecos River arm of International Amistad Reservoir provides a good fishery for largemouth bass, small mouth bass, white bass, striped bass, and catfish. Many miles of stream and tens of thousands of fish can be decimated by a series of blooms.
Besides the potential loss of angling opportunities, 12 threatened or endangered fishes are located in the counties surrounding the Pecos River. Additional stresses on these species could decrease their numbers even more, if not eliminate them.
Only two control strategies have been reported that successfully saved fish during a P. parvum fish kill. The first was removal of stressed fish from contaminated water. Gill function was restored when the fish were placed in uncontaminated water. The second was the addition of ammonia which has been extremely successful in halting die-offs in Israeli culture ponds. Neither of these strategies is practical outside of fish farming operations. Presently, there are no safe or economic methods of controlling outbreaks in the natural environment.
Prymnesium parvum’s recent appearance in Texas is a mystery. It may be a new inhabitant, introduced in some unknown manner. It is also possible that it has always been present, but for unidentified reasons has not caused a problem or been documented until now.
Environmental alterations, such as increasing salinities in these affected basins, may have created living conditions more suitable to toxic alga. For instance, salinity increases in the Pecos River have been attributed to several factors including: groundwater pumping, which has resulted in many springs going dry and allowed the intrusion of brine from underlying saline aquifers through reduced pressure; surface and subsurface release of brine into river systems from petroleum exploration and development; irrigation return flows which concentrate dissolved solids by returning the original mineral loading and soil leachate in a reduced volume of water; water use by salt cedar; and periodic releases of highly saline bottom water from stratified reservoirs.
Prior to a kill in the Colorado River, heavy rains washed out an ancient, natural salt lake in west Texas, sending a slug of salt water into the river. California Creek has historically been used for surface brine disposal.
Many questions pertaining to Prymnesium parvum are still unanswered. Through continual monitoring and study, Texas Parks and Wildlife hopes to fill in some of the blanks.
Gordon W Linam, Jack Ralph, and Joan Glass, Resource Protection Division, Texas Parks and Wildlife Department, 4200 Smith School Road, Austin, Texas 78744.