Golden alga Prymnesium parvum was first identified in Texas during a fish kill investigation on the Pecos River in 1985. Since then golden alga kills occurred sporadically in a variety of waters in the western part of the state until 2001 when the alga became endemic in the Brazos, Canadian, Colorado, Red, and Rio Grande river systems, including the water supplies of two public fish hatcheries, the Possum Kingdom and Dundee state fish hatcheries. The increasing area adversely affected by the alga and frequent massive fish kills heightened public and political awareness and concerns regarding the ecological and economic impacts of P. parvum blooms. The Texas Parks and Wildlife Department (TPWD), the wildlife conservation agency of the state, responded to these concerns with a program to assess the ecological and economic impacts and to develop management options. To date 33 water bodies have been affected and losses are conservatively estimated at 34 million fish valued at US$13 million. Several sport fisheries, including smallmouth bass Micropterus dolomieu, striped bass Morone saxatilis, channel catfish Ictalurus punctatus, and blue catfish Ictalurus furcatus, have been severely affected. Additionally, 26 imperiled fish species occur in the affected water basins and some have been adversely affected. Economic losses associated with reduced fishing and other water-based recreational activities appear considerable. The combined economic losses to three counties (Palo Pinto, Stephens, and Young) surrounding Possum Kingdom reservoir for 2001 and 2003 were estimated at US$2.8 million and US$1.1 million, respectively. This paper describes how the TPWD responded to public and political concerns relative to the emergence of golden alga, its harmful effects to fisheries, and its historic and current statewide distribution.(KEY TERMS: algae; aquatic ecology; environmental impacts; fish; toxicology; water resources economics.) Southard, Gregory M., Loraine T. Fries, and Aaron Barkoh, 2010. Prymnesium parvum: The Texas Experience.

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The minimum concentration of un‐ionized ammonia nitrogen needed to kill the alga Prymnesium parvum in 24–48 h was determined from ammonium sulfate treatments. Five treatments [0, 3, 5, 10, and 15 mg (NH4)2SO4/L] were tested at pH 8 and pH 9 and temperatures of 15, 20, and 25°C to generate various concentrations of total and un‐ionized ammonia nitrogen; their effectiveness in killing the alga was then monitored for 48 h. There was a strong relationship between treatment success and the level of ammonium sulfate or un‐ionized ammonia nitrogen. The ammonium sulfate treatments that killed the alga did so within the first 24 h, and their effectiveness at 48 h was not significantly different from that at 24 h. At pH 9, ammonium sulfate concentrations above 5 mg/L eradicated the alga approximately 90% of the time, compared with 33% for the other treatments. In terms of un‐ionized ammonia nitrogen, 0.055 mg/L was similar to the control in failing to kill the alga; however, 0.17 mg/L eradicated the alga 80% of the time. We suggest 0.17 mg/L of un‐ionized ammonia nitrogen as the minimum concentration for therapeutic control of P. parvum.

The toxigenic alga Prymnesium parvum has caused significant fish kills in Texas reservoirs and fish hatchery ponds since 2001. Copper sulfate and ammonium sulfate can control P. parvum, but they provide short‐term improvement and have undesirable side effects. Copper sulfate also kills desirable algae and invertebrates, and ammonium sulfate can be harmful to fish. Because dominance of the phytoplankton community and toxin production by P. parvum appear to be nutrient related, we evaluated two nitrogen (N) and phosphorus (P) fertilization regimes for their efficacy in controlling P. parvum populations and toxicity. The experiment included two N:P fertilization treatments (300:30 or 300:60 μg/L three times weekly) and an unfertilized control group conducted in limnocorrals suspended in a fish hatchery pond. Ammonium chloride (NH4Cl) and phosphoric acid (H3PO4) were used as the sources of N and P, respectively. In the unfertilized control, P. parvum cells and toxicity persisted throughout the 40‐d experiment. In both fertilization treatments, P. parvum cell densities and toxicity declined below detectable levels within 2 weeks and remained undetectable for the duration of the study. The fertilization regimes promoted pH and un‐ionized ammonia N levels that may be detrimental to the survival of crustacean zooplankton and sensitive fish species, such as fry and fingerlings of temperate basses Morone spp. However, this approach may be suitable for production of less‐sensitive species, such as advanced fingerlings of channel catfish Ictalurus punctatus. The next steps in refining this strategy are reducing the fertilization rates while still achieving P. parvum control, investigating alternative sources of N fertilizer to eliminate the potential confounding effect of un‐ionized ammonia, and integrating pH reduction measures for sensitive fish species.