Fish Kill Investigated on Fairfield Lake
TPWD fisheries biologists solve mystery of dying fish.
Anglers fishing the lake the morning of September 13th noticed the dead fish and notified the TPWD 24-hour
Main locations of dead fish were along the shoreline of a cove immediately northwest of the dam to the spillway and in the cove south of the dam, but also extending up the shoreline to mid-reservoir. The fish appeared to have died no earlier than Saturday, September 12th.
TPWD personnel returned to the lake on Monday the 14th and Tuesday the 15th to collect water samples and conduct a thorough investigation and enumeration of the kill. Water quality field data were measured both in and out of the area where dead fish were observed. Dead fish were distributed along approximately 10 miles of shoreline. A series of detailed counts were conducted along approximately 0.75 miles of that shoreline to allow estimation of the total kill.
Water quality datasondes (electronic data gathering devices) were deployed in the northwest and south coves where the main kills occurred. These datasondes were able to collect temperature and oxygen concentration every 30 minutes for the following 14 days.
Water quality field data collected on Sunday the 13th indicated extensive areas of lower than normal oxygen and abundant phytoplankton (chlorophyll a) in the areas where the fish kill occurred. Using information on oxygen concentration from the datasondes, water quality data collected the day of the kill, and information on sunlight level from a local weather site, TPWD biologists began to piece together a theory on the cause or causes of the kill.
Normally photosynthesis (oxygen production) by phytoplankton during daylight hours increases oxygen concentration enough to compensate for respiration (oxygen use) by those same phytoplankton as well as bacterial decomposition at night. However, during periods of cloudy weather sunlight (measured as solar radiation) is reduced; oxygen consumption remains high but oxygen production is greatly reduced. When cloudy weather lasts for several days and oxygen concentration falls below the minimum level to support aquatic life, fish begin to die.
A good rule of thumb is at oxygen concentrations below 5 milligrams per liter (5 mg/L) many species become stressed, and at concentrations below 3 mg/L most species can die from oxygen deprivation. These concentrations are also known as the daily mean and minimum dissolved oxygen criteria for high aquatic life use in the Texas Surface Water Quality Standards. Although oxygen levels may stay above the minimum level during the day, it only takes a few minutes below the minimum at night to be fatal to fish.
Cloudy weather in the
This reprieve was short-lived; cloud cover reduced solar radiation on the 15th, 16th and 17th and oxygen concentrations remained below the minimum level. By the 18th the weather pattern had improved and oxygen production in the north cove remained above the minimum level. Oxygen production in the south cove lagged several days behind due to its higher phytoplankton level, but by Sunday the 20th abundant sunshine was once again allowing phytoplankton to produce high levels of oxygen in both areas. Another short period of cloudy weather from the 22nd to the 25th reduced oxygen concentration again but was of short enough duration that no substantial fish kill occurred.
By combining oxygen data from the datasondes with solar radiation data from the weather station, TPWD biologists now had the critical information needed to understand the complex dynamics of the repeated kills at
In September, water temperature and bacterial activity are still high but day length has been getting shorter incrementally since the summer solstice on June 21 (the date in the Northern Hemisphere when daylight hours are longest relative to dark). Extremely high phytoplankton levels due to high nutrient levels produce sufficient oxygen during sunny days to compensate for lack of production at night; however, when early fall cool fronts and cloudy weather limit solar radiation, oxygen levels drop rapidly and fish may die.
Initial estimates from the 2009 TPWD fish kill investigation indicated that nearly 1 million fish died compared to an estimated 7,345 that died in September 2008 and an additional 114,223 that October. However, the species distribution in the current kill was considerably different than in September 2008. In 2009, 96% of the kill (an estimated 875,793 fish) were threadfin and gizzard shad and 3.3% (an estimated 30,168 fish) were sunfish species; the remaining fewer than one percent were bullhead minnow, inland silverside, channel catfish, flathead catfish, yellow bullhead, largemouth bass, blue tilapia, and red drum. In the 2008 kill, 51% were large red drum with an average length of 32 inches.
Water samples and tissue samples from live but stressed fish collected during the day of the kill have not shown the presence of any toxicant that could be responsible for the kill.
TPWD has stocked more than 5.3 million red drum in Lake Fairfield since 1984, and despite the estimated 3,750 red drum lost in the 2008 kill and the far lesser number lost in the present kill, anglers fishing the lake report excellent success for this popular fishery. Anglers spent over 9,000 angler hours seeking red drum at
The TPWD Kills and Spills Team is a group of biologists who respond to pollution reports or natural incidents that threaten state fish or wildlife resources. If you see dead or dying fish or wildlife or pollution threatening fish and wildlife, please contact the 24-hour
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