SAN ANTONIO – Last month, about a thousand dead fish washed up against the banks of Calaveras Lake.
The event was classified as a “fish kill” after officials tested the water and determined the fish died as a result of a fatal mix of weather phenomena.
A “fish kill” is a single event that causes a massive die-off of fish that can affect a single or multiple species.
KSAT reported the incident, which raised questions about why the fish — mostly red drum — died and how often fish kills happen at Calaveras Lake and elsewhere.
How common are fish kills?
Fish kills are not rare.
About 120 fish kills have happened each of the last 5 years in Texas, according to data provided by the Texas Parks and Wildlife Department (TPWD).
Calaveras Lake usually experiences one each year, the agency said.
In 2023, a fish kill happened at both Calaveras Lake and nearby Braunig Lake in August.
What caused the fish kill at Calaveras Lake?
KSAT meteorologist Adam Caskey and I set out to find answers from Texas Parks and Wildlife Department biologist Travis Tidwell, who works for the department’s Kills and Spills Team.
Texas has four regional Kills and Spills Teams that determine whether an event — like what happened at Calaveras last month — can be classified as a fish kill and determine what factors caused it.
In the case at Calaveras Lake, low-dissolved oxygen was the reason why the fish died, he said.
According to Tidwell, it is a pretty common cause of fish kills... but what does that mean exactly and what causes it?
“Fish breathe oxygen dissolved in the water,” Tidwell said. “... and there’s not enough of that in there, then you’ll often see them coming up to the surface and gasping for air. They’re trying to get oxygen.”
So what is stopping the oxygen from dissolving into the water? A deadly mix of hot temperatures, cloud cover and not enough wind, he said.
“When the water gets warm it loses its ability to hold that oxygen in the water,” Tidwell said. “The warmer the water, the less dissolved gases (like oxygen) that it can hold.”
This process of water not mixing due to different temperatures is called stratification. When the water is warm at the top, you get colder water below, he said.
The cold water layer has less dissolved oxygen. And while the surface water layer is getting oxygen because it’s closer to the atmosphere, the water can’t hold onto that oxygen because it is too warm.
As a result, the fish are effectively trapped in stagnant water unable to swim up or down to get oxygen, Tidwell said. That’s when they begin to die.
“It’s kinda like a can of Coke sittin’ out on a hot day and it goes flat,” Tidwell described. “All of that dissolved gas has left the liquid and that’s typically what happens in these low-dissolved oxygen events.”
Wind is the weather element that helps mix the two water layers together.
When it blows it agitates the water, allowing oxygen to be circulated, or dissolved, into the water.
While too much sun can lead to a fish kill by heating up the water, so can the lack of sun.
That’s because plants need sunlight to make food and/or energy, the process is known as photosynthesis.
During photosynthesis, plants use sunlight to convert water and carbon dioxide that is in the air and water into energy. As this process happens, the plants give off oxygen.
However, if the clouds hide the sun — or when night falls — plants can not produce oxygen, which means less oxygen in the water.
Tidwell said this is why fish kills typically happen overnight.
“There is no photosynthesis going on with the algae, with plants in the water the fish still need to consume oxygen. Overnight you’ll end up having this low oxygen case that’ll go on into the early morning hours and that’s when the fish die off,” Tidwell said.
Does the power plant at Calaveras Lake contribute to fish kills?
Both Calaveras and Braunig Lakes have coal-burning power plants operating on them. Those are owned by CPS Energy.
The lakes serve as necessary cooling reservoirs for the power plants.
The power plants draw in water from the lake, use it and return the water into the lake at a warmer temperature.
The warmer water from the power plant can lead to fairly seasonal, low-dissolved oxygen fish kills, according to Tidwell.
Last month, CPS Energy told KSAT in a statement that the Calaveras fish kill “is a result of a naturally occurring phenomenon when periods of cloudy conditions and calm winds persist for several days. These conditions limit the amount oxygen mixing in the deeper parts of the lake, potentially impacting aquatic life. The utility is aware of the situation, is checking the entire lake to assess the impacts and is in close communication with Texas Parks and Wildlife Department.”
Warm water added to the lake that’s already warm because of air temperatures and the sun may not be ideal for fish in the summer. However, during the winter it can make for good fishing since the fish the warmer water can make the fish more active, Tidwell said.
What else can cause a fish kill?
Despite the grim scene that people can come across, not all the fish in a body of water die.
According to Tidwell, the conditions associated with low-dissolved oxygen can be isolated to a specific area like a cove that is separated from the rest of the main body of water.
In addition, Tidwell offered something else that can cause a fish kill: too many fish in one spot.
A reported fish kill Quintana Beach near Freeport on the Texas Coast last June was due in part to the sheer number of fish that congregated in that area, he said.
“Having that massive amount of fish itself was enough to cause the oxygen to crash. There were more fish than oxygen in the water at the time within that area,” he said.
Can you eat fish that died in a kill?
The short answer: no.
Tidwell cautioned those who come across a fish kill.
All those fish may look good to some folks but it is probably best to leave them for the birds and the other animals out in the wild.
Tidwell stated that TPWD does not typically issue fish consumption advisories but said people may want to avoid eating a dead fish they come across.
“The fish have been sitting in warm water for who knows how long, it’s not going taste very good and there’s also you know, concerns about bacteria, contaminants,” Tidwell said.
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