The area around Mansfield Dam on Lake Travis has smelled of dead fish for several days now because there are a lot of dead fish in that portion of the lake. Unfortunately, it's not likely to get any better for the long Labor Day weekend.

No need to call a CSI unit. Investigators with the Lower Colorado River Authority and the Texas Parks and Wildlife Department have already determined that it's a natural — if unpleasant — occurrence this late in the summer.

The victims: striped bass, freshwater drum and carp.

The cause of death: asphyxiation by stratification.

The culprit: hot late-summer weather, more than anything else.

Here's how it went down: Persistent hot weather heats up the shallow areas of the lake. Less fresh water comes into the lake. Colder water is trapped in the deep reaches of the lake. The colder water runs out of oxygen, killing fish at those depths.

"It happens every year," said Dave Bass , an aquatic representative with the LCRA. "It's just kind of intense this year. Usually by October, when the lake starts to turn over, it will all be gone."

At last count, the dead fish numbered in the dozens. Most of the victims were striped bass, which normally seek deeper, cooler water. In this case, that cooler water was losing its dissolved oxygen.

Steve Twidwell of Texas Parks and Wildlife's Kills and Spills Team said the Travis event occurred when the stratification, or layering, of oxygenated and hypoxic (devoid of oxygen) water in the lake became so strong that the fish were trapped in the layer of water that was declining in quality. Ultimately, the fish didn't have enough oxygen to survive.

That layer is below 40 feet in Lake Travis right now.

Bass said the striped bass have been particularly vulnerable. "The larger fish need more oxygen than smaller fish. But as algae and debris begin to sink and then decay, it begins to use up more of the dissolved oxygen. The fish get stuck in that layer of water. They can't go up or down, and they can die."


There was a similar small kill on Inks Lake earlier this month when very cold but hypoxic water was released through Buchanan Dam to generate electricity. As that water moved downstream in the Colorado River, it killed the fish in Inks. "Those fish are squeezed in there, and they're stressed," Twidwell said.

Bass said that lake visitors probably shouldn't be walking around among a bunch of dead and decaying fish but that there's no immediate threat to human health as a result of the fish kill.