SAN ANTONIO -- Biologists with the San Antonio River Authority are keeping a close eye on the sky, after this week’s brief storm led to a significant fish kill in south San Antonio.

It happened in a tributary of the San Antonio River near Espada Dam.

Aquatic biologists said the runoff from an intense, but brief storm is what led to the kill, when dissolved oxygen levels plunged to zero.

The levels are supposed to be between five and eight milligrams per liter of water.

Senior Aquatic Biologist Shaun Donovan said the oxygen levels fell because the nutrient load in the water had a spike. He said the sudden rise was caused by an influx of common chemicals in storm water runoff.

Donovan said fertilizers, pesticides and herbicides can temporarily overwhelm the river.

“It hasn't rained in such a long time that a lot of the pollutants can pile up on the impervious surfaces, hardened surfaces, like streets, parking lots, sidewalks and roof tops. Then when we get a really, really quick rain event it just goes straight in the creek and there's not any time for anything to soak into the ground and for the pollutants to get dissipated,” said Donovan.

"The first thing we did yesterday morning when we came out here is we tried to rescue all of the fish that were struggling and gasping for air. So the first thing we did was take the live fish and put them in the main stem of the river," said

The SARA staff had help from the Texas Parks and Wildlife Department’s Kills and Spills team that has special training with these issues.

According to Donovan the rescued fish did well.

“Those guys, as soon as they hit the main stem, they recovered just fine and they swam off,” said Donovan.

The rescue team saved almost 900 fish but about 10-thousand were lost.

"The most abundant fish that was killed in the fish kill were gizzard shad, again about 7,000 of those. The next two were blue gill sunfish and long-eared sunfish," said Donovan.

The biologists have sorted and counted the types of fish that died and they are monitoring the health of the stream as oxygen levels continue to improve.

They're also focused on recycling. Donovan said the dead fish will be composted, turned into fertilizer - and the cycle of life will continue.