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The Battle for Lake Austin: Hydrilla vs Asian Grass Carp

The battle for Lake Austin continues: 6,000 sterile Asian grass carp were released into the lake this week in an effort to combat the invasive water plant hydrilla. That brings the total number of Asian grass carp stocked in Lake Austin to 11,000 this summer and 40,000 overall.

Hydrilla is a non-native aquatic plant that has spread rapidly in Lake Austin since it was first discovered in 1999. It grows to be very thick and can clog up pipes that carry drinking water from the lake. It can also cause problems for those who use the lake for recreation.

Mary Gilroy, an environmental scientist with the City of Austin, told KUT News earlier this year that if the plants get thick enough, they pose a danger to swimmers.

For several years, hydrilla had covered less than 80 acres of Lake Austin, held in check primarily by the periodic stocking of sterile Asian grass carp. But, earlier this year, a report by the Texas Parks and Wildlife Department showed that hydrilla reached a historic high of 580 acres of coverage this July. The city says that went down slightly to 560 acres of coverage in September.

Releases from Lake Travis for downstream irrigation have also decreased this year, limiting the water flowing through Lake Austin and contributing to hydrilla growth.

In response to this significant increase, more than 11,000 fish were stocked this summer, bringing the total number of fish stocked to over 40,000. 

The increase of hydrilla may well be due to the ongoing drought, which creates conditions that support aquatic vegetation but experts say its growth depends on many factors.

According to the City of Austin’s hydrilla webpage, “Harvesting and herbicides are options, but don’t provide the long-term, deep-water control of the fish.”

The latest restocking of the Asian grass carp is intended to replace fish lost to natural mortality to maintain a target population of 50 fish per acre of hydrilla, a rate that has controlled hydrilla in the past.

If you are interested in seeing how the release of Asian grass carp takes place you can watch a video here.

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