Invasive aquatic plant spreading again on Lake Austin

AUSTIN (KXAN) — An invasive aquatic plant that can grow up to an inch a day is spreading on lake Austin, but the city is working to fight it back.

The plant is called hydrilla and this is not the first time the City of Austin has invested time and money to get rid of it.

Dr. Brent Bellinger with City of Austin Watershed Protection says the first documentation of the invasive plant was in 1999. During the early 2000s, they began mitigation.

According to the city, at its peak in 2012 and 2013, hydrilla covered 500 acres — almost a third of the lake.

The goal now is to slow down the growth of hydrilla and reduce its density, especially in coves, without having a negative impact on the lake’s environment.

In 2003, the city and TPWD started adding sterile grass carp to the lake. That effort eventually worked. Hydrilla had almost disappeared and carp stocking stopped in 2013.

But hydrilla is back and it’s spreading fast.

Bellinger said the older carp are nearing the end of their lifespan so they’re not eating enough to keep hydrilla at bay.

“There was then a period of about a decade where Lake Austin was essentially devoid of aquatic vegetation and now, over the last couple of years, we have seen a resurgence of aquatic vegetation, native and non-native species,” Bellinger explained.

With that resurgence, Bellinger says the hydrilla has just kept growing and now the city is looking at ways to get it under control again.

“Unfortunately, the hydrilla, just based on its life history, is winning that race with everything else,” said Bellinger.

In July, 360 carp were added to Lake Austin to slow the spread.

“The thing that makes it such a nuisance aquatic plant is because it grows very densely and it can impact all kinds of recreational use and access out here,” said Bellinger.

The plant can get caught in boat rudders and propellers and make it difficult for people fishing along the banks of Lake Austin.

“If I want to fish, you are not going to be able to throw a line out. If your hook is exposed, you are going to lose your lures every time,” said Anthony Rodriguez who was out on his kayak.

Bellinger said Austin Watershed Protection and TPWD have relied on aquatic plant surveys to come up with their next steps.

“From what we have seen we will make some decisions on a measured thoughtful approach,” said Bellinger. “We don’t want to have a system devoid of all aquatic vegetation, that is not for water quality, so we are trying to be deliberate in our stocking, in our management.”

The city said in a recent press release that hydrilla is still localized to a few spots. Texas Parks and Wildlife conducted a vegetation survey in June and found the plant covering 74 acres, or 5% of the lake.

Fanwort vs hydrilla

Hydrilla can be mistaken for Cabomba caroliniana, or Carolina fanwort as it is more commonly known, which grows naturally in Lady Bird Lake.

“Cabomba is a native plant. You can see it if you are swimming around in Barton Springs Pool. It can grow pretty robustly, but it doesn’t quite reach the same density,” said Bellinger.

He said that, for some reason, hydrilla has not spread into Lady Bird Lake, possibly because fanwort already has established itself there.