Austin Poured 7,091 Sterile Fish Into Lake Austin to Eat 575 Acres of Hydrilla

The stocking happened at three places, chosen for the same reason: that is where the weed is thickest. On Oct. 29, Austin Watershed Protection released 7,091 sterile grass carp into Lake Austin at Jessica Hollis Park, the Walsh Boat Ramp and Bull Creek, an operation the department announced the same day. The fish are the treatment. The lake is the patient, and the diagnosis, from a Texas Parks and Wildlife vegetation survey completed in mid-September, is 575 acres of hydrilla.

Hydrilla is a submerged perennial that arrived in North America in the mid-to-late 1950s through the aquarium trade, according to the Texas Invasives fact sheet, and it has been a management problem in Texas reservoirs ever since. The plant in this lake is the female dioecious form, which means it does not produce seed here. It does not need to. It reproduces from fragments of stem that root wherever they land, from tubers in the sediment, and from turions — small buds that look like pine cones and stay viable for up to four years. Stems reach 30 feet and grow as much as an inch a day, filling the water column until mats reach the surface.

That growth pattern explains why mechanical and chemical control struggle to keep up, and why the city keeps coming back to fish. Grass carp are plant eaters with a documented preference for hydrilla, and they work the whole lake rather than the stretches a crew can reach. Austin's stated goal is deliberately modest: reduce dense growth while leaving enough aquatic vegetation behind to support a healthy fishery. A lake scoured clean of plants is a different problem, not a solution.

What is new this year is how the number was picked. Watershed Protection built a predictive model from 15 years of Lake Austin data, and the model says 7,091 animals should put the lake on a path toward less hydrilla. After each vegetation survey the department will compare what it finds against what the model predicted and adjust the next stocking accordingly. That is a step up from stocking by instinct and hoping, though it also commits the city to a public prediction it can be measured against.

The tool has limits, and TPWD's permitting guide is blunt about them. Grass carp are effective against hydrilla, bushy pondweed and American pondweed. They are not effective against Eurasian milfoil, coontail, water primrose, cattails or filamentous algae — a distinction that matters to anyone with a pond who assumes the fish is a general answer. Control can also take several years, which is a long time to look at a mat of weeds.

For private landowners the paperwork is the part nobody warns them about. In Texas a permit is required to possess, transport and stock triploid grass carp in public or private water. Private-water applicants pay a $16 processing fee plus $2 per fish, with the per-fish portion refunded if the request is partly or fully denied; public-water fees are waived. Permits run 36 months, fish must come from a permitted vendor, and every holder has to keep either a Texas exotic species transport invoice or an out-of-state aquatic product invoice along with U.S. Fish and Wildlife Service documentation certifying the fish as triploid — the sterile form — for a year or as long as the fish are in the water. Applications for public water get reviewed by department staff, and approval is not guaranteed where sensitive species are present.

The sterile part is not a technicality. A fertile grass carp population would be its own ecological event, so the certification is what keeps a biocontrol tool from becoming the next entry on the invasive species list.

Residents get a share of the work, and the city's list is unglamorous. Cut back on fertilizer, especially on property that drains toward the lake. Clean, drain and dry boats, trailers and gear before and after launch. Stay out of the dense patches rather than swimming or running a prop through them, because a chopped stem is a new plant. If you pull hydrilla off your own shoreline, dispose of it or compost it on land instead of leaving it at the waterline. The same logic ran through our Oct. 30 piece on waterfowl gear: the hull gets rinsed, and the fragments ride home in everything else.

Five hundred and seventy-five acres is the number to watch. Coverage has held roughly stable since June even as the plant shifted location, which is not the same as progress, and a stocking of sterile fish is a bet that a model built on 15 years of history can predict a lake's next five. The carp will eat. Whether the acreage falls depends on how much nitrogen and phosphorus keep arriving from the watershed, which is the part no fish can fix.