Anglers have been complaining about grass carp for nearly 40 years. Coincidentally, that's how long fisheries managers have been using the
exotic species as a control for another exotic, hydrilla, along with other invasive aquatic plants.
What's happened recently at Texas' Lake Austin provides a prime example of why they complain. Grass carp released there in 2012 and 2013 have consumed not just the hydrilla, but all of the aquatic plant habitat, which was beneficial for bass and other species. The stocking permit has now been revoked by Texas Parks and Wildlife Department (TPWD), with anglers and bowfishermen encouraged to catch and kill the carp.
"Anglers get angry at us when we stock grass carp, and we understand that," said Dave Terre, TPWD's Management/Research chief, who emphasized that his agency strives to involve fishermen in plans to control invasive plants.
"Everywhere we've used carp has resulted in a complete vegetation community crash," he added. "But it took 10 years for that to happen at Austin. And maybe someday we'll be able to extend that to 20 years. Just three or four years ago, we were promoting the success of grass carp in Austin."
Ten years is a considerable improvement over what happened in the early 1980s. Over the objections of anglers and TPWD, the Texas Legislature authorized the stocking of 270,000 carp into 21,000-acre Lake Conroe. In just two years, aquatic plants were gone, and the long-lived carp kept it that way until the late 1990s.
About the same time, much the same thing happened up in Kansas' Big Hill Reservoir. A heavy stocking of carp "set the reservoir back eight or nine years," said Doug Nygren, fisheries chief for Kansas Department of Parks, Wildlife, and Tourism.
"We regretted it," he added. "It was a wakeup call to be careful."
Yet, there is a flip side to this grass carp decimation story, starting with the fact that Conroe will be the site of the 2017 Bassmaster Classic March 24-26. The impoundment on the San Jacinto River now has not only grass and grass carp, but a first-class bass fishery.
"It's been producing 30-pound stringers all summer long," said Tim Cook, Conservation Director for the Texas B.A.S.S. Nation. "The lake has a significant number of 8-pound fish and I'm expecting that six to ten over 8 pounds will be caught each day. We've had five Toyota Texas Bass Classics on that lake, so anglers know how good it can be."
In short, fisheries do recover, as these controversial exotics remain the best biological method for controlling hydrilla in public fisheries, as well as filamentous algae in private and hatchery ponds. Sadly, though, their use is not a precise science. Inevitably results are cyclical, as they have been at both Austin and Conroe and other impoundments across the country, and often influenced by variables that resource managers have no control of.
Weather is the most prominent. Lower water levels prompted by drought and warmer water courtesy of hotter temperatures combined to power an unprecedented hydrilla growth spurt at Austin, and suddenly 600 acres spread across the 1,600 acre fishery. "It changed the dynamics and threw everything out of balance," Terre said.
Mechanical harvest wasn't an option, because fragmentation spreads the fast-growing exotic. Neither was herbicide, since Austin serves as a public water supply. Additionally, unchecked hydrilla inevitably would impede hydropower generation.
To knock back the plant, the city of Austin's resource managers stocked 33,000 carp from late 2011 through spring of 2013. Contrast that with only 20,000 that had been stocked incrementally for eight years, starting in 2003.
"We could live with 100 acres," said biologist Marcos De Jesus. "But 600 acres was just too much. It was going to cause problems with the turbines."
Terre added, "Anglers sometimes think that we're attacking the habitat of bass. We're not. We know that vegetation is important. But there are multiple users to consider. And grass carp are a tool."
He added that in lakes like Amistad, Falcon, Rayburn and Toledo Bend, with little development, "hydrilla is not a problem for anyone and we don't touch it."
Efforts already are underway to jumpstart aquatic vegetation again at Austin, by growing it in cages, De Jesus said. "There are going to be lots of efforts to restore habitat and to provide more options, with things like brushpiles."
Because it is smaller than Conroe and because state agencies have begun to partner with other entities, including bass clubs, in recent years to re-establish aquatic vegetation, Terre is hopeful that Austin's recovery will be rapid.
Conroe, meanwhile, serves as a model for restoration not only for Austin, but for the nation, with much of the credit going to Seven Coves Bass Club. The B.A.S.S. affiliate spearheaded growing and planting of native vegetation, even as officials continued to combat hydrilla with more stockings of grass carp.
As part of an overall management plan for the lake, the club was awarded a grant for about $45,000 from B.A.S.S. and the U.S. Fish and Wildlife Service to build a plant nursery on property provided by the San Jacinto River Authority. The latter and TPWD also helped finance the effort.
Additionally, occasional harvest by anglers and bowfishermen has been added to the overall strategy for using and controlling grass carp. In 2011, they removed more than 5,000 pounds of carp from Conroe during a tournament.
The event was intended to reduce carp population "to a number capable of preventing re-sprouting of hydrilla but which will allow us and our partners to better enhance important native aquatic vegetation for fish habitat and water quality improvement," said Craig bonds, TPWD's Director of Inland Fisheries.
While habitat initiatives and angler harvest have been added to help control carp and restore beneficial vegetation more quickly, not much has changed in terms of overall strategies for stocking grass carp to keep problematic vegetation under control, said John Biagi, Chief of Fisheries Management for the Georgia Department of Natural Resources.
Two of the most notable are smaller, incremental stockings as opposed to one massive release, and replacing diploid grass carp, which can reproduce in the wild, with triploid, which are sterile and have shorter live spans.
"And we are learning," said Texas biologist De Jesus. "We're refining the way we monitor the effects of carp on vegetation."
Terre added, "Instead of stocking carp per surface acre, we're looking into doing it per the biomass of the plants. Our goal always is to maintain good aquatic habitat for fisheries."
Most anglers think about grass carp only in terms the damage that they do to bass fisheries. But they've been used since the late 1960s by aquaculture facilities to keep filamentous algae blooms under control. They still serve that important purpose today, both in hatchery and private ponds.
A disturbing corollary to that, however, is that they also can cause algal blooms, explained Doug Nygren, Kansas fisheries chief. "When they eat the plants, that releases nutrients that can feed those harmful blooms," he said.
At Clarks Hill, meanwhile, grass carp could help save bald eagles, as well as coots, among their favorite prey on that Georgia-South Carolina border reservoir. Researchers say that a toxic cyanobacteria grows on submerged aquatic plants, especially hydrilla. Coots feed on the plants, contract Avian Vacuolar Myelinopathy, and die, as do the eagles that eat them.
"Native plants are coming on and hydrilla seems to be diminishing," said Georgia fisheries chief John Biagi. "We're in discussions with the Corps (of Engineers) on stocking rates. This is a tough one. We don't want to eliminate good habitat, but the eagles have to be considered."
In the Wild
Diploid grass carp are reproducing in Missouri's Truman Lake, and that's just one of many waters where this plant-eating exotic now has sustainable populations, according to Duane Chapman, Asian carp expert for the U.S. Geological Survey.
While Texas reports they are spawning in the Trinity River and Kansas indicates the same for the Missouri, Chapman said, "The Illinois River has a large wild grass carp population."
In fact, he added, they are reproducing in free-flowing waters from Louisiana up into Illinois and Iowa, including in the Mississippi River. And some have turned up on "the wrong side of the (electric) barrier" designed to keep Asian carp out of Lake Michigan. They're also in the Sandusky, a tributary of Lake Erie.
Although some might believe that dams will stop them, Chapman said, "I suspect that when we start looking, we're going to find diploids fairly common above dams."
He added that some believe that silver and bighead carp pose more of a threat to our waterways than grass carp, both because they are more prolific and because data already has revealed how they are outcompeting and replacing native species.
"But grass carp don't require huge populations to have detrimental effects," he said, adding that they could destroy Great Lakes wetlands, re-established through time-consuming and costly mitigation projects.
"Grass carp remove vegetation and they dig, causing destabilization and turbidity," the carp expert continued.
Additionally, diploid grass carp can live nearly 30 years, surviving on very little when plants are scarce. "They just shut down when the food is not there and don't expend energy," Chapman said. "They're just waiting for things to change, and then they gorge."
How did this happen?
"When grass carp were brought into this country by aquaculture facilities in the late 1960s, they weren't worried about security," he said. "The belief was that they wouldn't reproduce."
Sadly, that has proven to be wrong in waters all over the country.
Additionally, while some states now prohibit diploid grass carp, others do not. And anyone can buy them.
"It might be difficult to take them into some states, but it's tough to police that," Chapman said. "States might agree to make diploids less available. But in states like Iowa and Missouri, where grass carp already are all around you, it's not a big issue. There's no downside."
(A variation of this article appeared originally in B.A.S.S. Times.)