It has a B-movie-monster’s name and a reputation to match. Hydrilla, a fast-growing aquatic weed native to Asia, has been rampant in many North Carolina waterways since its discovery, near Raleigh, in 1983. During the past few decades it has spread steadily westward; by 1999, it had made its way into Lake James, near Marion.
The submerged plant’s furious rate of growth — more than an inch a day — and total lack of competition are the sorts of things that keep water-resource managers up at night. Unchecked, hydrilla can fill lakes and ponds with dense, green curtains that choke out native species and snare boats and swimmers. But in Lake James, at least, hydrilla appears to have met its match in another equally exotic species.
“They’re eating machines,” says Win Taylor, a fisheries biologist with the North Carolina Wildlife Resources Commission. He’s talking about the grass carp. In the summer of 2002, 21,000 of the vegetarian fish were turned loose in Lake James. By that time, hydrilla had claimed 500 acres of the lake; but since the carp’s arrival, there’s been little sign of the territorial weed. In certain circles, the grass carp’s savage eating habits have earned it the nickname “H-Bomb.”
“It seems like that initial stocking has taken care of it,” says Taylor. Given hydrilla’s ability to grow in water up to 20 feet deep, it would eventually have blanketed 1,400 acres of Lake James — nearly a quarter of the lake’s surface — he predicts.
Historically, herbicides have been the weapon of choice against aquatic weeds. But these potent chemicals are expensive, touchy to use and bring their own set of water-quality concerns. Accordingly, biological controls are gaining favor in the fight against weeds, and none is more cost-effective than the lowly grass carp.
Native to eastern Asia, the grass carp was introduced to North America in Arkansas in the early 1960s and has since spread to 35 states. A distant kin to goldfish, grass carp grow to 40 pounds or more and can eat their weight in weeds each day. Indeed, their habit of denuding ponds and lakes of vegetation makes them a nuisance in much of their adopted range.
But if the idea of introducing a troublesome, exotic species to control another troublesome, exotic species sounds somewhat dicey, if not downright dumb (particularly here in the land of kudzu), consider this: The grass carp introduced at Lake James are “triploid” fish, rendered sterile by a pressure treatment while still in the egg. Try as they might, they simply can’t reproduce. Triploid grass carp also tend to die sooner than their fertile counterparts, living only about 10 years. That’s good, because with hydrilla (their preferred food) gone, these insatiable consumers inevitably turn their attention to native species.
Down but not out
Perched at the foot of the Blue Ridge, the 6,812-acre Lake James is the first big reservoir along the Catawba River; there are 10 others, all managed by Duke Power. The largest of these is 32,475-acre Lake Norman, near Charlotte.
“When invasive plants move in, their growth is not limited in any way,” Ken Manuel, an aquatic biologist with the utility, explains. “They have no pests, no predators; they’re very aggressive. They can out-compete the natives.” Slow-growing native plant species (such as elodea and spatterdock in North Carolina) are simply strangled by these interlopers.
Being a business, Duke Power is most concerned with hydrilla’s habit of clogging up water intakes used to produce electricity. But the weed is also subject to periodic die-offs, leaving thick mats of vegetation that rot and strip oxygen from the water, souring water quality. And if that weren’t enough, hydrilla patches have also proven to be favored breeding spots for mosquitoes.
Hydrilla, however, is only one of a whole rogues’ gallery of invasive plants infesting North Carolina waters. The state’s 1991 Aquatic Weed Control Act identified 28 species as “noxious,” banning their importation, sale or cultivation. Among those of particular interest are alligator weed, creeping water primrose, Eurasian water milfoil and giant salvinia. But recently, concern has shifted to a gaily named but positively maniacal South American plant called parrot feather. (A patch of it detected at Lake Hickory several years ago exploded from 10 acres to 100 in the space of two years.) And despite its thuggish reputation among biologists, parrot feather remains a staple plant of the water-garden industry, sold legally at nurseries and garden centers across the state.
“Parrot feather isn’t regulated at all,” explains biologist Rob Emens of the N.C. Division of Water Resources in Raleigh. And in any case, much of the responsibility for stopping the spread of aquatic weeds lies with individual homeowners, he notes. But heedless water gardeners have been known to pitch overgrown plants from their ponds into neighboring lakes or streams, unwittingly inciting vegetative riots there. Floods can also flush invasive species from gardens into local waterways.
Hydrilla, meanwhile, has spread from lake to lake mainly via bits of stem and leaf that cling to boats and boat trailers, says Emens. Boaters can help slow the plant’s migration by carefully examining their rigs for “hitchhikers” before putting in.
Despite the steady stream of newcomers arriving in Asheville, however, this voracious weed has yet to put in an appearance here. Western lakes such as Fontana and Hiwassee are heavily used for hydropower, and hydrilla can’t stand up to the extreme fluctuations in water level that this entails. Fontana Reservoir, for instance, is subject to drawdowns of as much as 40 feet — enough to expose and kill even the most determined plant invaders. “Aquatic weeds just can’t make it here,” proclaims Powell Wheeler, a fisheries biologist with the Wildlife Resources Commission’s District 9, which covers all the counties west of Buncombe.
Elsewhere in North Carolina, though, alien weeds remain a growing threat. Hoping to enhance their arsenal of biological weapons, scientists are testing various aquatic insects and fungi for use against invasive plants such as hydrilla. But until better, cheaper controls become available, it looks as though we’re living in a carp-eat-weed world. And because even grass carp, gluttonous as they are, tend to leave hydrilla’s roots alone — and those roots can go more or less dormant on a lake bottom and survive as much as 10 years — the feisty plant may spring back as energetically as ever once the neutered carp pass on. Thus, restocking the sterile fish may be needed for long-term weed control.
“You don’t really get rid of these plants,” asserts Manuel of Duke Power. “It’s deceptive to talk in terms of eradication. This is a management issue that has to be addressed over time — maybe forever, as far as I know.”
[Freelance writer Kent Priestley is based in Asheville.]