A noxious aquatic weed has been identified as a suspect in the deaths of one of America’s most iconic symbols of freedom, the bald eagle.
The suspected culprit, hydrilla verticillata — or as it is commonly known, hydrilla — is generally considered by scientists to be among the most problematic of aquatic weeds.
The weed can impact lake navigation, swimming, industrial and municipal water uses and water recreation.
Now it appears that the federally-listed noxious plant species may also be linked to the deaths of eagles.
“Hydrilla has not been directly responsible but has been involved,” said Chris Page, South Carolina Department of Natural Resources Aquatic Nuisance Species program manager.
He said the plant is considered a host for blue-green algae, a neurotoxin that causes avian vacuolar myelinopathy.
Eagles contract AVM by preying on afflicted waterfowl who have eaten the algae. The disease has been linked through the food chain from plants to waterfowl to predators.
AVM also is suspected in the deaths of buffleheads (Bucephala albeola), northern shovelers (Anas clypeata), American widgeons (Anas americana) and other waterfowl, according to the DNR.
n Area lakes
Lake Marion and Lake Moultrie are no strangers to the plant and the disease. Hydrilla was first discovered in the state in the 1980s in the Santee Cooper Lakes, which include Lake Marion and Lake Moultrie.
“It could have been introduced by wildlife or by boats traveling from an infested water body into South Carolina,” Page said.
The plant has long, leafy stems that branch and form thick, floating mats.
Hydrilla reproduces by sprouting plants from stem fragments and can grow from the lake bed to a height of 18 feet.
“It exhibits very rapid early growth, mostly because it can grow in lower light conditions than other native species,” Page said.
There are two types of hydrilla. The one found locally is the dioecious, he said.
“Currently, dioecious hydrilla occurs in reservoirs and rivers from the Lowcountry to the Upstate,” Page said. “However, both types have been introduced to waters throughout the state. It can spread by fragmentation ... (a hardened seed pod) and the production of tubers.”
Hydrilla was introduced into the U.S. from Asia for its esthetic value and for use in aquariums.
In 2011, a DNR study cited the “exponential rate” of hydrilla growth in the Santee Cooper Lakes. According to the study, the total hydrilla acreage for both Lake Marion and Lake Moultrie is 3,244 acres or a 300 percent increase from 2010, while the total of submerged vegetation climbed to 16,025. The total acreage of the lakes is 170,000 acres.
Dr. Susan Wilde, assistant professor at the University of Georgia’s School of Forestry and Natural Resources and an expert in invasive aquatic species, has collaborated with biologists from the SCDNR, the Georgia DNR and the Southeastern Cooperative Wildlife Disease Study to further research this link.
The research has received funding from the U.S. Army Corps of Engineers and the U.S. Wildlife Service.
“We have a pretty strong field and laboratory trials that confirm the birds are getting sick consuming invasive aquatic species, typically hydrilla,” Wilde said, noting this has been the case in both South Carolina and Georgia.
Wilde said although there is no “absolute certainty” that algae bacteria on hydrilla is the cause of eagle deaths, it “is the most likely explanation.”
In lab trials, birds fed hydrilla without the algae were fine; those with the algae formed brain lesions, she said.
Sick or dead eagles are generally found from October to March, with a peak in eagle deaths occurring from mid-November through December.
AVM, first reported in 1994, is estimated to have caused the deaths of at least 100 bald eagles at 11 sites from Texas to North Carolina, according to the SCDNR website.
The majority of these deaths were found in the J. Strom Thurmond Reservoir at the South Carolina and Georgia border; however, the actual number may be higher since only about 12 percent of carcasses are recovered, according to DNR.
The number of eagle deaths that have occurred on Lake Marion is uncertain.
Today, the disease has been recorded in Texas, North Carolina, Georgia and Arkansas, and is typically reported in the winter months.
Wilde said the algae bacteria can make liver and neurotoxins that can kill cattle, wildlife and even humans.
“Hydrilla grows really densely and gets close to the surface,” she said. “It grows in a perfect location with lots of sunlight. When it is in a hydrilla bed, it traps the water. It is a great substrate for the harmful algae to grow.”
Wilde said in a laboratory setting, birds have died within two days of ingesting the algae. In the wild, death can occur within a one-week period.
“It is sad,” she said. “They get real lethargic, they have trouble walking, trouble swimming and trouble flying, for sure.”
The impact of algae-tainted hydrilla on humans is uncertain, Wilde said.
Feeding mammals, including mice and pigs, the algae-laced hydrilla did not have any impact, but she noted the studies are not 100 percent foolproof.
Wilde said the fact that humans eat fish that have consumed hydrilla tainted with the algae is disconcerting, but the potential impact is not clear.
n Tackling the problem
The SCDNR is well aware of the problem and is on top of the situation, Page said.
“We have done a variety of things,” he said. “First, we informed the public with an education campaign to help stop the spread by cleaning boats and trailers going from one water body to another.”
More directly, the department has used a chemical treatment.
“We have utilized herbicides, lake draw-downs and, most effectively, utilized biological control in triploid grass carp,” Page said. “It is sterile so that we may control the population to a certain density to guard against permanent alteration to the ecosystem.”
Herbicides toxic to the weed, including Komeen and SonarOne, are sprayed upon the waters where the weed is located.
The expanse of hydrilla has also forced the DNR to enter into a strategic grass carp stocking plan targeting the worst locations of hydrilla.
In the DNR 2012 Aquatic Plant Management Plan, about 3,200 acres are treated with the herbicides and the carp around electric power generation facilities, public and commercial access sites and residential shoreline areas.
Page said the carp has been quite expensive for the state but “very effective.”
A maintenance stocking plan was approved by DNR in 1999 and provided for stocking a small number of carp to control the plant and for increasing the population to the level at which hydrilla was under control and maintaining that level in subsequent years.
However, the stocking rate of about one fish for every eight surface acres did not suffice.
DNR, in its management plan, ramped up its carp stocking to 10 fish per acre of hydrilla.
With herbicides and carp, the estimated cost of control operations on the Santee Cooper Lakes is budgeted for about $900,000.
Wilde said that while the carp do get the disease, lab testing with chickens as the avian model revealed the chickens did not get the disease.
The South Carolina Public Service Authority and the DNR fund the programs.
The state has also adopted laws that prohibit the importation and distribution of specific exotic aquatic plant species known to cause problems. Those caught transporting the weed can be fined $500 and put in prison for a year.
Contact the writer: firstname.lastname@example.org or 803-533-5551.
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