Posted on: Friday, November 15, 2002

Lake Wilson choking on weed

By Jan TenBruggencate
Advertiser Science Writer

State and city crews are working jointly to free Lake Wilson in Wahiawa of Salvinia molesta, an aggressive fern that is now on the federal noxious weed list. Deborah Booker • The Honolulu Advertiser

City and state crews worked for a second day yesterday to try to clear a fast-growing aquatic fern from the surface of O'ahu's Lake Wilson, but it may be a lost cause.

"I don't think this stuff can be eradicated. It's probably going to need continual work," said Bill Devick, administrator of the state Division of Aquatic Resources.

The problem fern is Salvinia molesta, once commonly used in aquarium settings, but so aggressive an invader that it is now on the federal noxious weed list. It is illegal to possess the plant in the United States. The U.S. Army Corps of Engineers has referred to it as "possibly the world's worst weed."

Other communities have found that it grows so fast that they can't keep up with manual methods of eradication. A biological pest that attacks it is being worked on, but is not ready for use.

What's left is chemical treatment. While there are herbicides approved for use in water, Devick said his office hopes to avoid using chemicals in a water body whose fish are eaten.

In Hawai'i, salvinia has been reported in Lake Wilson and Enchanted Lake, probably the result of someone dumping unwanted aquarium contents into the lakes or into streams leading to them, Devick said.

The fern is free-floating, and while it is sterile, it reproduces readily from fragments. And that's the problem. Salvinia under good conditions can double its water coverage in just two to three days.

The South American native, sometimes called Kariba weed, water velvet or aquarium watermoss, is doing particularly well because of the high nutrient levels in Lake Wilson. The lake is manmade, created by damming up Kaukonahua Stream. The stream flows into the lake, and so does the treated sewage from the city's Wahiawa wastewater treatment plant.

"It has very high-quality treatment, but they don't take out the nutrients," which act as fertilizer for salvinia, he said.

The 300-acre lake is a popular fishing spot, with a catch-and-release fishery for large mouth bass and tucunare. Anglers can catch and keep any of two dozen other species found in the lake, including channel catfish and snakehead.

The mat of fern on the surface interferes with boating and fishing on the lake, but once it covers large parts of the water, there is a more insidious problem. The layer of plants can interfere with the exchange in oxygen between the air and the water, and as dissolved oxygen levels drop, fish die.

"There's an estimated 500 tons of fish in there," Devick said.

The state Division of Aquatic Resources, the city and landowner Dole arranged a plan to clear the lake. State crews and city crews and equipment would try to collect the stuff, and it would be dumped on a Dole pineapple field, where it would be allowed to serve as a mulch.

Crews were using oil spill containment booms to round up the fern and bring it to shore, where a loader was used to dump the plants into trucks.

"It looks like it's going to be an extended effort," Devick said.