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The Honolulu Advertiser
Posted on: Saturday, February 15, 2003

Assault on water weed to be costly

By Jan TenBruggencate
Advertiser Science Writer

Herbicide-sprayers and excavators will hit Lake Wilson next week in an attempt to gain control over the superweed Salvinia molesta, which satellite photos show has clogged the entire lake surface.



TOP: A photograph from the Ikonos satellite taken of Lake Wilson on June 28, 2000, shows a clear surface, before the invasive weed Salvinia molesta became established. The lake is a popular fishing spot.

ABOVE: Photo from the Ikonos satellite taken of Lake Wilson last Saturday shows virtually the entire surface and both arms of the lake covered with salvinia. The weed cuts oxygen flow in the water.

Photos courtesy Space Imaging

"I don't think anyone really appreciates the size of the job. You've got an estimated 2 million cubic feet of salvinia," said Honolulu Mayor Jeremy Harris. "It's thousands of truckloads."

Bill Devick, head of the state Division of Aquatic Resources, said the public-private Salvinia Working Group yesterday committed to a multi-agency program to try to reduce the weed coverage to 10 percent of the man-made lake by July.

"It's going to cost hundreds of thousands of dollars. It could turn into millions, depending on the conditions," Devick said. A $150,000 federal grant for the salvinia-control effort is nearly spent, and federal, state and city agencies are being asked to try to pay for the remaining effort out of their existing budgets, he said.

Devick said the cost of doing nothing would be far greater.

The Honolulu firm Science and Technology International presented the working group with satellite images that show the stunning spread of the weed, an aquarium fern banned in Hawai'i. In the three months from Nov. 8 to last Saturday, the aggressive plant went from covering a few acres near the lake's dam to coating virtually the entire 300-acre surface of the lake, including both of its extended arms.

"The magnitude of the problem is amazing," said Jonathan Gradie, chief technology officer at the STI firm.

Salvinia is an aquatic pest in several warm-weather areas of the Mainland, and it quickly became a problem here. In addition to Lake Wilson, it is present in Kawainui Marsh and Enchanted Lake on O'ahu. It has been cleared from an area downstream from the Ho'omaluhia Park lake and that region is being monitored, said city spokeswoman Carol Costa.

Devick said it has also been located near the Menehune Fishpond on Kaua'i and the Waiakea Pond in Hilo, and crews have been assigned to eradicate it in both places before it spreads.

At Lake Wilson, health and fisheries officials are concerned that continued blanketing of the water will kill the estimated 500 tons of fish that live there, creating a significant public health threat. The city sent a backhoe and trucks to the lake for a month late last year, hauling the weed out of the water and trucking it to a Dole Co. agricultural field where it could compost.

"We spent about $30,000 and we didn't make a dent," Harris said.

Lake Wilson is a nutrient-rich body of water, and when warmer summer weather arrives, conditions for salvinia will go from already very good to ideal.

"Under ideal conditions, it has a doubling rate of 2.2 days, which means your removal effort has to be aggressive and intense," the mayor said.

Devick said a helicopter pilot determined the conditions around the lake are too dangerous for aerial spraying. As a result, crews will try to develop a boat-mounted spraying program. They will use the chemical glyphosate, commonly known by its tradename Rodeo, in a form approved for aquatic applications. Devick said he had hoped to avoid using an herbicide, but "it's impossible not to do that at this time. Mechanically alone, it would be effectively impossible to control it."

Officials expect to have two giant excavators at the lake during the coming week to begin scooping up the material.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com or (808) 245-3074.