Alien seaweed choking reef off Waikiki Beach
By Jan TenBruggencate
Advertiser Science Writer
Not only is the noxious weed Salvinia molesta wreaking havoc in the fresh waters of Lake Wilson, but seaweed is being hauled away by the tons from the Waikiki reef.
Jennifer E. Smith • Special to The Advertiser
Volunteers have been hauling tons of the stuff by hand, but it grows so fast they can't keep up with it. In one day last weekend, 120 volunteers collected 13,000 pounds of Gracilaria salicornia, a non-native seaweed that is cousin to ogo and manauea, two popular eating species. Divers stuffed the limu into burlap bags while a chain of volunteers passed the bags to shore for sorting, weighing and disposal.
Volunteers have done four cleanups off Waikiki Beach in the past year, collecting 20 tons of invasive seaweed in the past six months alone. The algae is given to a farmer for fertilizer and to a greenwaste processor.
"We've removed more than 20 tons of this seaweed from this reef over the last six months," said Jennifer Smith, a University of Hawai'i graduate student whose doctoral research involves controlling alien seaweeds.
The alga is rich in nutrients, so there's a demand for it even though it's a marine pest.
"It went to a taro farmer who uses it to fertilize his fields, and to a greenwaste firm that composts it and sells it as Menehune Magic," said Cindy Hunter, acting director of Waikiki Aquarium.
Native seaweeds found among the aliens were separated by hand and replanted on the reef or sent to agencies that use them in fisheries or conservation work.
There have been four seaweed cleanups in Waikiki and two in Kane'ohe in the past year.
The Nature Conservancy of Hawai'i's marine and coastal conservation program is working with engineers to design a device to allow marine experts to clean alien seaweed species off the reef quickly and efficiently.
"We're working on developing an industrial suction device" that can be floated out to where the limu is, and some kind of a raft, barge or boat on which it can be deposited and sorted before the unwanted seaweed is taken to shore for disposal, said project coordinator Eric Co.
Jennifer E. Smith • Special to The Advertiser
The project is early in the design phase, and biologists are still reviewing different kinds of pumping devices and vessels, Co said.
Non-native algae covers the beach in front of the Waikiki Aquarium after a south swell. "Superweeds" can crowd out native species and alter the ecosystem.
Anyone walking down the shoreline in front of Kapi'olani Park and the aquarium after a south swell can see immediately the effect of the fast-growing algae.
"The beach gets covered with rotting, stinking seaweed," Hunter said.
Co said alien marine algae is a statewide concern, though unrelated to a separate battle being waged against the noxious aquatic weed Salvinia molesta, a fast-growing freshwater plant — not an alga — that is clogging up Lake Wilson.
"The five major problematic limu in the Hawaiian Islands are a real threat to the coral reef systems. Gracilaria salicornia, for example, will quickly overgrow corals and less aggressive marine algae, outcompeting them for space," Co said.
The abundance of Gracilaria is mainly an O'ahu problem — at Waikiki, Pearl Harbor and Kane'ohe Bay — but is now found also in a couple of locations off East Hawai'i.
Acanthophora spicifera, the most widespread of the five top alien algae, covers 11 percent to 50 percent of the sea floor off certain areas of Kaua'i, O'ahu, Moloka'i, Maui and a single small area off the Big Island's Hamakua Coast.
Hypnea musciformis, a feathery limu brought in for aquaculture, is found most heavily on Maui, with one dense population on south Kaua'i and smaller amounts on south O'ahu.
Several species of Kappaphycus are thick in Kane'ohe Bay, where it is expanding fast. The weed is so rigid it can damage corals by abrasion as well as shading them out.
Avrainvillea amadelpha, with bright green, leathery leaves, competes directly with the Islands' only native seagrass on sandy bottoms off south O'ahu and in one location off south Kaua'i.
Hunter said all but the Acanthophora were introduced for aquaculture. Some, such as Gracilariaare, are eaten without processing, although the native manauea and less aggressive introduced ogo are considered tastier.
Some seaweeds are processed into agar — used in laboratories to grow bacteria and in food production as a thickening agent — and carageenen, widely used as a gelling agent and to make ice cream and yogurt smoother.
Acanthophora is believed to have arrived in Hawai'i on the bottom of a barge from Guam in 1950.
"All of these are superweeds," Hunter said. In addition to trashing the marine environment and impacting native species and valued fish, the weeds pile up on shore after storms, marring beaches.
"I don't know if we can win, but we can certainly shift the advantage to native species," Hunter said.
A consortium of agencies has joined forces to try to control the non-native varieties, including The Nature Conservancy of Hawai'i, Hawai'i Coral Reef Initiative, state Division of Aquatic Resources, U.S. Fish and Wildlife Service, Project Reef Check, University of Hawai'i's Waikiki Aquarium, U.S. Environmental Protection Agency and UH Botany Department.
Reach Jan TenBruggencate at jant@honoluluadvertiser.com or (808) 245-3074.
