An aggressive alien seaweed that has damaged coastal tourism on Maui and choked reefs on several other islands is now spreading on the near-pristine reefs of Mokumanamana — Necker Island — in the Northwestern Hawaiian Islands.

Reef scientists, concerned about the possible spread of the red alga Hypnea musciformis, hope to begin diving off the island during a September research cruise to determine the extent of the problem.

"It's very disturbing news. We think of the Northwestern Hawaiian Islands as being relatively free of invasive species," said marine biologist Randy Kosaki, research coordinator for the Northwestern Hawaiian Islands Coral Reef Ecosystem Reserve.

University of Hawai'i botanist Isabella Abbott, Hawai'i's premier seaweed scientist, started alerting authorities during the past week after studying her most recent batch of algae from the northwestern end of the Hawaiian archipelago. Fisheries scientists who conduct lobster studies each year collect whatever seaweed is caught on the lobster traps and ship it to Abbott for identification and study.

The scientist said she first encountered hypnea from Necker samples in 2002 — but only a few sprigs. It was in roughly the same small amounts in 2003 and 2004. "In 2005, it was suddenly measured in pounds," she said.

The material was collected from traps deployed by researchers in 90 to 270 feet of water on the south side of the small island, about 350 miles northwest of Kaua'i.

"If it gets loose, the entire Northwestern Hawaiian Islands is in trouble," Abbott said.

Ironically, Hypnea musciformis was brought to the Islands in 1974 in hopes of establishing an economic crop, but it has instead ended up costing money for cleanup and research and in lost value of coastal lands. The plant some people call hookweed can be processed into a food thickener called carrageenan, but it has never developed into a commercial crop here.

"There are windrows of it on sandy beaches around Kihei on Maui," Abbott said.

A 2002 study for the Hawai'i Coral Reef Initiative on the value of coral reefs found significant reductions in property values, room rates and occupancy on Kihei properties fronted by the worst of the seaweed problem. The loss is in the range of $20 million, said Celia Smith, a limu researcher at the University of Hawai'i.

"The biomass builds up on beaches so much that people don't want to be there," she said.

The issue in the Northwestern Hawaiian Islands is not the impact on tourism, but what the weed could do to native ecosystems. The entire northwestern island chain is a wildlife refuge and the coral reefs are protected as reserves.

The only species that might welcome the weed is the Hawaiian green sea turtle, which eats it.

Hypnea has long, thin, twisting tendrils with hooked ends. It readily breaks up, and each new piece can hook on to a new surface and grow. Sometimes it hooks onto corals or rock, but often it snags native seaweeds and grows on top of them. Then, if waves and currents break it free, it can rip the native species away with it.

Abbott said the prized edible seaweed limu kala appears to be a preferred host.

"This is not a benign intruder," Smith said. "It is a point of concern when any alga begins to increase its biomass. This is a plant that has the propensity to undergo rapid growth under certain conditions."

Kosaki said he hopes divers, during a September monitoring cruise aboard the National Oceanic and Atmospheric Administration's research vessel Hi'ialaka'i, will be able to assess the spread of the weed and to determine, for instance, whether it showed up in Abbott's deep samples because it is tumbling down from shallower beds.

Smith said she feels an "ecological response may be appropriate." To her, that means divers with bags collecting the weed while its population is still controllable. Otherwise, hypnea threatens to wash on the currents up the island chain, infesting the coral reefs.

In the main Hawaiian Islands, hypnea has been a significant problem in specific areas where there are high nutrient levels in the water. Runoff from agricultural fields and outflow or seepage from sewage systems are fertilizer for marine plants.

The Northwestern Hawaiian Islands are not normally thought of as a nutrient-rich environment, and Smith said the appearance of large amounts of weed there may require a study of nutrient levels around the islands and atolls.