OFF THE SHORES OF LAKE VICTORIA, Kenya — The common water hyacinth, a floating weed that's spreading across the world's largest tropical lake like a moss-green carpet, is known to botanists as "Eichhornia crassipes." Fishermen call it something else.
"That thing is satanic," said Jim Otieno, an angular 33-year-old, to murmured assent from a handful of men gathered by the water's edge in the town of Mbita. They blame the feathery, fast-growing hyacinth for trapping their boats, choking off the fish supply and breeding malaria and other diseases along the southern rim of Lake Victoria.
Otieno isn't the first Kenyan to ascribe diabolical powers to the hyacinth. When it first appeared here a decade ago — introduced by an unwitting farmer and multiplying within months to cover 260 square miles of the lake's surface — a witch doctor was brought in from neighboring Tanzania to eradicate it.
According to local lore, a stiff wind blew in the next day and scattered the hyacinth from the lake like bits of confetti.
There's nothing mystical about the weed's resurgence this year. Experts blame a mix of sediment in the water and climatic changes, in the form of unusually heavy rains that helped the plant to proliferate. To scientists, it's another threat to one of Africa's most important natural resources.
"It's a clear sign that things are very far from healthy. This is not the natural state of the lake," said Louis Verchot, an ecologist with the World Agroforestry Center, a research agency based Nairobi, the Kenyan capital.
The first major infestation, in the late 1990s, devastated the lake's $600 million fishing economy and clogged a major hydropower plant in Uganda. After several years and tens of millions of dollars of investment, the hyacinth was finally beaten back by swarms of locally bred weevils — small snout beetles — deployed as biological control agents.
But countless hyacinth seeds remained buried in the lakebed. Over the past four decades, Verchot says, as rainfall in the lake basin has intensified, storms have eroded an average of 3.5 million tons of nutrient-rich sediment every year and swept it into the rivers that flow into Victoria.
The World Agroforestry Center estimates that in the narrow gulf that forms Kenya's share of the lake — most of Victoria straddles Tanzania and Uganda — sedimentation has increased by as much as fourfold over the past century, damaging water quality and creating an ideal environment for hyacinth to grow.
And can it ever grow. One of the most invasive species in the plant kingdom, it can double in size every two weeks.
After two months of epic rain last November and December, the hyacinth was back. For much of this year, parts of the gulf have resembled a Kansas prairie, making life ever more difficult for tens of thousands of poor Kenyans who derive their meager livings directly from the lake.
In dozens of fishing villages, empty wooden boats lie moored in masses of weeds, unable to reach the open water. Fewer boats ferry catches to market. In August, a trader got his motorboat so badly stuck in a thick green tangle that he and 12 others were marooned for four nights, surviving on raw fish and lake water, until a military helicopter airlifted them to safety.
Fishermen complain that the best catches — tilapia and Nile perch, local delicacies that can fetch $2-$3 apiece — have fled the lakeshore, where the weed sucks up most of the oxygen.
"There was a lot of fish here before this thing," said Steven Juma, a sinewy 21-year-old, gesturing at a knot of weeds as he waded in waist-high water near Kendu Bay.
One recent morning, he and another fisherman untangled a net to find only a few finger-length mudfish, wormlike brown creatures that serve as bait and fetch about 3 cents apiece. In that spot a few years ago, Juma said, you could count on at least one decent sized tilapia a day.
Which was why he developed a new plan: earning a taxi driver's license.
Health officials are worried about the impact on communities already reeling from HIV/AIDS and tuberculosis. The weed blanket creates stagnant pools, increasing the risk of malaria and parasitic infections such as bilharzia, which comes from swimming in infested waters.
But mudfish lurk in the weeds, and for many the hope of earning a dollar or two overshadows the risks.
"The water is quite smelly, and there are snakes, but we go in anyway," said Joyce Aoko, a copper-skinned 42-year-old, as she watched her nephews tramp through a field of hyacinth.
Lately, Aoko said, the boys have been complaining of rashes and colds, but then she proudly displayed their day's work: a two-gallon jug with a few dozen unimpressive-looking mudfish swimming in murky water. "A hundred shillings," she estimated — about $1.50.
Kenyan officials believe that the weevils will make a dent in the hyacinth within months. But they acknowledge that it's likely to come back because of the quantity of seeds and high levels of sediment in the lake.
Environmental groups say that Kenya has failed to protect land in the lake basin. A law prohibits farming within 10 yards of the lake, but as the population of the region has soared, many farms now run right to the water's edge, increasing the chance of soil runoff.
Vast tracts of forest also have been leveled for farming, making the ground more vulnerable to erosion, experts say.
"At this rate, within 20 years the gulf will be declared unfit for humans," said Peter Mireri, an environmental officer with Friends of Lake Victoria, a local nonprofit group. "It will have turned into a pond."