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Alien "Water Weed" in Africa Choking Lakes, Killing Fis

Dan Morrison in Kayago, Uganda
for National Geographic News
May 2, 2007
 
Paul Okecho stood on a hard spit of land jutting into central Uganda's Lake Kyoga late last fall and looked over a dozen empty wooden fishing boats trapped in a floating field of aquatic plants.

"Here we have a problem," he said. "This hyacinth, this weed."

Although its dark green leaves and lavender flowers might look pleasant, water hyacinth is a menace that is once again on the rise, decimating fisheries and choking vital shipping lanes.

(Related news: "On Africa's Largest Lake, Fishers Suffer Falling Stocks, Rising Demand" [March 13, 2007].)

The plant is a prodigious breeder, doubling its mass every two weeks. Its leaves starve the water of sunlight and cut off access to oxygen, which suffocates fish and encourages blooms of deadly green algae.

The hyacinth was virtually eliminated from East Africa's waterways in the 1990s. But now increased pollution has allowed the fast-growing plant to return.

Agricultural runoff and sewage both dump excess nutrients in the water that further the plant's spread.

Rain loaded with nitrogen from the smoke of countless wood cooking fires also pours into the lake, feeding the scourge.

"If this continues, the boats won't be able to land at all," said Okecho, leader of Kayago, a tiny fishing village near Lake Kyoga.

"Without fish, we won't have a shilling left. Kayago would dry up."

"Planting" Weevils

A native of South America, water hyacinth was introduced into Africa in the late 1800s.

The plant was first noticed on Lake Kyoga in 1988 and on Lake Victoria to the south in 1989 (Uganda map).

At its height in the '90s, the hyacinth blanketed more than 10 percent—46 square miles (119 square kilometers)—of Lake Victoria, the world's second largest lake.

It reached from shore to shore at Lake Kyoga's outlet, obstructing boat traffic on the Victoria Nile River, which connects the two lakes.

Officials fought the last hyacinth outbreak by dispersing tens of thousands of tiny weevils, or snout beetles, into the lakes.

The weevils introduced here eat only the hyacinth, so the bugs would not be a threat to other plants on the lake.

Fishers would carry weevil-infested hyacinths onto the water and throw them into clumps of healthy plants.

"The weevils feed on the leaves and lay their eggs in the stalks. Their larvae move into the roots and pupate," said Omar Wadda, chief of Uganda's Water Hyacinth Control Unit. "In time, the plant dies and sinks to the bottom."

(Related news: "Tiny Bugs Enlisted to Fight Invading Water Hyacinths" [March 3, 2003].)

Meanwhile, floating mechanical harvesters cleared vital ports of the weed, while fishers were given machetes and other tools to hack the dense growth from their landing sites.

"In 1998 the hyacinth collapsed," Wadda said.

Today a spike in pollution levels has revitalized the water hyacinth, which once again covers 2.7 square miles (7 square kilometers) of Lake Victoria.

Already giant clumps of hyacinth have prevented ships from docking at Kisumu, Kenya's main port on the lake, blocking transit of goods and passengers to Uganda, Burundi, and Rwanda (Kenya map).

High and Dry

Water hyacinth is a double threat: It can live and reproduce both on open water and on land.

Lake Victoria is in the middle of a prolonged drought, with the waters of the upper Nile Basin at historic lows.

Long-dormant hyacinth seeds on the exposed banks have erupted, ringing the shoreline in pernicious green.

This puts the plant-munching weevils at a disadvantage, Wadda said.

"They reproduce slowly. It takes about five years for them to catch up." And unlike their prey, the weevils can only survive on water.

At Migyera, another town on the shores of Lake Kyoga, a joint Egyptian-Ugandan project has been working to keep the Nile flowing smoothly and the shoreline unobstructed.

But the project recently found itself high and dry: Barges and dredging equipment were trapped on mud banks that just weeks before had been part of the lake.

Still, the effort continues. Last December the Egyptian government donated the equivalent of 4.5 million U.S. dollars to keep the hyacinth-control project alive.

Standing on Kyoga's shore, project supervisor Fred Kutesa surveyed the beached machines.

"It's really a constant battle," he said.

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