Vast swaths of an invasive weed are creeping their way along the Kenyan and Ugandan shores of Lake Victoria, severely disrupting the local fishing economy and sending scientists and policymakers scrambling to find a way to staunch the leafy green threat.

The great floating mats of water hyacinth are reducing fish stocks, blocking wharves where boats are launched, destroying nets in deeper waters, and fouling drinking supplies for beachside communities.

And all this from a non-native plant that scientists thought they'd successfully booted from the world's second largest lake more than a decade ago.

The weevil fix

In the 1990s, the weed began to spread across the lake, eventually covering an area of more than 12,000 hectares.

Then, a team of Australian, Kenyan, and Ugandan scientists came up with an innovative fix that appeared to have the problem licked. They introduced South American beetles that feed only on water hyacinth, which was itself brought from South America to East Africa in colonial times for ornamental gardens.

The insects quickly began to eat their way through the plant and some stretches of water were cleared, prompting claims of an environmental and humanitarian miracle for the 30 million people who survive on the lake’s bounties.

Now the invasion is back, and in denser quantities than before.

Scientists say this is because of a failure to keep the beetle program going and a surge in human population that has increased pollution in the lake.

“The weevils worked, in places, for a while, but the idea was not properly followed up and the water hyacinth has now found the perfect conditions to thrive again,” says Henry Ndede, the head of the United Nations Environment Programme’s Kenya office.

Among the worst of the affected areas is the bay where Kenya’s fourth largest city, Kisumu, sits.

Its 350,000-strong population has boomed in the last decade, and many newcomers live in slums with poor sewage where effluence runs straight into the lake.

Major rivers flowing past the city rise in Kenya’s western highlands, where its sugar and tea estates lie and where agrochemicals and fertilizers run off into streams and eventually to the lake. The result is inshore, shallow waters that are "exceptionally high" in nutrients, allowing the hyacinth to breed at speeds that means it can double its biomass in as little as eight days.

Now the body of water colloquially known as Kisumu Bay is almost entirely clogged with water hyacinth, and looks more like a wide green meadow than a large inlet.

A problem of oxygen and corruption

Other areas along Kenya’s Lake Victoria shoreline, and further north in Uganda, are also battling the same problem.

For the fishermen seeking tilapia, Nile perch, and tiny silver cyprinids known here as omena, the weed has dramatically cut their catches.

“It is a problem in part of deoxygenation,” says William Oweke Ojwang, a senior scientist with the Kenya Marine and Fisheries Research Institute. Shallower parts of the lake, he says, are literally being choked of oxygen as dead hyacinth biomass sinks to the bottom and rots, a bacterial process that sucks oxygen from the water.

The heavy plants also calm the lake’s surface, cutting its wave flow and further reducing the air getting into its waters. That creates conditions where major species that need oxygen-rich water are dying out, says Mr. Ojwang.

But there is more to it than that, argues Michael Nyaguti, chairman of a fisherman’s lobby group in Kisumu called Magnam Environmental Network.

“Our problem with the fish stocks is illegal fishing, and it is corruption,” he says. “Yes, the water hyacinth blocks us sometimes from going into the water, but other times I think it just gives an excuse.”

Years ago, Mr. Nyaguti says, boats would use bright lights to attract lakeflies, which in turn would attract the fish, especially the omena. That, he says, “is how to fish.”

Now, nets with a mesh almost as fine as a mosquito net are trawled behind boats, pulling up omena but also juvenile tilapia and Nile perch, killing them as bycatch before they can grow.

Fishermen too afraid to go to deeper waters for fear that the hyacinth will float in and cut them off from shore are instead mining shallow waters, where the species breed.

Both of these practices are illegal, but Nyaguti says enforcement of the rules is easily neutered with bribes.

A new way forward?

Attempts have been made to clear the hyacinth mechanically, using engine-driven conveyers that pull it out of the water and dump it on the beach. But there, it germinated in the rainy season and seeds sluiced straight back into the waters.

What is needed, says Ndede of the UN’s Environmental Programme, is for Kenyan and Ugandan policy-makers to find a cash value for the nutrient-rich but water-heavy plant.

“We import fertilizers and blend them with farm mulch,” he says. “Why not investigate whether the hyacinth can be used for the same? Then people here would have an interest to help clear it. It’s only a matter of this menace being prioritized at the highest levels of government.”

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Alongside this, argues Ojwang, people living near the lake need to be better educated about the effects their waste and pollution is having ultimately on their own earnings.

“Yes, the hyacinth needs tackling,” he says. “But it has only grown because of human activities combining to change that lake’s conditions so it is perfect for the plant. Changing people’s behaviors is the most important path we must choose.”