But the unsparing mesh, with holes smaller than mosquitoes, traps much more life than traditional fishing nets do. Scientists say that could imperil already stressed fish populations, a critical food source for millions of the world’s poorest people.

Scientists are hardly the only ones alarmed. Fistfights are breaking out on the beaches of Madagascar between fishermen who fear that the nets will ruin their livelihoods, and those who say they will starve without them. Congolese officials have snatched and burned the nets, and in August, Uganda’s president, Yoweri Museveni, threatened to jail anyone fishing with a mosquito net.

“We need a regulation to deal with these people,” Mr. Museveni said.

Many of these insecticide-treated nets are dragged through the same lakes and rivers people drink from, raising concerns about toxins. One of the most common insecticides used by the mosquito net industry is permethrin, which the United States Environmental Protection Agency says is “likely to be carcinogenic to humans” when consumed orally. The E.P.A. also says permethrin is “highly toxic” to fish.

Most scientists say the risks to people are minimal, because the dosages are relatively low and humans metabolize permethrin quickly. But with coldblooded animals, it’s a different story.

“If you’re using freshly treated nets in a smallish stream or a bay in the lake, it’s quite likely you’re going to kill fish you don’t intend to kill,” said Dan Strickman, a senior program officer for the Bill and Melinda Gates Foundation, which has invested heavily in malaria research and development. “That’s definitely an environmental hazard.”

The leading mosquito net manufacturers insist that their products are not dangerous. Still, many nets are labeled: “Do not wash in a lake or a river.”

Some labels go even further, warning people to pour any water used in washing a net into a hole in the ground, “away from home, animals and wells.”