This sprawling, jade-hued lake in eastern China is pleasant enough on a cool spring day. But when spring warms into sultry summer, Chaohu turns slimy and stinky as algae fed by sewage, farm and factory runoff bloom, leaving it toxic and undrinkable.
China's pollution busters, banking on a rather unorthodox approach, are hoping this summer might be different.
Across the country, officials desperate to meet a national goal of restoring China's severely polluted lakes by 2030 are dumping tons of voracious fish into lakes in hopes they will gobble up the algae infestations.
Other countries have tried this in sewage treatment pools or drinking water reservoirs — with mixed success — but nowhere else has it been attempted on such a large scale.
Workers dumped 1.6 million silver carp fry into Chaohu Lake in February in the largest such project in China. They expect each fish to eat as much as 100 pounds of algae as they grow, helping to ensure clean drinking water for more than a million people.
"We're trying to restore the ecological balance. That's the main principle," Che Jiahu, a local fisheries official, said in his chilly office in Zhongmiao, a small temple town on Chaohu's north shore. The village is a tourist attraction — when the algae is not in full bloom.
Officials also hope the carp will revive a local fishing industry nearly wiped out by pollution. They shrug off questions about the wisdom of consuming fish that feed on pollutant-laden algae.
"We've never heard of anyone getting sick from eating Chaohu's fish or aquatic products," Che said. The fish are not as tasty as when he was a child, he concedes, but "still, I believe fish that eat the algae are safe."
Taihu Lake also tested
About 125 miles east of Chaohu, fisheries workers released 100 million whitebait fry in Taihu Lake in March, hoping they will eat up the nitrogen and phosphorous that feed algae blooms that forced the cutoff of water to millions of residents last summer.
Another 50 tons of whitebait and carp fry were dumped into Taihu in April to counter an unusually early algae bloom, said Fan Xiao, an official with the Taihu Fishery Administration.
"We didn't really expect the first attempt to work right away," he explained. "This algae bloom makes us even more determined to carry on."
It's not the first time China has resorted to novel strategies to combat stubborn problems.
In the 1950s leader Mao Zedong ordered farmers to bang pots and pans to scare sparrows away from grain fields. The experiment backfired. All sorts of birds too frightened to alight dropped dead from exhaustion, allowing an explosion of crop-devouring pests that they might otherwise have eaten.
In America, the use of carp to control algae in sewage treatment pools created problems when the nonnative fish escaped into waterways.
In China, that's less of a problem, because carp are an indigenous species that have been fished for centuries.
What's the right number?
The tricky part is figuring out how many carp to put in a lake. Too few, and the algae will still prevail. Too many, and the waste from the fish themselves may simply feed more algae blooms, experts warn.
The silver carp thrive on blue-green algae, which lurk in microscopic form until rising temperatures trigger a foul-smelling, often toxic bloom that saps the water of oxygen, killing fish and making it unhealthy to drink.
From the United States to Australia, such blooms are flourishing across the globe, fueled by warming temperatures and pollution.
In Israel, the use of silver carp and other filter-feeding fish in drinking water reservoirs has worked in some cases, and failed in others, says Ana Milstein, an aquaculture expert there.
Others are more skeptical.
China's undertaking "sounds like a big, artificial fix, which from my experience doesn't often work and often leads to more unplanned problems," says Paul Csagoly, an expert who worked on a project to clear fish that had been introduced to eat grass in the Danube River.
Some bad examples
History is replete with examples of the dangers of messing with Mother Nature.
Toxic cane toads imported for beetle eradication on sugar cane plantations are a threat to Australia's indigenous wildlife. Mongooses introduced to the Hawaiian islands in the 19th century ended up doing more harm to native birds than to the rats they were meant to kill.
Still, the folks in Chaohu seem to figure they have little to lose.
A decade ago, the lake was already rated dangerously polluted. Loans from the Asian Development Bank helped pay for upgrading some heavily polluting factories and building sewage treatment plants.
All to little avail. Two industrial cities of 5 million — Chaohu to the northeast and Hefei to the northwest — flank the lake, providing a steady diet of nitrogen, phosphorous and other nutrients to algae that last summer turned wide swaths of the lake a brilliant algae green, and then putrid black.
"Economic development has had a negative impact on the lake. We're just finding ways to counteract that," Che says.
The cleanup involves more than just stocking the lake with carp fry, says Ding Zhisong, deputy director of Chaohu's environmental bureau. He shows off a grove of trees planted in contaminated silt dredged from the lake.
Dozens of fishing boats are moored nearby, their occupants busy mending nets and painting, since the lake is closed to fishing until mid-June.
The water's edge is a soapy froth mixed with trash; the only sign of aquatic life, a tiny freshwater shrimp meandering through thick green fronds of algae.
Most environmental experts warn against consuming carp and other bottom feeders from lakes such as Chaohu that are contaminated not just with algae but also with toxins such as lead, cadmium, mercury and arsenic.
Though the official government line is that the fish are fine, Yvonne Sadovy, a Hong Kong University professor who sometimes meets with fisheries experts in China, said some of them have expressed concern.
While silver carp sometimes can consume toxic algae without becoming poisonous themselves, they also may absorb other contaminants, says Celia Chen, a Dartmouth College professor who has researched how pollution affects the food chain.
"I would ask myself as a scientist and as a consumer, 'What would make me comfortable eating the fish?' and that would be knowing the fish tissue did not have contaminants in it," she said in an e-mailed response to an inquiry.
She noted that most fish in China are never tested because of the expense involved.
"I wouldn't eat them on a regular basis," she said.