Roberto Altamirano has the lake to himself as he casts his glistening net onto the still water in a perfect circle, lets it sink, then slowly pulls it in.

It comes back bearing a large haul of tilapia and carp -- and that is exactly the problem.

Altamirano is one of just 20 or so fishermen who remain in the floating gardens of Xochimilco, an idyllic network of lakes, canals and artificial islands improbably tucked into the urban sprawl of Mexico City.

At 42, he has watched the number of fishermen here plunge over the years, leading to booming populations of tilapia and carp -- invasive species that are threatening the already strained ecosystem of Xochimilco, a green lung vital to the health of smog-choked Mexico City.

"There's more Xochimilco than there are fishermen," says Altamirano.

First settled by the Aztecs, who created the original artificial islands, Xochimilco is today a UNESCO World Heritage Site visited by millions of tourists each year, who ply its maze of canals in colorful tour boats.

Chinese carp and African tilapia were first introduced here in the 1970s in what turned out to be a misguided plan to supply a new food source for local residents.

Today, no one in Mexico City will eat them, since the rampant growth of the Latin American mega-city has badly polluted the waters of Xochimilco with heavy metals.

Altamirano and his colleagues sell the fish to be ground up and used as compost.

The carp and tilapia are meanwhile threatening to wipe out a small, critically endangered salamander called the axolotl that is found only in Xochimilco.

The fish eat the eggs of this small amphibian, whose peculiar ability to regenerate its body parts has led researchers to study it for possible biomedical applications in humans with organ damage or missing limbs.

Carp and tilapia "were introduced as a resource for human consumption. However, since they are invasive species, they turned out to be a problem," said researcher Maria Figueroa.

- 'Last crazy one' -

Altamirano's response to the crisis is to do what he has done all his life: fish.

"I learned to fish from my grandfather, who's 98 years old. The tradition had died out because it was no longer profitable. I'm the last crazy one in the family line," he told AFP.

Eight years ago, he launched a group to fish as many tons of carp and tilapia as possible out of these waters. Their record: nine tons in half an hour.

With these fishermen, Xochimilco's ecosystem would face collapse -- something scientists have warned could raise average temperatures in Mexico City by as much as four degrees Celsius.

Altamirano says it is getting harder to find recruits to join his crusade.

Fishing here is hot, dirty work. And the salary is meager: around $145 a month, paid by the city.

"When we die out, that will be the end of the fishermen. Young people don't want to get into this line of work anymore," he said.

Fishing at his side, 23-year-old Ramses Coloapa said he was doing the job "mostly out of necessity, because I have to buy food."

Altamirano hates the idea of watching the axolotl go extinct.

The little salamander -- whose name means "water monster" in the indigenous Nahuatl language -- was considered sacred by the Aztecs, who believed it was the last incarnation of their fire god, Xolotl.

"There was a local legend here that said the day the axolotl disappeared, that would be the end of Xochimilco," said Altamirano.

"We're almost there."

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