WIJK BIJ DUURSTEDE, Netherlands » The hiss of gas, released by a red lever turned by Arie den Hertog in the back of his white van, signaled the start of the massacre. The victims, crammed into a sealed, coffinlike wooden case, squawked as they struggled to breathe. Then, after barely two minutes, they fell silent.

Glancing at the timer on his cellphone, Den Hertog, declared the deed done. "Now it is all over," he said proudly of his gruesomely efficient handiwork, on a gloriously sunny day beneath a row of poplar trees on the banks of the Lower Rhine.

Reviled as a Nazi by animal rights activists but hailed as a hero by Dutch farmers, Den Hertog, 40, is the Netherlands’ peerless expert in the theory and practice of killing large numbers of wild geese.

On his recent outing to Wijk bij Duurstede, a village in the Utrecht region southeast of Amsterdam, he killed 570 graylag geese in his portable gas chamber, fitted with two big canisters of carbon dioxide. That brought his death toll to more than 7,000 for the week. "It is not fun, but it has to be done," he said of his work.

The Dutch authorities insist it must be done, too. They pay Den Hertog to keep a ballooning geese population from devouring the grass of cow pastures and flying into planes taking off from Amsterdam’s Schiphol Airport, a major hub in Europe. He is the unpleasant answer to what has become a problem on a grand scale for the Netherlands.

Geese populations here have skyrocketed, buoyed by a 1999 ban on hunting them; farmers’ increasing use of nitrogen-rich fertilizer, which geese apparently love; and the expansion of protected nature areas. That combination, plus an abundance of rivers and canals, has made the country a "goose El Dorado," said Julia Stahl, head of research at Sovon, a group that monitors wild bird populations in the Netherlands.

Birds that used to return to the Russian Arctic and elsewhere in the summer are increasingly staying put. Stahl estimates that the graylag goose, which seemed to be dying out in the Netherlands in the 1970s, now accounts for three-quarters of the goose population, which can reach 800,000 in the summer and double that in the winter.

As long as animal rights activists do not interrupt the gassings, Den Hertog said, passengers flying to and from Schiphol Airport do not need to worry whether their pilots have the same skills and sangfroid as Chesley B. Sullenberger III, the US Airways captain who managed to land in the Hudson River in 2009 after geese knocked out his plane’s engines.

"If I can do my job, pilots don’t need to worry about doing theirs," Den Hertog said.

Fears that geese pose a threat to airline safety secured Den Hertog his first big break, in 2008, a year before the Hudson River landing, when he won a government contract to deploy his homemade gas chamber. Until then, his family’s pest control company had focused on more conventional tasks, like ridding homes and schools of pigeons, moles, foxes and other pests.

While a big boost for business, the airport contract also put Den Hertog in the sights of animal rights groups, which accused him of reviving Nazi methods of slaughter and began a series of court cases to stop him.

Activists from the Animal Liberation Front broke into his company offices, setting fire to a back room and scrawling abusive graffiti on the walls. No arrests were made.

With his offices and adjacent home since fitted with security cameras and a fence to keep out intruders, Den Hertog hopes that the worst is over, but still worries about hate mail.

"You are like the Nazis in the war," a recent, unsigned, email read. "I hope you get a deadly disease and die slowly."

The abuse, he said, got him down but was compensated for by "all the happy farmers who like what I am doing."

"I have been called every name in the book," Den Hertog said, rifling through a black folder filled with abusive messages. "I get called a Nazi nearly every day."

Bart Krol, a vice governor of the Utrecht region responsible for geese policy, said he, too, received hate mail because of his role as the region’s "minister of geese." He said he did "not enjoy giving permission to kill geese, but I have the task of implementing what our democracy has decided is the policy for dealing with this problem."

Stahl, the bird researcher, said she did not oppose reducing the number of geese, but doubted whether gassing really worked as a form of population control.

The comparisons to Nazi actions packed such an emotional punch that government officials for a time backed off from supporting Den Hertog’s methods.

But work has picked up sharply since a June 1 decision by the European Chemicals Agency to formally approve the use of carbon dioxide as a biocide, a ruling that lifted a cloud of uncertainty about the legality of Den Hertog’s approach.

"A lot of people are too emotional about animals," Den Hertog said, scoffing at what he dismissed as sentimental attitudes shaped by modern urban living. "They give them names and think they are humans. But nature itself is very hard. When a bird gets sick, a fox will see it and kill it immediately. But if humans see it, they want to take it to a hospital."

Since he started in 2008, he estimates that he has killed more than 25,000 geese around Schiphol Airport, and 50,000 to 60,000 in all. All are donated to a butcher in Amsterdam who specializes in game.

After the initial uproar over his methods, Den Hertog was barred from using carbon dioxide and had to resort to other methods, like shooting, or, in a few instances, bludgeoning them with a hammer.

"It was very ugly, with blood everywhere," said Hugo Spitzen, a conservation area ranger here who watched an earlier gas-free slaughter.

The use of gas, Spitzen said, posed a public relations problem because "the link with World War II is pretty difficult," but was "much better because there is no blood and no panic and only takes a minute or so."

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