Electro-fishing here is not the user-friendly procedure used by fisheries scientists. Here, poachers hook two wires to overhead power lines, then — being very careful where they put their feet — dip the other ends into the river.

“As well as blowing the system, they wipe out everything in that piece of water,” said Vinay Badola, 32, a fish conservationist. “And if one day somebody is taking a swim when the 11,000 volts goes in, they’ve had it too.”

In February 2003 Mr Badola found a giant “goonch” catfish, or Bagarius bagarius, that had been killed using this method. “It was so big that three of us couldn’t lift it. It must have weighed between 100 and 150 kilos.”

These hideous predators were described by A. St J. Macdonald in 1948 as “the vermin of the water”. Now, because of poachers, they have become a rarity.

However, it is the golden-scaled Himalayan mahseer (Barbus tor putitora), a member of the carp family, that the poachers are after. Their favoured method is dynamite. “But fuses are expensive, so they can be a bit on the short side,” Mr Badola said. “You can tell some old poachers by their lack of a hand.”

Corbett National Park, 120 miles from Delhi, has introduced an experimental scheme. Since 2004 tourist resorts have been given control of stretches of the West Ramganga River, where they play host to sport anglers. This makes the fish more valuable alive than dead.

Elsewhere, Mr Badola is employing former poachers as guides for anglers and encouraging some line-fishing by riverside families. “That way they have a real stake in the state of the river instead of seeing it as the preserve of outsiders,” he said. “But just one stick of dynamite could undo years of work.”

Although the fish are returning in places, the giants described by Victorian and Edwardian writers are still elusive — with one exception.

Locals claim that in the Mahakali river a goonch has grown to such monstrous proportions from feeding on corpses from riverside funeral pyres that it no longer waits for its food to die. It has allegedly made off with a carpenter, a small boy and a fully grown buffalo. Man Singh, the buffalo’s owner, said: “It was as big as the buffalo.”

• Jeremy Wade is the presenter of Jungle Hooks — India, a series about Himalayan fish, on Discovery Real Time from Friday, February 24, at 10pm.

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