“Just a wee bit further now,” said Dougy, stepping backwards into space and vanishing from view. I clung to a slimy branch and peered after him. He was already 50 feet below me, abseiling down the crumbling cliffside on an old nylon rope. We had descended 200 feet and there was still no water visible through the forest below us, although there was no mistaking the thunderous noise echoing off the sandstone walls of the gorge.

I had met Dougy, an ex-gillie and professional fly-fishing instructor, while fishing on the river Tay in Perthshire the week before. By the end of that afternoon, he had offered to lead me—on the strict understanding that I would not reveal its precise location—to his “secret hidey-hole,” a stretch of highland river containing so many salmon that it was almost possible to scoop the fish from the water with a landing-net. He added that only he knew how to reach the spot, on account of its outrageous inaccessibility.

For anglers, wild Scottish salmon is probably the most highly prized quarry in the world. Enthusiasts pay as much as £1,500 a day to fish the best river-beats at the best times of the year—with no guarantee of catching anything. In 2004, it is estimated that salmon anglers spent £73m on their sport and related services, and caught 93,000 fish—which works out at £800 each. Yet here was Dougy promising super-abundance. His claim seemed improbable. This river was fished by paying anglers up and downstream where the topography was less demanding. And the spot was near a main road. Poachers, I thought, would surely have cleared out such a place long ago.

At length we reached the bottom, 350 feet down. We had to yell to make ourselves heard above the noise of the dark and frothy torrent, as it surged between the rocks. It was noticeably warmer down here. Gnats and dragonflies patrolled the humid air. The gorge, Dougy told me, is an “SSSI,” one of Defra’s sites of special scientific interest, on account of certain rare types of vegetation that grow there.

Dougy retrieved a long metal ladder that he kept padlocked to a tree, laid it over the water, and scuttled to the other side. I followed more slowly, the ladder sagging beneath me. “Extreme fishing,” he had called our expedition earlier, and he…

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