IT IS 40 years ago this summer since Grafham Water, in Cambridgeshire, opened as a trout fishery. It is 50 years this summer since Chew Valley Lake, in the West Country, did likewise. Together, they launched a movement that has changed the face of British fly fishing. Grafham proved much the stronger influence.

Neither Grafham nor Chew was the first supply reservoir to be stocked with trout and made available to the public. Blagdon, in the West Country, had become that in 1901. Much smaller reservoirs — notably two in Northamptonshire — had also opened earlier. What made Grafham, near Buckden, so special was its size and location, the quality of the fishing it offered — and the timing.

Up to the mid-1960s, most trout fishing was where God had made it. The chalk streams of the South were beyond most anglers’ financial reach. The teeming waters of Wales, the West Country and the North were, like the lochs and the loughs, far removed from the centres of population. They catered for the mobile few.

Grafham opened in the heart of England as the motorway system was being developed and more people were gaining access to cars. Its 1,500 acres were within reach of millions.

The lake had been stocked with half-pound trout 18 months before opening. In the summer of 1966, visiting anglers could not believe what they found. Pretty well wherever they cast their flies, tight-muscled and ferocious trout leapt on them.

The smallest fish, having gorged on the sticklebacks and snails that had quickly colonised the water, had reached 3lb. Four-pounders and five-pounders were little more than average. Six-pounders were common. Chew had opened with a fanfare and fine fishing, but not on this scale. And Chew was more remote.

The sport Grafham offered was, by any measure, some of the finest in the world. It was slap in the heart of England, it was accessible and it was cheap. The publicity that resulted was huge and anglers who had never fly fished before — among them me — converged on the new water. Demand outstripped supply.

Other water authorities saw what was happening and followed suit. Soon, the 3,100- acre Rutland Water opened close by. So did Draycote, near Rugby. More huge, new reservoirs were opened to trout fishing in Kent, Essex and Northumberland. As the resource expanded, so did the numbers wanting to enjoy it.

Most of the newcomers came from the ranks of coarse fishing and they included many of coarse fishing’s finest brains. They brought insight and free thinking to a branch of angling long burdened by tradition.

Although most developments concentrated on boat-fishing techniques, others gave attention elsewhere. Soon, the first nymph fishermen appeared on the banks — anglers skilled in imitating the kinds of bugs fish eat and presenting them on long, disturbance-free nylon leaders to fish in the shallows. They brought a delicacy back to fly fishing on lakes that some new techniques had removed.

On the back of all this, a second layer to the market was being created. In Romsey, Hampshire, Alex Behrendt, a former German prisoner of war who married locally and decided to settle there, was showing how smaller lakes and ponds could be custom- designed as trout fisheries and run at a profit. He shared his knowledge freely and even ran how-to-do-it courses.

Estate-owners, farmers and smallholders saw an opportunity. By the 1980s, small still-water trout fisheries were glinting in the landscape. These, too, led to tactical developments and, wherever the water was clear enough, techniques for stalking individual, sighted trout one-on-one came to the fore.

Inevitably, this maelstrom of activity led to excesses. The appearance of so much fishing eventually produced overcapacity and fierce competition at the smaller end of the market. Some of the gimmicks designed to attract anglers at that time dismayed thinking fly fishers everywhere.

Some lake owners stocked ever-bigger trout to attract publicity and gain an edge over their rivals. Before long, trout bigger than the wild fish in the old angling records were being put into ponds straight from the fish farm. They were going into the water in the morning and being caught by lunchtime. Record claims were in the post by early afternoon, undermining the integrity of the ancient record lists. Eventually, separate record lists had to be created — one was for wild fish and one for farmed fish.

Other waters were deliberately overstocked to make the fish easier to catch. A handful were stocked with salmon — a madness because the stomachs of Atlantic salmon atrophy in fresh water and the fish become impossible to catch.

But over time, things settled down and most of the gimmicky fisheries went to the wall. The waters that remain offer a quality of fishing that is still second to none. It is still varied, still reasonably priced and still easily reached from pretty well anywhere. A once minor branch of angling is now a multimillion-pound business catering for about 600,000 fly fishers. And Grafham Water can take credit for much of that.

• Brian Clarke’s angling column appears on the first Monday of each month

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