When the founding father of modern angling wrote: "Is it not an art to deceive a trout with an artificial flie?" he could never have imagined the cunning lure would be more populous than the living insect it pretends to be.
But 350 years ago, when Izaak Walton's The Compleat Angler was first published, environmental disaster was not something fishermen - or trout - worried much about.
Nowadays, Walton's acolytes have increasing grounds for concern that their "art" is slowly being destroyed by the disappearance of the very insects whose life they seek to impersonate at the end of a majestic arc of nylon line.
A comprehensive survey of river fly populations across England and Wales has found that falling numbers of the aquatic insects on which game fish such as trout, grayling and salmon feed, have gone into even steeper decline.
Figures released yesterday show that fly populations now stand at just 25 per cent of the levels before the Second World War - and have declined by a third compared with the population of just two years ago.
The findings are so alarming that the Environment Agency has set up a scientific team to try to explain the mystery alongside the first national study to plot the distribution of declining fly species.
Game fishing experts say the virtual disappearance of some species, such as the Iron Blue, is the result of environmental damage over the past decade that is endangering not just fish but the fly-fishing industry, which generates £1bn a year.
Paul Knight, director of the Salmon and Trout Association, which represents Britain's 800,000 fly-fishing enthusiasts, said: "This is a grave danger to our past time and anglers all over the country are trying to save fly-fishing as we've always known it. Anglers have always been the best defenders of rivers."
The association is working with the Environment Agency by asking its members to complete the first systematic survey of river fly numbers, which should be completed by early next year.
Each species of fly hatches according to the season, the water temperature and even the hour of the day and requires an angler to have an intimate knowledge of the river and its insect life to tailor his fly - carrying such names as Gold-Ribbed Hare's Ear or Orange Partridge - to the feeding habits of the target fish.
The fly is a hook dressed with feathers and hair to imitate the species of insect upon which fish may be feeding.
That, in the words of Walton, is what the fisherman must cast in hope and "let no part of your line touch the water, but your flie only".
Conservationists believe that a catalogue of environmental factors, ranging from climate change to pollution caused by changing farming practices, are ruining the natural habitat for the river flies and thus playing havoc with the diet of the fish and a fly- fisherman's ability to seek his prey.
A study by the Environment Agency based on data ending in 1999 found that some species, such as the Mayfly, were apparently resisting the worst declines by benefiting from the increased silting of riverbeds. Even so, Mayfly numbers have fallen by nearly 60 per cent compared with pre-war levels.
Other stock fly-fishing species, such as the Iron Blue, have suffered a catastrophic decline of up to 80 per cent, pinning back the population to strongholds in two of Britain's most famous fly-fishing rivers, the Test and the Itchen.
The 1999 study found general river numbers stood at just 34.2 per cent of that 60 years earlier but figures from a subsequent survey released to The Independent show that the number has plummeted to 25 per cent for 2000 and 2001.
Experts admit they are baffled by the insect loss, which is worst in the heavily managed chalk streams of the South of England favoured by the wealthy. Ian Johnson, national fisheries policy manager for the Environment Agency, said: "At this stage we don't have an answer - we're investigating all hypotheses. Fly-fishing wouldn't be able to continue if all the flies disappeared."
Current theories include the cycle of summer droughts and winter floods brought on by climate change wiping out the species of weed, such as water crowfoot or ranunculus, that play vital roles in the lifecycle of many species of fly. Sudden flooding may also be responsible for washingpesticide and other chemicals off farmland and into rivers before it can be broken down, forming a high concentration that kills off water weed and larvae.
Peter Hayes, a conservationist leading efforts to gather data on the problem, said: "Environmental factors are playing a role and we need further work to find out just how.
"In the short term, it is more difficult for fly-fishermen to catch wild fish because they have switched to other sources of food below the surface. But if things continue, then the fish population itself will suffer the same sort of decline."
Philip White, a river keeper for 20 years who now teaches fly-fishing, said: "There has been a decline in river fly everywhere. But you only have to look elsewhere to see this is part of a more general problem - where are all the beetles, bees and wasps? We are paying the price for modern life."Reuse content