THERE can be very few anglers — indeed, there can be very few folk of any persuasion — who have not been aware of the heroic attempt to bring salmon back to the Thames.

This project, set up 27 years ago, was not conceived for anglers but as a means of demonstrating that the capital’s river, for a century and a half a running sewer, was more or less clean again. What better way to demonstrate it, the water authorities, the tourist industry and London’s city burghers decided, than to have salmon leaping and forging past Westminster and salmon anglers coming from all over Britain to catch them?

Their plan was to put immature fish from elsewhere into the Thames, have these fish go to sea the way all salmon do and then return as big fish ready to spawn naturally, kick-starting a self-sustaining stock.

Alas, the news is not good. Last year turned out to be the first since the Thames Salmon Rehabilitation Scheme was launched in 1979 when not a single fish was counted in from the sea. Just as disappointing, no salmon, not even in the heady days of the 1980s and 1990s, when sometimes hundreds of fish a year were returning and anglers were catching them regularly, has been known for certain to spawn in the river. Every returning fish trapped so far has been one previously stocked. Now, 27 years and £6 million later, the project is not dead, but its future is in the balance.

“It’s been an incredibly disappointing year,” Darryl Clifton-Dey, who runs the project for the Environment Agency (EA), said. “There is still time for a turnaround. In 2003 we thought we were getting there. We tracked fish right back to the River Kennet, a tributary of the Thames where we had put them in as juveniles. But we don’t know if they actually did the business.”

If they did spawn, Clifton-Dey said, their progeny should be in from the sea this year. “But if they didn’t, and if we haven’t had a successful spawning by April 2009, when the five-year programme ends, we’ll have to look again. We’ll have to determine whether there’s a future for salmon in the Thames.”

In medieval times, tens of thousands of salmon ran the river every year, but the growth of the capital and mounting pollution steadily choked them off. No salmon had been seen in the Thames for 140 years when, on November 13, 1974 a single stray salmon from another river was found dead. After that, the odd stray salmon was seen every year. Five years on, the Thames Water Authority (TWA) and others came up with the restocking plan.

The hope was that the young fish put in would imprint on the river, go out to the feeding grounds off the Faeroe Isles or Greenland and come back big, vibrant, silver and sexually mature a year or so later. They would hopefully spawn in the river, their progeny would repeat the cycle, the artificial stocking could be phased out — and bingo.

Tens of thousands of juvenile salmon have been released every year since. The Thames Salmon Trust, a charity part resourced and funded by anglers, embarked on a programme that was to result in 39 fish passes being built into the weirs and other obstacles between the lower Thames, where spawning potential was not good, and the middle reaches of the Kennet.

The numbers of returning fish increased. The TWA offered a prize for the first salmon angler to land an authenticated fish on rod and line — and on August 23, 1983, Russell Doig, a Londoner, obliged with a six-pounder. The publicity picture of Doig and his fish, cannily taken with Tower Bridge in the background, even though the fish was caught upstream at Chertsey, proved a winner. It put the project on the map. In 1993, 338 fish were counted in — an all-time high. Three years later, anglers took 34 fish in the season — another high.

But after that, returns collapsed abruptly. No fish has fallen to an angler since 1999. The project faded from public sight.

Potential reasons are legion. Up to 1994, young fish were released into the lower Thames and that is where the returning adults were counted. From 1995, the fish were released over potential breeding grounds far upstream in the Kennet and to get back there, the fish had to negotiate 70 more miles of river hazards.

Water quality in the Kennet has plummeted in recent years. The Kennet and the Thames have been hit by a series of droughts since the mid-1990s. Abstraction for housing and industry has soared, reducing their flows still further.

Long, hot summers have warmed remaining water, lowering the oxygen content at the very time the salmon, which need lots of oxygen, were trying to get upstream. A series of freak summer storms has flushed sewage into the water.

No one can point the finger for certain at any one cause — not even the storm pollution of August 2004 that killed fish of many kinds downstream from the capital. But as returns dwindle, so does hope — and with it the justification for pouring large sums into a dream that maybe came before its time.

Come 2009, Londoners may have to look elsewhere for their environmental credentials. Salmon anglers will be doing likewise for their sport.

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