Nature Studies by Michael McCarthy: Of all our conservation failures, this is the saddest

It's a hoary old cliché, the dream that died, but perhaps we may be allowed to write the dream that is dying: for such is the situation facing anyone who has supported the noble aim of restoring salmon, the finest of all freshwater fish, to the River Thames.

After more than 30 years of trying, and a colossal amount of thinking, effort, and expense, the current custodians of the dream, the Environment Agency, has more or less accepted that it is not going to be realised, and this year's stocking of young fish in the river – in the hope that they might go down to the sea and then return as adults, to breed – will be the last.

The salmon have not come back. A native breeding population has not been established, and only a tiny handful of adult fish are now turning up in the river each summer. The project, which many people set their hearts on, has effectively failed.

Hard to credit, really, when we think of other conservation successes: in Britain we've been able to bring back a number of very special organisms from extinction or its brink – birds like the sea eagle, flowers like the lady's slipper orchid, butterflies like the large blue – but Salmo salar in the Thames has defeated us.

And it was such a prize, such an exhilarating vision. This legendary, righteous fish, this icon of aquatic purity – with its need for a high dissolved oxygen content in all the fresh water it passes through – was to be the supreme symbol of the Thames reborn, of a great watercourse brought back to life. For as recently as 1957, a survey showed that the London section of England's longest river held no viable fish populations at all between Kew in the west and Gravesend in the east, as it was a filthy, stinking, dead ditch.

The salmon themselves, which once silvered their way up the river in substantial numbers, were long gone, driven to extinction in a mere 25 years at the start of the 19th century, as a result of the pollution which surged as the capital entered the industrial age; a fish caught in June 1833 is believed to have been the last Thames salmon of all.

Bringing them back only became a possibility 141 years later, when, to general amazement, an 8lb 12oz salmon was caught in the intake screens of West Thurrock power station, near Dartford, on 12 November 1974 – 10 years after a major cleansing of the river began.

It was obvious that the clean-up was working, and the effort to restore the Thames salmon began formally in 1979, when smolts – juvenile fish six inches long – were released in the tidal section of the river, to see if they could pass down to the sea through the once impossibly polluted estuary, and successfully return. They did, in eye-catching numbers, and made headlines.

But the real challenge was to get them to swim a further 75 miles all the way up to potential spawning streams such as the River Kennet in Berkshire, and for this to happen, fish passes had to be built in every one of the 37 weirs along the way. In a remarkable and largely unsung achievement, this was done, over a period of 15 years between 1986 and 2001, and at a cost of several million pounds, most of which was privately raised.

So for the past decade the way has been open for Thames salmon; but they have not taken it. The water is clean enough now, but no fish, as far as is known, have swum down to the sea after being stocked in the intended spawning grounds (a lovely section of the Kennet known as the Wilderness Water) and then swum all the way back up, and bred.

Although that is not quite true: a single fish, just the one, is known at least to have made the journey from the Wilderness to the sea, and then all the way back. It has a name, or, at least, a number: 00476. It was caught in the salmon trap at Sunbury weir on 14 July 2003, as a fish newly returned from the sea, and fitted with a radio tag and given its number; and its signal was picked up in the Wilderness on the following 28 November, thus proving that the arduous Thames spawning journey is possible. Even if it hasn't happened.

The man who picked up the signal of 00476 was Darryl Clifton-Dey, the Environment Agency biologist who has been running the Thames salmon project for the last decade. Having followed his work for all of that time I regard him as a hero, as indeed I regard the Environment Agency, which gets its fair share of stick, as pretty heroic in the way it has persevered; but both Darryl and the agency ruefully recognise – even if they don't know why – that the great fish are not going to return, or at least, not yet. "The decision [to stop releasing juvenile fish] saddens me," Darryl told me, "because I feel the burden of all the other people who have worked on the salmon scheme and believed in it since 1979, and because I have a passionate and romantic feeling for the project and a fundamentalist belief that it could work, if I could just identify that X factor that is stopping it at the moment. But in my few rational moments I know that the correct decision was made."

I'm sad too, but I don't blame them for stopping. The episode teaches us something profound: when we foul up the natural world, there are sometimes limits to what we can do to repair it.

m.mccarthy@independent.co.uk