Fish have been seen trying to jump the weir at Warrington, where the tidal section of the Mersey ends, raising the prospect that they may start breeding again in waters from which they have been absent for over a century. Coarse fish such as roach, perch, bream, chub and pike are returning in large numbers to the once-lifeless middle section of the river, encourged by a massive clean-up of the whole Mersey basin that began in 1985.
Increasing numbers of sea fish species - 42 so far - aresteadily returning to the estuary. But the salmon's comeback as a breeding fish would be the most remarkable revival of the waterway, which has long been a byword for pollution.
A series of sightings has convinced Environment Agen-cy staff the fish are already back in the lower, tidal part of the river, below the smaller tributaries they would use as spawning streams. They have been seen trying to get over Howley weir in Warrington, the first big obstacle for migratory fish swimming upstream from the sea, and there have been other sightings of them "porpoising" in the river.
"We all think it's highly likely that they have returned," said Bernie Chappel, the agency's area fisheries manager. "It's the consensus of the fisheries staff. We've had so many reports now that we're sure something has been happening."
The agency has already picked the River Bollin, which joins the Mersey at Lymm, above Warrington, as potentially the first tributary that might allow salmon to spawn. The fish need fast-flowing, clear water and gravel bottoms in which they make "redds", beds used for laying and covering their eggs. The Bollin, which flows through Cheshire after rising high in the Pennines above Macclesfield, has been surveyed for its salmon potential and would be perfectly suitable, Mr Chappel said.
Were it not for its gross pollution, the Mersey would be an ideal salmon river, with its high volume of water and many tributaries, he added. It sits between two other large rivers renowned for salmon - the Ribble and the Dee. The estuary of the Dee, one of Britain's premier salmon rivers, is a meresix miles from the Mersey estuary on the other side of the Wirral peninsula.
Historically the Mersey was a good salmon river but the fish disappeared with the industrial revolution of the 19th century. And the pollution from industry and sewage became so terrible that twenty years ago more than 80 per cent of the entire river basin system, meaning the main river and all its tributaries - about 1,200 miles of waterway - was unable to support fish life.
It was believed to be the most polluted river system in all of Europe. The clean-up campaign was prompted by Michael Heseltine, appointed minister for Merseyside by Margaret Thatcher after the riots of 1981 and who described the state of the river as "an affront to a civilised society".
The Mersey Basin Campaign, begun in 1985, has now presided over a startling change in the river's water quality, with remarkable declines in both industrial and sewage pollution. In the past five years, for example, the 200 industrial discharges at the bottom of the Irwell, the tributary that flows through Manchester and Salford, have been reduced to six. The amount of mercury, one of the most harmful of heavy metal pollutants, flowing down the river, has gone from 300 tons a year in 1985 to less than one ton now. And a pounds 500m improvement programme by North-West Water means the 28 outlets that used to discharge Liverpool's raw sewage into the river have been closed and the city's sewage is all taken to one of the world's biggest treatment plants, at Bootle.
Because of those changes and others, said Tony Jones, the chief executive of the Mersey Basin Trust, between 60 and 70 per cent of the river system can now support fish life and by 2001 that will rise to more than 80 per cent. "But salmon are partic-ularly sensitive, and it would be a great achievement if they were to reappear," Mr Jones said. "We think they could be back soon, if they're not already."
There is a precedent for a grossly polluted English river getting its salmon back - the Thames. In the 19th century the fish were killed off, with the last one being caught in 1833. But a clean-up similar to the Mersey's, begun in the Sixties, prompted a return, and the first new fish was caught at West Thurrock power station, Essex, in November 1974.
Since then a reintroduction programme has brought a steady stream of salmon back - about 160 a year - and after a programme of fish passes on weirs is completed next year, it is hoped that salmon will once again spawn in a Thames tributary, the Kennet, and run the river as they once did.