Plan is hatched to fill Thames with salmon

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It's almost like preparing the nursery for baby. Except this is for thousands of babies.

An isolated spot on a Berkshire stream has been chosen as the epicentre of Britain's most ambitious species restoration project - the return of the Thames salmon.

London's river was once packed with salmon, but the pollution of the Industrial Revolution together with the building of weirs, to make the river deeper for boats, saw them off more than 170 years ago.

The dream now is to have them spawning again in this Thames tributary, the river Kennet, which is one of southern England's classic chalk streams, famous for its trout fishing.

The young salmon born in the river would go down to the sea - passing right through central London - and come back after one or two years to spawn for themselves, thus giving the Thames a self-sustaining salmon population for the first time since 1833, when the last native-bred fish was caught in the river.

One particularly wild and lovely stretch of the Kennet, between Newbury and Hungerford, is considered the ideal salmon spawning habitat in the whole Thames region, because of its gravelly bottom. Hen (or female) salmon need the gravel of a stream bed, where they dig out their spawning areas, or redds, laying their eggs in among the stones.

Last week the Environment Agency began an experiment to prove that the gravels of the Kennet can indeed prove a suitable nursery area for the salmon.

About 20,000 salmon eggs were fertilised on its banks with milt from male salmon - both brought from fish caught in the river Tyne in Northumberland. The bright orange, quite large eggs (familiar to anyone who eats sushi) were then placed, 50 at a time, in small wire mesh baskets which were buried in artificial redds, created in the gravels of the river bed with high-pressure hoses. (A cast-iron hollow dibbler hit with a sledgehammer is the way it's done.)

Some of the baskets will be opened at successive intervals to see how well the eggs are progressing towards hatching. The danger being watched out for is that they may be choked by the increased silt in the water that has affected the Kennet in recent years, and die from lack of oxygen. But the Environment Agency scientist in charge of the project, Darryl Clifton-Dey, says earlier analysis of the gravel makes him expect a reasonable survival rate, and that the eggs will hatch. "I'm pretty confident, but then I'm an optimist," he said.

He needs to be, as he has seen a number of setbacks in recent years in the Thames Salmon Rehabilitation Scheme (to give it its proper name) which he has been running since 1998.

While reintroduction projects for several British species have proved spectacularly successful, such as those for the red kite, the sea eagle, the ladies' slipper orchid and the large blue butterfly - all once extinct or extremely rare, and now soundly re-established - the attempt to bring salmon back to the Thames has hit several obstacles.

It has been in progress since 1979, when young fish were first stocked in the tidal section of the river, and caught the public's imagination by returning from the sea in substantial numbers, thus proving that the lower Thames was much cleaner than people realised.

A key part of the project has been the building of 37 fish passes in all the weirs between the tideway and the Upper Kennet spawning grounds, which took from 1986 to 2001, and cost more than £3m. But in recent years fish have struggled to return to the Thames because of insufficient water flows and renewed pollution from sewage pouring into the river when old sewers are overwhelmed by storms.

However, the agency is not calling it quits, partly because salmon are a unique indicator species for the health of the river.

"There's no other species that starts in the headwaters, goes all the way out to sea and requires good quality water along its entire migration," Mr Clifton-Dey said. "If you didn't have salmon, what would you have? No other species of mammal, bird or insect gives you such a good indication of water quality. If we can get a river like the Thames clean enough for salmon, we can do it anywhere."

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