LONDON -- British fishery scientists are trying to create a strain of salmon tough enough to survive in the Thames River.
They say they're succeeding.
Salmon have to be downright streetwise to live in the urban Thames. In London, the river is brown, busy, slow-moving and flecked with debris.
Still, it is considered "clean," with about 60 percent of London's tap water coming from the river -- after treatment.
Salmon and about 100 other species of fish already live in the Thames. The river is restocked every year with about 180,000 young salmon. But so far nearly all of the salmon in the river have been spawned elsewhere, often in Scotland.
The National Rivers Authority and the Thames Salmon Trust are now trying to renew an indigenous strain.
Before the 1820s, Thames salmon thrived.
A fish caught in London in 1789 weighed 70 pounds, and in 1810, more than 3,000 Thames salmon were brought to market.
But river locks, installed to facilitate barge traffic early in the 19th century, cut salmon off from their spawning beds. And effluents from industrialization and urbanization polluted the river.
Thomas Crapper had invented the water closet in 1810, and Londoners flushed raw sewage into the Thames, to join wastes from slaughterhouses, coal wharves, breweries and fish markets.
In London, it was the time of "The Great Stink."
The pollution removed oxygen from the water. Salmon literally drowned.
By 1920, no fish at all could survive in 30 miles of the river from London to Tilbury near the mouth of the Thames.
Massive rehabilitation of the Thames began in the 1960s with strict pollution controls and construction of huge sewage treatment works.
Conservationists say the results were rapid and dramatic.
And in 1974, one lonely stray female salmon was discovered stranded but alive on the intake screens of a power plant, the first salmon in the Thames for 140 years.
She was enough to make fishery scientists think about renewing the salmon run.
Greg Armstrong, a fisheries manager for the National Rivers Authority, says the first step was to restock the river to see if salmon really would come back.
The Thames salmon is essentially an Atlantic salmon "imprinted" with local characteristics -- or will be if the breeding program is successful.
The salmon spawns in gravel in shallow water, spends about two years in fresh water as a "parr," then heads for the open sea as a "smolt."
The smolt grows to an adult salmon in northern waters as far off as Greenland and Iceland. They return after one, two or three years to spawn in the fresh water of the river to which they have been imprinted.
"In 1982," Mr. Armstrong says, "we had the first significant return from stocking, 128 salmon confirmed by ourselves."
The biggest year so far has been 1988, when 323 salmon were recovered in a fish trap; 283 came back last year, 180 so far this year. Mr. Armstrong thinks that actual return might be 20 per cent higher.
Satisfied that the fish would come back to the Thames, the Salmon Trust began a program to build 25 fish passes around locks and weirs to allow salmon to reach favorable spawning grounds. So far, 14 have been built.
"The third phase," says Mr. Armstrong, "is the attempt to create a strong Thames River salmon strain adapted to local conditions."
They're helping natural selection along.
"The fittest ones come back," Mr. Armstrong says. The return rate is about 1 percent of the 60 thousand to 80 thousand smolt stocked.
The fishery managers pull these adults out to be bred in controlled situations. Parr, or young salmon, from fish that returned in 1991 were returned to the Thames this year.
"The run has to rely on us to help it along in spawning," Mr. Armstrong says.
They want to restore a self-sustaining population of salmon genetically adapted to the Thames. They envision a run of a thousand salmon-- with lots of urban river smarts.
"It takes time," says Mr. Armstrong. "It takes a great many years to get right what we only took a few years to destroy."