British efforts to restore salmon to Thames River are going swimmingly

November 21, 1993|By New York Times News Service

LONDON -- Scientists are a step closer to nurturing a self-sustaining strain of salmon in the Thames River, 160 years after the indigenous species were wiped out by pollution and impassable dams.

The scientists' efforts, a mixture of engineering and breeding, passed a crucial test this year when the first group of adult salmon of Thames parentage returned, swimming upstream past Big Ben and Parliament to complete their life cycle after feeding at sea.

Matching fish with river

While dams and altered river beds still prevent these second-generation fish from reaching spawning grounds in the Thames' tributaries, the returnees are the keystone in attempts to breed a strain of salmon whose genetic traits match the river's altered chemistry and terrain.

"These are very important fish to us," said Greg Armstrong, a fisheries scientist at the National Rivers Authority. "It's certainly an encouraging sign that our efforts are working."

Mr. Armstrong said that more than 300 salmon, including 30 bred from earlier Thames returnees, have returned so far this year. Fish counted at an upstream trap are identified by tags or the absence of the adipose fin, a small fleshy lump near the tail removed to mark stocks. Since not all returnees are caught, Mr. Armstrong said, the count is conservative.

Salmon face great pressure to survive, depending as they do on two fragile environments for the completion of their complex life cycle. They spawn and spend their youth in fresh water, then migrate to sea. Later they return to areas in which they hatched to spawn and die.

Vanished by 1834

North Atlantic salmon have dwindled in this century because of overfishing, the depletion of species on which salmon depend and the destruction of salmon habitats in dozens of rivers in Europe and North America. Two hundred years ago the Thames teemed with salmon. By 1834, none remained.

Salmon require clean, well-oxygenated water and easy access to shallow gravel breeding grounds.

After the Industrial Revolution and the proliferation of indoor toilets turned the Thames into a sludge-filled sewer, the estuary remained uninhabitable to all species but eels until the 1960s, when the government made a concerted effort to improve water quality.

More than 100 species of fish now live in the Thames, but `D man-made changes to the river have prevented the natural return of salmon. The deepening of the river for boat traffic destroyed shallow breeding areas, and the construction of locks and dams created insurmountable barriers to upstream migration.

The National Rivers Authority stocks the Thames with 200,000 young salmon each spring. As each generation of adults returns from the sea to spawn, scientists catch the most robust ones, perpetuating the genetic strains most compatible with the river and, in effect, breeding a new indigenous strain.

"It's natural selection with human help," Mr. Armstrong said.

About a third of the fish stocked in the Thames are bred from returnees; the rest come from other rivers and hatcheries in Britain. By 2000, scientists hope to stock only progeny from Thames-born returnees and to find some successful reproduction occurring without human help.

Scientists started a small salmon stocking program in the Thames in the late 1970s, encouraged by the return of other species and the surprise finding of a stray salmon caught in a power station screen. It was the first salmon seen in the Thames in 140 years.

As adult stocked salmon started trickling back to spawn, a trust was formed to raise money to build ladders and fish passes at dams so salmon could get to breeding grounds. So far, 14 passes have been completed on the Thames. Three more are to open by the end of the year, and another five by March 1995. Others are being installed in tributaries as maintenance on dams is completed.

Once the chain of ladders is completed, the returning Atlantic salmon will be able to swim upstream unimpeded, make their way to the tributaries in which they were stocked and then spawn naturally.

Only when this occurs can the project be deemed a success. Currently, the returning salmon are netted at the passes and their eggs are removed for artificial breeding.

"We hope by end of the century a good percentage of the returning fish will spawn successfully on their own," Mr. Armstrong said. "A self-sustaining population should rapidly grow from there."

Dr. Geoffrey Tingley, a fisheries scientist at Imperial College in London, said that constructing passes is only half the battle. The quality of gravel beds in tributaries must be improved because in many areas, a deep blanket of silt now covers natural gravel beds.

"I can see no reason why the Thames can't become a salmon river again," Dr. Tingley said, "as long as salmon have easy access upstream to good spawning areas, and as long as the river downstream in the estuary remains clean."

If so, Britain's success will be counted as a milestone.

Bad luck in the States

In the United States, three decades of efforts to restore salmon to rivers in the Northeast have not yet produced the hoped-for success, said Larry Stolte, a coordinator with the Fish and Wildlife Service in New Hampshire. Only an effort on the Penobscot River in Maine has produced a handful of salmon that continue to spawn in the wild.

In American rivers, hydroelectric dams as high as 80 feet present returning salmon with a Herculean challenge. Mr. Stolte said that because most dams on the Thames are not more than 9 feet high, it might be easier to make the river completely passable for salmon again.

He said that efforts to breed the salmon that return offer the best chance for success, but, he added, "We've learned that it takes a lot less time to destroy a wild resource than to bring it back."

Mr. Armstrong said, "Restoring salmon is not just a grand conservation exercise."

In the long run, the survival of the whole struggling stock of North Atlantic salmon is at stake.

Baltimore Sun Articles
|
|
|
Please note the green-lined linked article text has been applied commercially without any involvement from our newsroom editors, reporters or any other editorial staff.