It is not quite a return to the days when dolphins and even whales were seen making their way upstream to Paris, but the Seine is welcoming back at least one long-lost visitor: the Atlantic salmon.

A fish that in the Middle Ages abounded in the river is returning in healthy numbers after efforts to clean the polluted waters.

“There are more and more fish swimming up the Seine. This year the numbers have exceeded anything we could have imagined,” said Bernard Breton, the secretary-general of the National Federation for Fishing. “I would not be surprised if we had passed the 1,000 mark.”

It may be a while, however, before le pavé de saumon Notre Dame becomes a local delicacy. Salmon may be allowed to frolic in the Seine but levels of pesticide, lead and bacteria remain too high for people to risk it.

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Even overheated patrons of Paris Plage, the summer beach on the Right Bank, are warned to stay clear of the waters in which Louis XIV used to take his mistress for moonlight dips.

Those who ignore the rules are taken to hospital for checks once they have been hauled out by police.

Not so long ago, the Seine was connected to the famed sewers that the writer Victor Hugo called “a world of slime without human form”.

Efforts have been made in the past 30 years to end the fouling of the Seine with organic pollution and chemical run-off from industry and agriculture. Most species of fish had disappeared from the city’s waters by the 1920s.

Today the surface is cleaned regularly and there is a system that pumps oxygen into fish-friendly stretches when floodwaters run into the river.

Bertrand Delanoë, the Mayor of Paris, called the return of the Atlantic salmon a triumph for the Seine purification effort after the INRA, the leading state research institute, issued its study on the fish.

A statement on the mayor’s website said: “Without any project to reintroduce them we see that several species of migratory fish, including salmon, have come back up the Seine. This is a sign of a clear improvement in the quality of water in the river.”

The gradual return of about 30 species of fish — the Atlantic salmon made a tentative appearance a few years ago — has encouraged an urban angling boom. At weekends the Île Saint Louis, opposite Notre Dame, and the Left Bank by the Eiffel Tower are thick with anglers casting for bream, carp, pike, perch, catfish and sea trout.

“They do not eat them,” Stephanie Hofer, of the fishing federation, told The Times. “They throw them back — but that’s what 80 per cent of the anglers in France do.”

Central Paris hosted the world angling championship in 2001. No salmon were caught and few have since been hooked by the registered 7,000 fishermen in Paris.

The return of the salmon was spectacularly illustrated last October when a 15½lb (7kg) fish was caught in Suresnes, on the western downstream edge of the capital.

From genetic samples and age measurements, the INRA reported that all the Atlantic salmon were found to have swum up, or back up, the Seine via the estuary to the Channel in Le Havre after months or years in the sea. Some were born in French rivers, others farther afield.

The researchers said that the return was significant because the Atlantic salmon were bioindicators — creatures whose choice of habitat indicate a healthy environment.

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