Farmed salmon is having what is becoming known as its mad cow moment.

Just as BSE provoked a boycott of beef, now public confidence in the fish is being dented.

The reason? A study by American scientists claiming Scottish farmed salmon is so full of pollutant chemicals it should be eaten only sparingly.

Researchers advise only 8oz of farmed salmon bought in Edinburgh and London should be eaten per month and wild salmon consumed as often as eight times a month.

According to the study, consuming more than these amounts could boost the risk of cancer by at least one case in 100,000.

Researchers found levels of 14 toxins, including PCBs (polychlorinated biphenyls) and dioxins, were significantly higher in European and North American farm-raised salmon compared to fish caught in the wild.

The scientists, from six research centres in the United States and Canada, tracked the source of the pollutants to the fish meal fed to intensively-farmed salmon.

In response, the Food Standards Agency (FSA) pointed out the dioxins and PCBs were within safety levels set by the World Health Organisation and European Commission.

However, with 99 per cent of salmon sold in fishmongers and supermarkets coming from Scottish farms, just the mere whiff of controversy could be devastating.

The industry is likely to have been buoyed by initial evidence yesterday from supermarkets showing shoppers have so far not been deterred.

Asda reported sales up 20 per cent, Sainsbury's up nine per cent while only Marks & Spencer noticed a fall at the weekend.

Some in the fish industry admit there is a miniscule health risk which is outweighed by the benefits, such as salmon's high protein, vitamin and mineral content.

Omega-3 fatty acids found in fish such as salmon are also said to reduce the risk of heart attacks.

Family-run Springs Smoked Salmon in Edburton, near Henfield, uses wild and farm fish, importing wild Pacific salmon and frozen wild Scotch salmon.

With brick-built kilns, it uses salt to preserve and oak logs for smoking to give salmon the aromatic flavour that has won favour with celebrities like former Olympic athlete Sally Gunnell and chef Delia Smith.

Springs partner David Harris, 49, believes any impact on the industry would be short-lived.

He said: "I don't think there will be a long-term effect, at least I hope not.

"There is a lot of confusion among the public. Many people just don't know what to believe."

Jay Hunter, manager of Sussex Fish Sales on the Riverside Industrial Estate in Littlehampton, said the firm normally sold ten cases of whole salmon - the equivalent of 70 to 90 fish - each week.

However, in the wake of the cancer scare stories, both fresh and smoked salmon sales had plummeted by 25 per cent.

Mr Hunter said: "About80 per cent of people who have come here have mentioned the study so it has obviously had an impact.

"But we are finding most people are still buying."

Freia Sayers, owner of Fish at the Square in St George's Road, Kemp Town, Brighton, said she had noticed an increase in sales of her organically-farmed salmon.

She said: "I think if we can address the problem of the way fish are kept, it's a good thing. "

Dr Elizabeth Hill, senior lecturer in ecology at Sussex University, said: "Anything that adds to our consumption of PCBs and toxins is problematic.

"Some of these particular compounds have been implicated in causing certain cancers and in abnormalities in reproductive health.

"If the compounds are below the specified levels then that's the most important thing but we must reduce our levels of these compounds."