Salmon farmers are trying to reduce toxic contamination
In a small pilot study, Canadian researchers using a comparison with wild Pacific Salmon found farmed fish contained significantly higher levels of polychlorinated biphenyls (PCBs).
The concerns were raised in a programme made for BBC TV, Warnings from the Wild, The Price of Salmon.
The production of PCBs is banned in most countries - but the chemicals accumulate in oceans after being released in industrial waste.
In high levels the chemicals are thought to affect human nervous, immune and reproductive systems.
Nutritional value
Scientists traced the contamination back to the feed that includes fish meal and oil which come from wild fish trawled from the world's oceans in vast quantities by industrial fleets.
Concentrating the nutritional value of these fish into pellets to produce a high-protein diet for farmed salmon multiplies the minute traces of toxins present in each individual fish to a more significant level.
Once ingested, PCBs build up in body fat and take years to break down.
The programme makers say they recognised that oily fish, which includes farmed salmon, is recommended as a food with positive health benefits, and that one portion a week would be regarded as good for your health.
The documentary aimed to raise a concern that there are no clear guidelines for those who regularly consume more than this.
The programme reported that Dr Michael Easton carried out a pilot study for Canada's David Suzuki Foundation.
He analysed four types of fish feed, four farm salmon from different sources, and four wild Pacific Salmon.
The BBC documentary said that although the sample was small, "it seems to confirm that contamination in farm fish comes from the feed."
Dr Easton said: "The results were very, very clear. Farmed fish and the feed they were fed appeared to have a much higher level of contamination with respect to PCBs, organo-chlorine pesticides and polybrominated diphenyl ethers than did wild fish - in fact it was extremely noticeable the difference."
Toxic pollutants
Greenpeace scientist Dr Paul Johnston said: "Ultimately all these practices lead to products that are consumed by human beings.
"We are maximising human exposure to these chemicals by promoting an artificial food chain."
PCBs are among the most toxic and persistent pollutants in existence.
They are also thought to be among the chemicals responsible for so-called "gender bending" effects because they mimic the female sex hormone oestrogen.
Studies indicate the chemicals can cause cancer, decreased sperm counts, deformed genitals and sterility.
The World Health Organisation is sufficiently concerned about the potential consequences to have cut its guidelines on the recommended intake of dioxins.
Limits cut
But UK Government food watchdog, the Food Standards Agency, has not followed this lead.
And food safety expert Professor Hugh Pennington told the BBC: "Salmon is an extremely good food, and some studies show it can help prevent heart attacks.
"But if there are small amounts of chemicals then one must counsel moderation."
He said that one portion of salmon a week was unlikely to cause any harm, even to young people.
However, he added: "We should be monitoring the salmon perhaps more often and we should be making sure that what goes into the fish doesn't contain these chemicals, then there isn't a problem".
A spokesman for the Fishmeal and Oil Manufacturers Association said it was aware of chemical concentration in feed.
Salmon farmers were looking at ways of reprocessing fishmeal to reduce toxic contamination, he added.
Health benefits
The farmed salmon industry has also pointed to the health benefits of eating the fish.
Scientist Dr Hole Reid said: "It is a well documented, safe and healthy product.
"The special thing with Atlantic salmon is basically that it is known by the consumer ... and I think there is more and more concern today about human health.
"From that perspective, the Atlantic salmon contributed to improve the health status of the consumer.
"You can look at cardiovascular diseases - it is proven to have an effect on that, it has a reduction on cholesterol levels in the blood and it in general strengthens the immune system."
In an earlier version of this story, it was stated that the World Health Organisation was "sufficiently concerned about the potential consequences of dioxins/PCBs to have cut its guidelines on the recommended intake of salmon to just one tenth of the previous figure". We are happy to make it clear that although the WHO did reduce the level of its acceptable daily intake of these substances in 1998, it has never made a recommendation about salmon, or singled it out in any way.