Published last week in the journal Science and claiming to be 'the most comprehensive analysis to date of salmon toxin concentrations,' the study has set the cat among the pigeons with environmental campaigners, organic associations and government organisations staking their positions.
The production of farmed salmon has increased 40-fold over the last two decades, thanks in large part to the world's salmon farms. Over half the salmon sold globally are raised in Northern Europe, Chile and North America.
After a study of more than two metric tons of North American, South American and European salmon the researchers found that PCBs (polychlorinated biphenyls) and other environmental toxins are present at higher levels in farm-raised salmon than in their wild counterparts. The UK TDI (Tolerable Daily Intake) for dioxins anddioxin-like PCBs is 2 pg WHO-TEQ/kg bodyweight/day.
Quick to respond, Sir John Krebs, chairman of the UK government's independent food watchdog - the Food Standards Agency - stressed that the levels found were safe.
'This study shows that the levels of dioxins and PCBs in salmon are within internationally recognised safety limits and confirms previous studies by the FSA,' he said in a statement. They are in line with safety levels set by the World Health Organisation and the European Commission, added the chairman.
Others may disagree. ' The FSA should be trying to encourage British farmed salmon to be as healthy as possible. They should recognise this point, rather than accept the status quo,' Mary Taylor, a spokesperson for the environmental campaigners Friends of the Earth told FoodNavigator.com.
The group is calling for stricter controls and an improved food labelling regime for fish. Their call echoes the study's authors who note that findings demonstrate the importance of labelling salmon as farmed and identifying the country of origin.
The researchers at Indiana university, together with five other research centres, say that increased toxin levels in farm-raised salmon may pose health risks to people who eat the economically important fish.
Dioxins and PCBs are implicated in causing cancers, hormone disruption, developmental and neurological problems.
The term 'dioxins' refers to two groups of closely related compounds - 75 polychlorinated dibenzo-p-dioxins (PCDDs) and 135 polychlorinated dibenzofurans (PCDFs). Dioxins have developmental effects on young children, and are believed to disrupt the endocrine systems in humans and wildlife.
PCBs are another group of closely related organic chemicals. A few exhibit toxicity similar to those of the toxic dioxins and are therefore described as being 'dioxin-like'. They tend to bioaccumulate and aregenerally present at low concentrations in most foods, especially fat-containing foods such as milk, meat, fish and eggs.
"We think it's important for people who eat salmon to know that farmed salmon have higher levels of toxins than wild salmon from the open ocean," said IU school of public and environmental affairs professor Ronald Hites, who led the study.
Selecting farm salmon from supermarkets all over the world and wild from the waters of North America to make a total of 700 samples, the scientists used only PCBs, dioxins, dieldrin and toxaphene to calculate consumption safety guidelines, deeming these four toxins to most strongly impact human health.
Breaking it down by region, the researchers found levels of all 14 toxins were significantly elevated in both European and North American farm-raised salmon when compared with wild Pacific salmon. Levels of only 6 toxins were significantly elevated in South American farm-raised salmon.
Europe seems to have fared particularly badly, with the study finding toxin levels in European farm-raised salmon were significantly higher than in North American or South American equivalents.
'Levels of PCBs, dioxins, toxaphene and dieldrin were highest in farmed salmon from Scotland and the Faroe Islands (Denmark) and lowest in farmed salmon from Chile and Washington state,' pointed out the authors of the paper. Although Hites added that even these comparatively uncontaminated South American salmon had high levels of other toxins.
In reverse, levels of two toxins (HCB and lindane) were actually significantly lower in farm-raised South American salmon than in wild salmon species.
Seeing a window of opportunity, the UK's organic body, the Soil Association, encouraged consumers this week to buy organic salmon.
'Organically farmed fish are fed with the by-products (trimmings, et al) of fish caught for human consumption, not industrial fishmeal. The permitted oil content (where the majority of PCBs and dioxins are found) is less than that fed to intensively farmed fish,' said the group.
Francis Blake, standards director at the organic body added :"Chemical contaminants are found in the environment because of pollution from industry and intensive agriculture. By buying organic, including organic farmed fish, consumers are supporting better systems of production that aim to cut out such sources of pollution."