|Hites, RA, JA Foran, SJ Schwager, B Knuth, MC Hamilton and DO Carpenter. 2004. Global Assessment of Polybrominated Diphenyl Ethers in Farmed and Wild Salmon. Environmental Science and Technology, in press.
Background on PBDEs
Analysis of approximately 2 tons of salmon from around the world finds that farmed salmon have significantly greater levels of brominated flame retardants than wild salmon, with European farm-raised salmon the highest. Among wild salmon, Chinook had the highest levels. In fact, wild Chinook from British Columbia averaged higher than farmed salmon.
This study is a follow-up to work earlier in 2004, in which Hites et al. reported that farmed salmon have higher levels of dioxin and PCBs than wild salmon. For this new report, they measured tissue from the same fish samples collected in the prior study.
Hites et al. point out that current levels of PBDEs in humans may so high as "to leave little or no margin of safety." They recommend selective consumption of salmon containing relatively lower concentrations of PBDEs, which as their earlier work showed, also contain lower levels of many chlorinated organic contaminants.
What did they do?
Hites et al. measured polybrominated diphenyl ether levels approximately 700 salmon obtained from 8 major salmon-producing regions Europe, North and South America.
The farmed Atlantic salmon came from wholesale suppliers in distributing salmon from British Columbia, the state of Washington, Maine, Eastern Canada, the Faroe Islands, Scotland, Norway and Chile. Wild salmon (135 individuals) of five species were obtained from three regions along the US and Canadian Pacific coast. No wild Atlantic salmon were measured because it is rarely available in commercial fish markets, nor were farmed Pacific salmon studied because they are not raised commercially in substantial amounts.
In addition to measuring contaminants in 594 whole salmon obtained from fish wholesalers, 144 filets of farmed salmon were obtained from supermarkets in 16 major cities in North America and Europe. At the supermarkets, purchasers asked specifically for farmed salmon.
Samples of food fed to farmed salmon were also obtained for analysis from European, North and South American outlets of salmon food used by the salmon farming industry, and analyzed for contamination.
Analyses were carried out using a high resolution mass spectrometer equipped with a gas chromatograph.
What did they find?
Farmed salmon averaged much higher than wild salmon (figure to right; note the log scale).
European (EU) farmed salmon had significantly higher levels than North American (NA), which in turn were significantly higher than Chilean. Wild salmon averaged significantly lower than Chilean-farmed salmon.
Store-bought farmed salmon also averaged higher than wild salmon.
||A closer look at the wild salmon samples revealed, however, that wild Chinook salmon had PBDE levels comparable to farmed salmon. The three Chinook collected from BC, in fact, had the highest average of all fish measured. Oregon wild Chinook carried PBDE in the middle of the levels measured, while Alaskan Chinook had the highest concentrations of all wild Alaskan salmon measured.
Hites et al. found elevated levels of PBDE in the salmon feed, similar to levels found in the farmed salmon themselves.
What does it mean? No public health agency has established a risk-based threshold for exposure to PBDEs. Concern about them has risen only recently, driven both by growing awareness of how pervasive PBDE contamination has become, including in human tissues, and by a small body of scientific research indicating they alter gene expression and act as developmental neurotoxicants via endocrine disruption.
Within the last two years, a series of studies have drawn attention to human contamination by PBDEs, including research in Indiana and California, and two surveys of mothers' milk across the US (survey 1 , survey 2). Previous work had also reported finding PBDE in salmon from Lake Michigan and 87% of fish sampled in Virginia.
Why wild Chinook carry levels comparable to farmed salmon is unknown. Hites et al. suggest it must somehow relate to Chinook diets--in adulthood they eat mainly other fish, compared to the other species' greater reliance on invertebrates. In that way the Chinook's diet may be more similar to farmed salmon, because the chow fed to farmed salmon contains a high percentage of ground-up fish.
Hites et al. conclude by supporting efforts to label salmon as farmed or wild and to identify the country of origin. Consistent with a recent recommendation by the Institute of Medicine, which recommends steps to reduce levels of dioxin-like contaminants in the human food supply, Hites et al. also call for more research on the sources of contamination, as this may identify ways to develop less-contaminated feeds and food products.
They recommend selective consumption of salmon containing relatively lower concentrations of PBDEs, which as their earlier work showed, also contain lower levels of many chlorinated organic contaminants.