The first extensive look at chemical contamination pathways in aquaculture systems points to fish meal and oils as primary sources of persistent organic pollutants (POPs) in farm-raised fish. The study, published on ES&T’s Web site under Research ASAPs (10.1021/es011287i), turned up a wide range of POPs, including polychlorinated biphenyls (PCBs), polybrominated diphenyl ether (PBDE) flame retardants, and organochlorine pesticides in farm-raised and wild European Atlantic salmon, aquaculture feed, and fish oils. High levels of PCBs and evidence of recent usage of the pesticide DDT in the fish samples suggests that dioxins are not the only contaminants in foods that regulatory bodies should be concerned about.

“In most of the EU deliberations, there is concern with dioxins in food,” says lead author of the study, Miriam Jacobs of the University of Surrey in the United Kingdom. “There have been discussions that levels of dioxins are decreasing in food. PCBs are far, far greater than the dioxins’ content,” she says.

In the United States, regulatory bodies are also focusing on dioxins in foods. At the request of the U.S. Department of Agriculture and the Food and Drug Administration (FDA), the National Academies has formed a committee to investigate the implications of dioxins in the food supply (Environ. Sci. Technol. 2002, 36, 93A–94A). A report from that committee is expected next spring.

Chemicals in the U.S. food supply are monitored as part of FDA’s ongoing Total Diet Studies (TDS), which date back to the 1960s, according to Richard Canady of FDA’s Center for Food Safety and Applied Nutrition. As part of the TDS program, levels of pesticides, industrial chemicals, radionuclides, toxic elements, nutrients, and folic acid are measured in food each year. In 1999, 17 dioxins/furans were added to FDA’s TDS list, and this year, three dioxin-like coplanar PCBs were added.

Although dioxins are much more potent and toxic than PCBs, in terms of their ability to bind to the aryl hydrocarbon (Ah) receptor, that is only one route of toxicity, Jacobs emphasizes. Many PCBs and polybrominated flame retardants are weak endocrine disrupters and can act through the estrogen and thyroid receptors, she says.

With the help of Adrian Covaci and Paul Schepens of the University of Antwerp’s Toxicological Center in Belgium, Jacobs analyzed both wild and farm-raised British and Norwegian salmon samples, aquaculture feed, and fish oils used to supplement the feed. High levels of PCBs (145–460 ng/g lipid) and moderate levels of PBDEs (1–85 ng/g lipid) and organochlorine pesticides ( DDTs = 5–250 ng/g lipid) were found in both the farm-raised and wild fish. Contamination was generally highest in fish, followed by aquaculture feed, and then oils.

For comparison, during the Belgian food crisis of 1999, in which animal feed was contaminated with PCBs and dioxins, total PCB levels in chickens were as high as 10,000 ng/g lipid (10 ppm). The U.S. government currently requires further testing of a food when levels of PCBs are >0.1 ppm, because of concern over dioxins in food. When there are high levels of PCBs, there are often high levels of dioxins.

The aquaculture industry commonly supplements feed with marine fish oils to increase the levels of omega-3 fatty acids. These oils are healthy from a nutritional standpoint, but they are likely to contribute to POPs’ contamination in farm-raised fish. Not all fish oils, however, are high in POPs. In general, researchers have found that fish oils obtained from the southern hemisphere have less POP contamination than those from the northern hemisphere. “There are big differences depending on the source of the fish meal. Fish oils from around the Baltic Sea have much higher levels of POPs than those from Peru,” Jacobs says. That same trend has been seen by other researchers investigating fish meal fed to chickens, pigs, and sheep, she adds.

The aquaculture study has insufficient samples to compare levels of POPs in wild fish with those in farm-raised fish. In addition, “it wasn’t clear whether the wild fish were truly wild or whether they were farm escapees,” Jacobs says. Although there is a strong likelihood that one of the wild samples was truly wild, because it had much lower fat levels and a different PCB congener profile than the farm-raised fish, at least one of the wild samples appeared to be a farm escapee.

For the most part, samples with higher PCBs also had higher levels of pesticide residues, but this was not true for PBDEs, suggesting that these flame retardants were coming from a different source. “In the north of England, around the River Tees, there is a great deal of flame retardant production,” Jacobs says. However, there are lots of other point sources, she adds. For example, PBDEs are widely used in computer and textile production.

Although PCBs and many organochlorine pesticides have been banned throughout most of the world, they are still ending up in the food supply. As reported by Jacobs and colleagues, European farm-raised salmon can be a significant source. In some salmon samples, ratios of DDT to its metabolite, DDE, suggest recent usage of the pesticide. “This should be looked at a lot more closely. Without regular monitoring, it cannot be controlled effectively,” Jacobs advocates. “With the emphasis more on dioxins, this sort of thing gets missed,” she says. —BRITT E. ERICKSON