Technology▼Solutions Improving fish food
even more comprehensive study that compares wild and farmed salmon. The aquaculture industry is reeling Ikonomou says it mirrors the size and in the wake of a widely publicized scope of the Pew study but also exreport showing that farmed salmon amines heavy metal and pesticide contain significantly more contamicontamination as well as hormone nants than wild salmon. The report, and nutrient profiles. A major aim is published in January (Science 2003, to determine the sources and magni303, 226–229), has focused worldtude of the contaminants of concern wide attention on fish nutrition by by examining their uptake patterns implicating the food fed to farmobserved in the tissues of various raised salmon as the source of the stocks of wild and farmed salmon. contamination. Unfortunately, no results are yet Experts in the field say that many available. technologies have the potential to The more economical way reduce the concentrations of to reduce the amount of conPCBs, dioxins, and pesticides in taminants in farmed fish is to fish food. Until now, however, use plant-based materials as much of the research on reducing a significant source of some the amount of contaminants in of the protein and oils in fish food formulations has their diet, Ikonomou says. foundered because of a lack of Plant-based oils such as funding, they say. canola are not expected to The Science study, which was contain dioxins or other funded by the Pew Charitable global environmental contaTrusts, represents the most comminants, he explains. prehensive analysis of contami“We could make dramatic nation in farmed fish conducted reductions in the amount of Because farm-raised salmon that comprises the bulk of to date. A group led by Indiana dioxins and PCBs in farmed the salmon eaten today has been shown to harbor contaminants like PCBs, scientists are researching ways to University researcher and ES&T fish,” says David Higgs, the include more plant-based proteins and oils in fish diets. Associate Editor Ron Hites anaresearch scientist who heads lyzed more than 2 metric tons of DFO’s fish nutrition program. farmed and wild salmon and found International Fishmeal and Fish Oil He has been researching ways to use that aquacultured salmon from Organization, an industry group. Five plant-based materials in fish food for Europe and North America contained European fish-processing plants are more than 25 years. According to resignificantly higher levels of 14 now using carbon filtration systems search conducted on trout by North organochlorine contaminants than that reduce PCBs and dioxins, he American and European investigathe wild fish. says. However, Barlow acknowledges tors, oilseed and protein concentrates According to the U.S. EPA’s riskthat the filters don’t target PCBs as efbased on canola and soy offer great based guidelines for cancer, the ficiently as dioxins, and they are unpromise to extensively replace fish combined concentrations of PCBs, likely to reduce pesticide levels. meal in diets for salmon, he says. toxaphene, and dieldrin in all the Although filtering can reduce the Higgs says that he and Ikonomou tested samples of farmed salmon amount of contaminants in the fish have tried unsuccessfully to get fundwere so high that consumers could oils fed to the farmed fish, which ing for a study that would investigate only safely eat no more than one serve as the source of most of the fatjust how much contaminants could serving of the fish per month. Salmon soluble contaminants in their diets, be reduced in salmon that were fed from farms in the European Union the approach is generally too expendiets with higher percentages of (EU) had the highest levels of consive to be practical, says Michael plant-based materials. taminants. The EPA guidelines are Ikonomou, a research scientist for Barlow agrees that additional much more stringent than the levels Canada’s Department of Fisheries and sources of fish meal and oil need to set by the U.S. Food and Drug AdOceans (DFO). In collaboration with be developed. “There’s a limit to how ministration or Health Canada, which Canada’s University of British Colummuch fish meal and oil can be prowould give both wild and farmed bia (UBC), DFO is in the process of duced [from fish]. Our worldwide salmon a clean bill of health. conducting what they believe is an production has been more or less staRHONDA SAUNDERS
Hites and his colleagues stressed that a number of the contaminants they documented in the farmed fish are also associated with noncancer heath issues, including endocrine disruption, which were not considered in their analysis. The contaminant levels found in European fish would be lower if the researchers had tested salmon on the market after July 1, 2002, when EU legislation limiting dioxin levels in fish went into effect, says Stuart Barlow, director general of the
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tic for the last eight years…. That’s not going to change unless something very surprising happens,” Barlow explains. Because the percentage of fish raised by aquaculture is projected to grow steadily (Environ. Sci. Technol. 2003, 37, 429A−430A), fish food formulators must find ways to use even more alternative oil sources, Barlow says. Plant oil sources can also help fish farmers during El Niño years when drops in the supply of pelagic fish caught off the coast of South America, which are the main source of world’s fish oil and fish meal, cause prices for these commodities to skyrocket, Higgs says. In fact, he says that the program he has been working for since 1977 to formulate fish diets that are based on more protein and lipid from plant sources originated in response to a particularly difficult El Niño year. Salmon farmers can never get away entirely from fish oil, Barlow claims. Fish oil is used to supplement the diet of fatty fish like salmon to ensure that they contain a sufficiently high percentage of the two longchain, highly unsaturated, omega 3 fatty acids found in fisheicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), he explains. These fatty acids are in high demand by consumers because they have been linked to various human health benefits, including reduced risk of heart disease, he says. That may not always be true. In the last 10 years, researchers have shown that humans can metabolize and produce long-chain fatty acids from shorter-chain vegetable sources, says Miriam Jacobs, a nutritionist and toxicologist who has also studied the contaminants in salmon (Environ. Sci. Technol. 2002, 36, 267A–268A). But she acknowledges that the process “is not terribly efficient—you need to have 8 or 10 times as much quantity-wise.” Scientists like Higgs are focusing their efforts on using oils from oilseeds that are rich in linolenic acid, an omega 3 fatty acid that is a precursor to EPA and DHA. The oils include flaxseed, hempseed, soy, and canola. They’re also looking at wheat, barley, corn, oats, canola, sunflower, and soybeans as sources of dietary proteins for salmon and other fish. “We have reduced the [amount of fish in] fish meal level [from the lev-
els] of a few years ago by 60%; we can get it down to 17 or 20% of the meal fed to the fish without breaking a sweat,” says Ron Hardy, director of the Aquaculture Research Institute at the University of Idaho. “Getting the fish oil level down presents a bigger challenge,” he says. If he received as much money for addressing the problem as the Pew Charitable Trust donated to defining it, he could solve it easily, he claims. Reducing the amount of fish material used in the oils is challenging because the price of fish oil tends to be lower than that of vegetable-based oils in the non-El Niño years, Higgs says. Getting away from the oils is also difficult because “the feed companies want to make sure that the omega 3 acids, especially EPA and DHA, are there at the right levels in the salmon going to market,” he adds. Higgs says that UBC and DFO researchers have been able to replace up to 75% of the supplemental fish oils fed to fish with cold-pressed flaxseed oil. But he is particularly enthusiastic about an approach that he and co-workers from CSH Innovations Ltd., of Vancouver, B.C., hope to patent, which uses animal proteins and lipids from fish or poultry together with pretreated oilseeds. This technology, which involves co-processing the oilseeds with the animal-based materials, is unique because animal and fish feeds generally include protein and lipid from oilseeds, fish, or poultry that are processed separately, not together. The process capitalizes on what Higgs refers to as “under-utilized or poorly utilized animal sources—processing wastes, or poultry offal, cod heads, and associated viscera.” The animal wastes are cooked with the oilseeds to denature proteins and free the cellular water bound in the animal cell proteins. This cellular water is then used to wash the watersoluble oilseed meats. The heating and washing processes can markedly reduce the anti-nutritional factors, such as protease inhibitors, phytates, oligosaccharides, and glucosinolates, that can be found in oilseeds, he says. If some fish or fish offal is used in the process, the oil rendered out of the process will contain the beneficial EPA and DHA fatty acids. “You can even use different combinations of
oilseeds to create all kinds of neat blends of lipids with different fatty acid compositions with different purposes in mind,” he says. The process may be cost-effective because fish meal is generally two times as expensive, on a per-kilogram-protein basis, as poultry lipids or plant-based meals made from grains like soybeans. Higgs says that he and his colleagues can use the technology to produce a protein concentrate that derives 90% of its protein from oilseeds and is also enriched in minerals from the animal protein. “We’ve also done in vivo digestibility work using Atlantic salmon as the test species and found that the availability of protein in these concentrates was among the best ever seen, relative to fish meal protein—up around 94–96% available protein— but we haven’t taken it past that due to lack of available funds,” he says. Higgs does not know how consumers will react to salmon raised on feeds containing these new protein and lipid sources. Fish fed diets too high in vegetable matter can taste different. Jacobs says that fish producers with whom she has spoken say that such fish have been rejected by Japanese consumers because they don’t taste fishy enough. However, Higgs says that researchers throughout the world are trying to determine the best nutritional strategies so that fish like salmon can be raised using diets that are largely plant-based and then “finished” during the last months of their lives with diets higher in the EPA and DHA omega 3 highly unsaturated fatty acids. Using plant-based material or products that would otherwise not be directed into the diets of fish increases the eco-efficiency of fish farming, according to environmentalists. At present, it takes 1.3 kilograms of wild fish to produce each kilogram of aquacultured fish, says Rebecca Goldburg, a senior scientist with Environmental Defense, an environmental group. The small pelagic fish used to produce fish meal and oil are important parts of the marine food web, she says. The future of farmed fish is bright, Higgs claims. In the long term, farmed salmon have the potential to contain lower levels of contaminants than their wild counterparts, he says. —KELLYN S. BETTS
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