Farmed salmonids are an excellent source of omega-3 fatty acids and contain very few residues of environmental contaminants

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Richard Morin, Innovation and Technologies Branch, ministère de l'Agriculture, des Pêcheries et de l'Alimentation du Québec (MAPAQ)

 

The quality and safety of farmed salmonids was the subject of a study conducted in Quebec in 2003 and 2004.

Media reports in 2002 and 2004 about scientific studies that reported the presence of environmental contaminants in wild and farmed salmon have given rise to serious concerns among the Quebec public about the quality and safety of these fish. Moreover, the studies have been heavily criticized by other scientists. In response to the first controversial study and surrounding media coverage, the Quebec Department of Agriculture, Fisheries and Food (MAPAQ) published a brief article which attempted to set the facts straight regarding the low level of risk of contamination for consumers of farmed salmonids (Morin, 2002). In the article, MAPAQ relied on the results of analyses of fish meal and feed fed to farm-raised salmonids conducted by the Canadian Food Inspection Agency (CFIA) in 1998-99 and by MAPAQ in 2001. The analyses showed that PCB and mercury concentrations were significantly lower than the maximum values set out in Canadian guidelines.

The results demonstrated that salmonid feed used in Canada contained concentrations of environmental contaminants well below the permitted limits. However, at that time, MAPAQ had no analytical data on the contaminant residue levels in the tissue of farmed salmonids sold in Quebec. In 2003, a study on the subject was proposed to MAPAQ by a team of researchers from the Unité de recherche en santé publique [public health research unit] (Centre de recherche du CHUL¹, CHUQ²) and the Institut national de santé publique du Québec [Quebec public health institute]. This team has been interested for a number of years in the nutritional value and safety of aquatic products and particularly their omega-3 fatty acid content and beneficial health effects.

MAPAQ contributed financially to the study in partnership with the Quebec Department of Health and provided technical support through the involvement of MAPAQ's food analysis laboratories, which took samples of farmed fish and conducted analyses of contaminants in the samples. The public health research unit team, under the supervision of Carole Blanchet for the purposes of the project, assayed the levels of fatty acids in the samples, compiled and analyzed all the results with respect to nutritional value and contamination risks for consumers, and drafted the report. The study provided data on the fatty acids composition and PCB, dioxin, furan and mercury residues in the tissue of wild and farmed Atlantic salmon and rainbow trout. Samples of farmed fish were also tested for pesticide, lead and cadmium residues in order to provide a complete picture of the fatty acids composition and environmental contaminant residue levels in the tissue of wild and farmed salmonids.

Omega-3 fatty acids are essential

Following the observation that Inuit populations had very low rates of heart disease, even though their diet was composed mainly of lipid-rich fish and marine mammals, numerous studies revealed that omega-3 fatty acids are essential for the proper development of the human body and have beneficial health effects (Dewailly, 2001; Demers, 2001). The term "lipids" is used to designate fats, of which fatty acids are the main biochemical constituent (Samuel, et al., 2002).

Eicosapentaenoic (EPA) and docosahexaenoic (DHA) fatty acids are long-chain omega-3 fatty acids that protect against cardiovascular and inflammatory diseases and are essential to brain development (Blanchet and Dewailly, 2002). They accumulate in the membranes of the heart, blood cells and other tissues (Nudds, 2002). They help to maintain membrane fluidity and ensure normal cell and tissue function. DHA is the most abundant omega-3 fatty acid in the brain and retina, accounting for more than 50% of the total unsaturated fatty acids present (Nudds, 2002).

Scientific studies have also demonstrated that consuming two servings of fish a week has protective effects on cardiovascular health (Harris, 2004).

Farmed salmonids are rich in omega-3 fatty acids

Fish and seafood are very important sources of omega-3 fatty acids. Vegetable oils are also a significant source of omega-3 fatty acids, but they contain mainly α-linolenic acid, whereas aquatic products are much richer in omega-3 fatty acids with a long carbon chain, such as EPA and DHA, which help to significantly lower the risk of heart disease (Demers, 2001; Harris, 2004).

Salmonids are carnivores and are fed a diet of dried feed in fish-farming operations. The feed consists mainly of fish meal and fish oils (Morin, 2002). The fish from which the feed is made are generally classified as fatty or moderately fatty, depending on the species, and their lipid content varies from 2% to 15%.

Farmed salmonids have a higher fat content than wild salmonids and thus contain higher levels of omega-3 fatty acids per serving. The diet of wild fish is lower in fat and more limited in quantity than that of farmed fish, which are fed a diet of lipid-rich feed to satiety. Wild fish are also more active, travel greater distances and thus expend more energy, whereas farmed salmonids are confined in a restricted space and expend less energy. The fat content of wild rainbow trout is 3.5% (g/100 g), while the fat content of farmed trout averages 5% to 6% (Samuel et al., 2002). The same is true for wild Atlantic salmon, which contains approximately 6% fat compared to 10% to 12% for farmed salmon (Samuel et al., 2002).

Environmental contaminants

Polychlorinated biphenyls (PCBs) are a group of chemical compounds consisting of chlorine. The low inflammability and very high boiling point of these compounds make them excellent insulators for electrical processes. They had hundreds of industrial applications and more than 700,000 tonnes of PCBs were manufactured in the United States before production was halted in 1977 (Canadian Environmental Law Association, 2004). The chemical stability of these substances, which was the essential property sought by industry, is also responsible for their persistence in the environment.

Dioxins are considered the most toxic chemical substances ever produced by man and furans are approximately one-tenth as toxic as dioxins. Agent Orange, a defoliant used in tropical forests during the Vietnam War, contained dioxin. Dioxins and furans are produced mainly by combustion of chemical substances in industrial processes and municipal incinerators. Actions by the U.S. Environmental Protection Agency have led to a 90% reduction in environmental emissions of dioxins and furans over the past 30 years (U.S. Food and Drug Administration, 2004). However, atmospheric transport of these substances and their very slow degradation have resulted in their ubiquitous presence in the environment and, consequently, in food.

There are a large number of pesticides, which have a wide range of agricultural, industrial and household uses. Pesticide use is now strictly regulated by governments in order to reduce the risk of contamination of the environment, food and humans.

Mercury is found in the environment mainly as a result of the burning of coal and the incineration of certain wastes, which release it into the atmosphere. Once it reaches the aquatic environment, microorganisms transform it into methylmercury, which is much more toxic, where it accumulates in this form in the food chain. Mercury's adverse effects on health mainly involve the nervous system, affecting individuals who have been exposed to high concentrations. Research shows that the consumption of fish by most individuals does not cause health problems (U.S. Environmental Protection Agency, 2004).

Lead was widely used in the paint industry as a pigment and in gasoline for its anti-knock properties. This resulted in the extensive dispersion of this metal in the environment. The major sources of contamination are water, food and dust. Lead ingested or inhaled is stored in soft tissues such as the brain and especially in bone. It has toxic effects on the central nervous system (L'Expertise collective INSERM, 1999).

Cadmium is a relatively rare element and is not found in the pure state in nature (Johnson, 1997). The main sources are phosphate fertilizers, the burning of fossil hydrocarbons and certain industrial activities (International Cadmium Association, 2000). Once transported into the atmosphere, it is deposited on plants and absorbed by animals and humans, mainly by ingestion of contaminated foods. The kidneys and liver are the main organs that accumulate cadmium. It is for this reason that wildlife managers in Quebec warn hunters about the risks of contamination through consumption of organ meats (kidney and liver) of cervids (Quebec Department of Natural Resources, Wildlife and Parks, 2004).

Risks of contamination of the flesh of farmed salmonids

The potential pathway of contamination of farmed salmonids is through feed and, as we explained in a previous article (Morin, 2002), fish meal and fish oils, which are the major constituents of their feed, are manufactured mainly from pelagic fish. These are species such as sardines, pilchard, sand lance and herring, which are of little commercial value for human consumption. Based on our understanding of the principle of bioaccumulation of toxic substances through the food chain, we know that the most contaminated fish are the predators at the top of the food pyramid. The pelagic marine fish used in the manufacture of fish meal have a lower risk of containing high concentrations of toxic environmental contaminants because they are first-level consumers which feed on plankton.

The fish meal and fish oil used in salmonid feed are also regularly tested by the Canadian Food Inspection Agency (CFIA) for environmental contaminant residues.

Study method

The purpose of the study was to analyze and compare the fatty acid content and environmental contaminant content of wild and farmed Atlantic salmon and rainbow trout (Blanchet et al., 2005).

In 2003, samples of farmed salmon and rainbow trout were collected by MAPAQ in supermarkets in various regions of Quebec. The wild salmon and rainbow trout samples were obtained from fishermen in the Gaspé Peninsula, the Centre interuniversitaire de recherche sur le saumon atlantique (CIRSA) [inter-university Atlantic salmon research centre] and the Freshwater Fisheries Society of British Columbia. In total, 46 farmed salmon and 37 farmed trout and 10 wild salmon and 10 wild trout were analyzed for their fatty acid content and environmental contaminant content.

The fatty acid analyses were conducted at the CHUL's Centre de recherche en maladies lipidiques [lipid disorders research centre]. They included saturated, monounsaturated and polyunsaturated fatty acids, including omega-3 and omega-6 fatty acids. The environmental contaminants were measured at MAPAQ's food analysis laboratories and included polychlorinated biphenyls (PCBs), dioxins and furans, pesticides and heavy metals: mercury, lead and cadmium.

Lipid and fatty acid content

The results of this study revealed that the fat (total lipids) composition of the flesh of farmed rainbow trout (5,576 mg/100 g) is 5.6 times higher than in wild rainbow trout (953 mg/100 g) (Table 1) (Blanchet et al., 2005). A surprising result was obtained for Atlantic salmon, which has a more or less equivalent fat level for both the farmed (7,421 mg/100 g) and wild fish (6,967 mg/100 g). Fatty acids make up slightly more than half of the total lipids for all the fish analyzed.

Table 1: Total lipids and fatty acids composition of wild and farmed salmon and rainbow trout (mg/100 g)

Lipides/acides gras	Rainbow trout			Atlantic salmon
	farmed	wild	farmed		wild
Total lipids Total fatty acids	5 576.3 3 188.4	952.7 593.0	7 421.3 4 022.1		6 966.8 3 974.2
PUFA MUFA SFA	1 221.6 1 106.0 860.8	342.7 106.0 144.4	1 634.8 1 473.7 1 037.7		1 052.5 2 161.4 760.2
EPA + DHA n-3 HUFA n-3 PUFA	731.4 861.3 930.7	232.4 255.0 268.2	855,2 1 065.6 1192.4		749.1 911.2 960.9
n-6 PUFA	290.8	74.5	442.4		91.7

PUFA: polyunsaturated fatty acids; MUFA: monounsaturated fatty acids; SF: saturated fatty acids; EPA: eicosapentaenoic acid; DHA: docosahexaenoic acid; HUFA: highly unsaturated fatty acids

Figure 1 shows that the fatty acid composition of farmed rainbow trout and farmed Atlantic salmon is very similar (Blanchet et al., 2005). Saturated fatty acids account for the lowest proportion at 26%-27%, monounsaturated fatty acids are at 33% and polyunsaturated fatty acids are the highest at 41%. This result was foreseeable because both species are fed a feed of similar composition (Morin, 2002). On the other hand, there is a more pronounced difference between the wild and farmed congeners of the same species. Wild rainbow trout has the highest proportion of polyunsaturated fatty acids (58.6%), while wild Atlantic salmon has the highest proportion of monounsaturated fatty acids (53.7%). The proportion of saturated fatty acids is virtually identical, at 24% to 27%, in wild and farmed rainbow trout and farmed Atlantic salmon, but it is lower in wild salmon, at 19%.

Figure 2 presents in greater detail the proportions of the various types of polyunsaturated fatty acids. Compared to farmed rainbow trout, wild rainbow trout has a higher proportion of polyunsaturated n-3 fatty acids (PUFA), highly unsaturated fatty acids (HUFA) and eicosapentaenoic (EPA) + docosahexaenoic (DHA) fatty acids. This difference is less pronounced between wild salmon and farmed salmon, and salmon contains a lower proportion of n-3 fatty acids than wild trout. The proportion of n-6 PUFA is higher in wild trout (12.5%) than in farmed trout (8.5%) and, conversely, farmed salmon has a higher proportion of n-3 PUFA (9.8%) than wild salmon (2.3%).

Researchers have determined, based on the results indicated in Table 1, the contribution of farmed and wild salmon and trout to the recommended daily intake of omega-3 fatty acids. These figures are at least 500 mg/day of EPA + DHA in healthy individuals and 1,000 mg/day of EPA + DHA in individuals with heart disease (Harris, 2004). A 180 g serving provides more than 1,500 mg of these fatty acids in the case of farmed Atlantic salmon (855.2 mg/100 g x 180 g), approximately 1,300 mg in the case of wild salmon (749.1 mg/100 g x 180 g) and slightly more than 400 mg in the case of wild trout (232.4 mg/100 g x 180 g) (Blanchet et al., 2005).

Figure 1: Proportion of fatty acids of farmed and wild rainbow trout and salmon (% of total fatty acids) (Blanchet et al., 2005)

Figure 2: Composition in n-3 and n-6 fatty acids of farmed and wild rainbow trout and salmon (% of total fatty acids)

Environmental contaminants content

In general, concentrations of contaminants are very low in both species, whether farmed or wild (Table 2). In Quebec, the rules governing fish consumption are based on the Canadian Food Inspection Agency (CFIA) guidelines for the marketing of fishery and aquaculture products. These guidelines stipulate a maximum concentration of 2 mg/kg of PCBs in fish flesh. The average and maximum PCB concentrations measured in each group of samples did not exceed 0.014 mg/kg and 0.039 mg/kg respectively, which constitutes a tiny fraction (less than 2%) of the acceptable standard. The average and maximum concentrations of dioxins and furans measured did not exceed 0.150 ng/kg and 0.480 ng/kg respectively, which represents 1% to 3% of the maximum standard (15 ng/kg). Mercury was measured at average concentrations, which did not exceed 0.056 mg/kg, and maximum concentrations of 0.090 mg/kg in the four groups of fish sampled, which represents less than 20% of the maximum standard of 0.5 mg/kg.

Table 2: Concentrations of PCBs, dioxins and furans, and mercury in farmed and wild rainbow trout and Atlantic salmon, and maximum permissible concentrations in fish according to CAIA guidelines

Contaminants	Rainbow trout			Atlantic salmon
	farmed	wild	farmed		wild
PCBs (mg/kg)	0.006	0.006	0.014		0.006
	0.013*	0.011*	0.039*		0.017*
Dioxins+furans (ng TEQ/kg)	0.041	0.098	0.082		0.150
	0.175*	0.285*	0.480*		0.440*
Mercury (mg/kg)	0.021	0.045	0.018		0.056
	0.040*	0.090*	0.030*		0.080*

* maximum concentration measured

No trace of the pesticides tested for (180 different compounds) was detected in the 68 samples of farmed fish analyzed. Two farmed fish yielded a positive result for lead, namely one trout at 0.15 mg/kg and one salmon at 0.19 mg/kg. The CODEX and EEC maximum residue limit is 0.2 mg/kg for this metal. A few samples contained traces of cadmium ranging from 0.02 mg/kg to 0.04 mg/kg.

To facilitate comprehension.
Units				Values
kg		kilogram	1 x 103 g				1,000 g
g		gram	1 g				1 g
mg		milligram	1 x 10-3 g				0.001 g
µg		microgram	1 x 10-6 g				0.000 001 g
ng		nanogram	1 x 10-9 g				0.000 000 001 g
pg		picogram	1 x 10-12 g				0.000 000 000 001 g
Units						Values			Examples
ppm ppb ppt	parts per million parts per billion parts per trillion				1 x 10-6 1 x 10-9 1 x 10-12			µg/g or mg/kg ng/g or µg/kg pg/g or ng/kg

Since fish are a source of exposure to environmental contaminants, toxicological reference values (TRVs) have been established by various agencies, including Health Canada, in order to prevent the risks associated with contaminants. These values make it possible to establish a relationship between a dose of a contaminant and an effect and they are defined in units of contaminants (m g, ng or pg) per unit of weight of an individual (kg). These units are shown in Table 3 for the contaminants measured. The authors of this study have therefore calculated, based on the TRVs and on a weekly basis, the doses of contaminants that could be ingested based on the frequency of consumption of a serving (180 g) of farmed rainbow trout or Atlantic salmon.

Figure 3 provides an example of the graphs which show, for each contaminant, the percentages of the toxicological reference value attained based on the frequency of consumption of fish meals per week (Blanchet et al., 2005). Figure 4 presents a summary of all these results for the most problematic case, namely a 60 kg woman who consumes seven 180-g fish meals a week. The percentage of the TRV is given for each contaminant, including two values for PCBs, the first at 1 m g/kg and the other, more stringent, at 0.13 m g/kg. These results indicate that with a consumption of seven meals of farmed trout or salmon a week, the TRV is never reached for any of the contaminants. The maximum values calculated are 38.5% of the TRV at 0.13 m g/kg for PCBs and 25% of the TRV for dioxins and furans. All the other values are below 15% of the TRV.

Table 3: Toxicological reference values (TRVs) established for PCBs, dioxins and furans and mercury

Contaminants	Values
PCBs (mg/kg)	1.0 and 0.13
Dioxins and furans (ng/kg)	1.0
Mercury (mg/kg)	0.47 (men and adult women) 0.20 (women of child-bearing age)

Figure 3: Example of a histogram which shows the percentages of the toxicological reference value attained for a contaminant based on the frequency of consumption of meals of a type of fish (from Blanchet et al., 2005)

Figure 4: Percentage of the toxicological reference value (TRV) attained for a 60 kg woman following consumption of 7 meals/week (180 g/meal) for PCBs, dioxins and furans, and mercury

Table 4: Concentrations of PCBs, dioxins and furans, and mercury in the flesh of farmed Salmon in Canada from various studies and maximum permitted values for consumption

Contaminants	Blanchet 2005 (Quebec)		Health Canada 2002 (Canada)		Cassidy 2002 (Ontario)	Easton 2002 (Western Canada)	Hites 2004 Foran 2004 (Eastern Canada)	CFIA Guide-lines
	trout	salmon	brook trout	salmon	trout	salmon	salmon
PCBs (mg/kg)	0.006*	0.014*	0.0065*	0.0175*	0.024*	0.051*	0.038*	2.0
Dioxins and furans (ng/kg)	0.041*	0.082*			1,44 (median)		1.6*	15.0
Mercury (mg/kg)	0.021*	0.018*			0.01-0.07 (min-max)	0.02*	0.5

* Average values

Farmed salmonids are good for your health

The concentrations of PCBs, dioxins and furans, and mercury in this study conducted in Quebec were compared to the results of other studies conducted elsewhere in Canada and in the world (Table 4). All these studies clearly show, even those that have given rise to incorrect interpretations of the toxicological risks associated with the residues measured: Easton et al., (2002) and Hites et al. (2004), that the concentrations of environmental contaminants are very low. The average concentrations of PCBs, dioxins and furans, and mercury measured all represent fractions of the values established to protect health in Canada.

The results of this study demonstrate that the consumption of farmed Atlantic salmon and rainbow trout provides consumers with a significant intake of omega-3 fatty acids and low intakes of environmental contaminants (Blanchet et al., 2005). A 180-g serving of farmed rainbow trout or Atlantic salmon provides the recommended daily intake of EPA and DHA. The consumption of these fish, even in several meals per week, never results in attaining the toxicological reference values that have been set for PCBs, dioxins and furans, and mercury.

In conclusion, the relatively low concentrations of contaminants and the relatively high concentrations of omega-3 fatty acids observed in farmed salmonids give us good reason to encourage Quebecers to consume these fish (Blanchet et al., 2005).

References

Blanchet, C., and E. Dewailly (2002). Le Guide alimentaire du Saint-Laurent, Un guide sur le potentiel alimentaire des ressources aquatiques du Saint-Laurent. ISBN 2-89496-234-7. http://www.slv2000.qc.ca

Blanchet, C., M. Lucas and E. Dewailly (2005). Analyses des acides gras oméga-3 et des contaminants environnementaux dans les salmonidés. Unité de recherche en santé publique. Centre de recherche du CHUL (CHUQ) et Institut national de santé publique du Québec. 45 pages.

Canadian Environmental Law Association and Great Lakes Centers (2002). Environmental profile of PCBs in the Great Lakes. http://uic.edu/sph/glakes/pcb/whatarepcbs.htm

Cassidy, M., A. Matu and G. Downing (2002). Baseline risk study of potential chemical contaminants in Ontario farm-raised rainbow trout. Ontario Ministry of Agriculture and Food. http://www.aps.uoguelph.ca/~aquacentre/aec/publications/contaminants.pdf

Demers. E. (2001). Les acides gras polyinsaturés oméga-3 en bref. Émergence 5(1):3, Bulletin d'information du Centre technologique des produits aquatiques, Quebec Department of Agriculture, Fisheries and Food (MAPAQ), 96 Montée Sandy Beach, Gaspé.

Dewailly, E,, C. Blanchet, S. Lemieux, L. Sauvé, S. Gingras, P. Ayotte and B.J. Holub. (2001). N-3 fatty acids and cardiovascular disease risk factors among the Inuit of Nunavik. Am J Clin Nutr 74(4): 464-73.

Easton, M.D.L., D. Luszniak and E. Von der Geest (2002). Preliminary examination of contaminant loadings in farmed salmon, wild salmon and commercial salmon feed. Chemosphere 46(7): 1053-1074.

U.S. Environmental Protection Agency (2004). Mercury, Basic Information. http://www.epa.gov/mercury/about.htm

U.S. Food and Drug Administration (2004). Questions and Answers about Dioxins. http://www.cfsan.fda.gov/~lrd/dioxinqa.html

Foran, J.A., R.A. Hites, D.O. Carpenter, M.C. Hamilton, A. Matthews-Amos and S.J. Schwager (2004). A survey of metals in tissues of farmed Atlantic and wild pacific salmon. Environ. Toxic. and Chem. 23(9): 2108-2110. http://entc.allenpress.com/entconline/?request=get-document&doi=10.897%2f04-72

Harris, W.S., and C. Von Shacky (2004). The omega-3 index: a new risk factor for sudden cardiac death? Prev Me 39: 212-220.

International Cadmium Association (2000). Cadmium Products, The Issues and Answers. http://www.cadmium.org/environmental.html

Johnson, B.T. (1997). Cadmium Contamination of Food. UCD Extoxnet FAQ Team. http://extoxnet.orst.edu/faqs/foodcon/cadmium.htm

L'Expertise collective INSERM (1999). Plomb dans l'environnement: quels risques pour la santé? http://www.inserm.fr/servcom/servcom.nsf/0/a1e5f17d2201769ac1256746003e1aca?OpenDocument

Morin, R. (2002) . La truite d'élevage est bonne pour la santé. L'Aquicole 8(1):3.

Nudds, K. (2002). Fish, Omega-3 fatty acids and you. Aquaculture Centre, University of Guelph. http://www.aps.uoguelph.ca/~aquacentre/aec/publications/FattyAcids.pdf

Samuel A., J. Boyet, M.-É. Carbonneau, F. Coulombe, N. Coulombe, M. Desbiens, L. Leclerc, C. Martin and R. Morin (2002). Guide Élevage des Salmonidés, Fascicule 12, Transformation. Quebec Department of Agriculture, Fisheries and Food (MAPAQ). ISBN 2-551-21368-1, 133 pages.

Quebec Department of Natural Resources, Wildlife and Parks (2004). La chasse d'hiver au caribou, Saison 2004-2005. http://www.fapaq.gouv.qc.ca/fr/publications/caribou_chasse/caribou_04_05.pdf

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[ 1 ] Centre hospitaliers de l'université Laval;

[ 2 ] Centre hospitaliers universitaires du Québec

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