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Food and Nutrition

Fish and Seafood Survey - 2002

Background

Fish and seafood are rich in many essential nutrients and can contribute to a healthy diet for Canadians. However, these and other foods sometimes contain environmental contaminants, usually at very low levels.

As a result, Health Canada undertakes regular surveillance activities to monitor the level of contaminants in foods. These surveillance reports enable estimates of the current exposure of Canadians to these contaminants to be conducted and are a valuable tool to improve risk assessments and to develop the appropriate strategies to manage risks that may be associated with these contaminants.

In addition to the yearly Canadian Total Diet Study, which focuses on determining background levels of contaminants in food, Health Canada conducted a specific survey on fish and seafood. Farmed and wild caught fish and seafood products sold at the retail level were sampled in Vancouver, Toronto and Halifax during March, 2002. Samples of farmed and wild caught char, oysters, salmon, shrimp and tilapia were analysed for dioxins, furans, polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and veterinary drugs. Shark, marlin, swordfish, tuna (canned and fresh/frozen), salmon, crab, oysters, and shrimp samples were analysed for total mercury, and shark, marlin, swordfish, and tuna (canned and fresh/frozen) samples were analysed for methylmercury. The resulting data are being used in Health Canada's health risk assessment updates for persistent organic pollutants and other chemical contaminants in the Canadian food supply. It should be noted that at the time of sampling, there was only limited availability of wild caught oysters, salmon, shrimp and tilapia in retail outlets and that only edible portions (skin and bone removed) were analysed for the various targeted contaminants. This report presents data on PCBs, dioxins, PBDEs, and mercury in these samples along with the results of preliminary human health risk assessments based on exposure to these contaminants through fish consumption. Information on other contaminants will be provided as the data and health risk assessments become available.

PCBs (Polychlorinated biphenyls)

PCBs are found in small quantities in all food commodities of animal origin, including fish. Since PCBs can accumulate in body tissues, they are also found in humans. PCB levels in the sampled fish and seafood products (Table 1) were quite low, with average values not exceeding 18 ppb (1 ppb or "part per billion" is equal to 0.000000001 grams of total PCBs per gram of product).

This is consistent with the findings of other Health Canada monitoring activities that have shown steady declines during the past 20 years in both dietary intake and human tissue levels of PCBs. There is no consistency with respect to the relative PCB levels in farmed and wild caught fish species- levels present in farmed and wild caught char are very similar; wild caught tilapia contained PCB levels approximately 3 times higher than the farmed tilapia; and farmed salmon contained approximately 2.5 times more PCBs than wild caught salmon.

While diet remains the predominant route of exposure to PCBs for the majority of the Canadian population, based on the highest average concentration of PCBs in fish found in this survey, Health Canada has determined that Canadians are not exposed to PCBs in foods at levels that pose a health risk. Health Canada believes that there is no need for specific advice regarding fish consumption vis--vis PCB exposure. The data generated through this study and others are being to update Health Canada's risk management strategies associated with exposure to PCBs from food and fish in particular. This study and its results have been peer reviewed and made available in the scientific literature (Rawn et al., 2006)

Table 1: Contaminants in Retail Fish and Seafood Products: Summary Statistics for Total PCB Levels (results expressed in ppb
Species Source Number of samples tested Mean Standard Error of the Mean Standard Deviation Minimum Maximum
ppb is parts per billion, and represents 0.000000001 grams of total PCBs per gram of sample (edible portion)
Char Farmed 6 6.5 1.5 3.7 3.5 13.5
Wild 5 5.4 1.1 2.5 3.1 9.7
Oysters Farmed 12 1.6 0.6 1.9 0.2 6.7
Wild 4 0.4 0.03 0.06 0.4 0.5
Salmon Farmed 19 17.5 2.4 10.6 4.4 45.1
Wild 3 6.6 3.4 5.9 2.8 13.5
Shrimp Farmed 13 0.3 0.1 0.5 0.04 2.0
Wild 4 0.4 0.2 0.5 0.1 1.1
Tilapia Farmed 15 1.8 1.1 4.2 0.06 16.6
Wild 3 5.3 4.3 7.5 0.3 14.0

Differences in total PCB levels between farmed and wild species were not statistically significant (p > 0.05). Total PCB levels between species were significantly different (p < 0.0001) with farmed salmon containing the highest average level (17.5 ppb) followed by farmed char (6.5 ppb).

PBDEs (Polybrominated diphenyl ethers)

PBDEs are man-made chemicals that are used as flame retardants in a wide variety of consumer products. They have similar chemical properties to PCBs, and are also regarded as being environmentally persistent and bioaccumulative, meaning that they accumulate in body tissues. Measurements in animals (fish, marine mammals) and humans (milk) have indicated that levels have been increasing during the past 10-15 years. Human milk levels are reported to be higher for North-America than Europe. Recent dietary surveys from European countries ( Netherlands, Spain, Sweden, UK ) and Canada have identified food as one possible route of exposure to PBDEs.

However, as dietary intake estimates of PBDEs for Canada (Canadian Total Diet study) are similar to those estimated in several European countries, other sources (consumer products, air, dust) than food may contribute significantly to human exposure in Canada.

PBDE levels in the fish and seafood products sampled (Table 2) did not exceed 5.5 ppb. While there is some limited evidence suggesting that the concentrations of PBDEs are higher in farmed fish and seafood products than in wild-caught products, Health Canada's opinion is that the current levels found in retail food are not considered to be a health concern. Health Canada will continue to update the health risk assessment for PBDEs as more data become available. This study and its results have been peer reviewed and are available in the scientific literature (Tittlemier et al., 2004)

Table 2. Contaminants in Retail Fish and Seafood Products: Summary Statistics for Total PBDE Levels (results expressed in ppb )
Species Source Number of samples tested Mean Standard Error of the Mean Standard Deviation Minimum Maximum
ppb is parts per billion, and represents 0.000000001 grams of total PCBs per gram of sample (edible portion)
Char Farmed 5 1.0 0.4 1.0 0.4 2.7
Wild 5 0.6 0.1 0.3 0.3 1.1
Oysters Farmed 11 0.7 0.1 0.5 0.006 1.4
Wild 4 0.4 0.08 0.2 0.3 0.6
Salmon Farmed 19 2.2 0.3 1.4 0.4 5.5
Wild 3 0.6 0.2 0.3 0.1 1.3
Shrimp Farmed 13 0.2 0.06 0.2 <0.001 0.7
Wild 4 0.1 0.05 0.09 0.009 0.2
Tilapia Farmed 12 0.6 0.4 1.4 0.04 5
Wild 3 0.1 0.09 0.2 0.01 0.3

Differences in total PBDE levels between farmed and wild species were not statistically significant (p > 0.05). Total PBDE levels between species were significantly different (p < 0.0001) with farmed salmon containing an average level of 2.2 ppb followed by farmed char (1.0 ppb).

PCDDs/PCDFs (Polychlorinated dibenzo-p-dioxins/Polychlorinated dibenzofurans)

Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans, commonly known as dioxins and furans, respectively, are chemicals produced in small amounts as by-products of various industrial processes, such as incineration of wastes, production of pesticides and bleaching of wood pulp. Similar to the PCBs and PBDEs, these compounds are ubiquitous in the environment and are known to bioaccumulate in the fat of organisms. They are, however, only found at very low levels in the environment and in food. PCDD/PCDF concentrations measured in a recent survey of 129 fish products available on the Canadian retail market in 2002 were below 8.5 parts per trillion (Table 3) in all fish product samples tested (one part per trillion is equal to 0.000000000001 gram of total PCDD/PCDFs per gram of fish tissue). These results are consistent with the residue levels observed in a wide variety of food samples collected as part of Health Canada's food surveillance program, including the Total Diet Study.

In theory, there can be up to 210 different dioxins and furans, all with a similar chemical structure. However, only 17 are known to be toxic and to bioaccumulate in food. The most toxic congener is 2,3,7,8-tetrachlorodibenzo-p-dioxin or simply TCDD; all other dioxins and furans are compared to TCDD using internationally-recognized numerical toxic equivalency factors (TEFs). The concentration of each congener is multiplied by its specific TEF and summed to give a TCDD toxic equivalency value (TEQ). Some PCB compounds have been shown to possess dioxin-like properties and have been assigned TEFs in a similar manner as with PCDD/Fs. A total TEQ can be estimated by summing the separate contributions from both PCDD/Fs and PCBs.

Using this TEF method, mean PCDD/F TEQ levels for fish/seafood products in this survey ranged from 0.02 pg TEQ/g whole weight in farmed shrimp to 0.36 pg TEQ/g whole weight in wild crab samples. Farmed salmon and wild salmon had TEQ values of 0.24 pg TEQ/g whole weight and 0.07 pg TEQ/g whole weight, respectively. When PCDD/Fs and dioxin-like PCBs were considered together, geometric mean total TEQs ranged from 0.06 pg TEQ/g whole weight in farmed shrimp to 1.1 pg TEQ/g whole weight in farmed salmon. For some fish products, the dioxin-like PCBs contributed much more to the overall TEQ than the PCDD/Fs.

Taking into consideration the total estimated TEQ values, Health Canada has determined that Canadians are not being exposed to dioxin-like contaminants in fish/seafood products at levels that pose a health risk and that there is no need for specific consumption advice at this time.

Based on the results from total diet surveys conducted by Health Canada, TEQ intakes, while exceeding the WHO provisional tolerable monthly intake for children under 5 years old, have been declining over the last 20 years.

Table 3: Contaminants in Retail Fish and Seafood Products: Summary Statistics for total PCDD/FsTable 3 footnote 1 (results expressed in pptTable 3 footnote 2 )
Species Source Number of samples tested Geometric Mean 95% Confidence Interval Minimum Maximum Total TEQ (Geometric mean)Table 3 footnote 3

Table 1 footnotes

Table 1 footnote 1

? PCDD/F Congeners: 2,3,7,8-TCDD; 1,2,3,7,8-PeCDD; 1,2,3,4,7,8-HxCDD; 1,2,3,6,7,8-HxCDD; 1,2,3,7,8,9-HxCDD; 1,2,3,4,6,7,8-HpCDD; OCDD; 2,3,7,8-TCDF; 1,2,3,7,8-PeCDF; 2,3,4,7,8-PeCDF; 1,2,3,4,7,8-HxCDF; 1,2,3,6,7,8-HxCDF; 2,3,4,6,7,8-HxCDF;1,2,3,7,8,9-HxCDF; 1,2,3,4,6,7,8-HpCDF;1,2,3,4,7,8,9-HpCDF; OCDF

Return to table 1 footnote 1 referrer

Table 1 footnote 2

ppt is parts per trillion, and represents 0.000000000001 grams of total PCDD/Fs per gram of sample (edible portion)

Return to table 1 footnote 2 referrer

Table 1 footnote 3

Compounds used to calculate TEQ = PCB: 77, 105, 114, 118, 126, 156, 157,167, 169, 189, PCDD: 2,3,7,8-TCDD; 1,2,3,7,8-PeCDD; 1,2,3,4,7,8-HxCDD; 1,2,3,6,7,8-HxCDD; 1,2,3,7,8,9-HxCDD; 1,2,3,4,6,7,8-HpCDD; OCDD; PCDF: 2,3,7,8-TCDF; 1,2,3,7,8-PeCDF; 2,3,4,7,8-PeCDF; 1,2,3,4,7,8-HxCDF; 1,2,3,6,7,8-HxCDF; 2,3,4,6,7,8-HxCDF;1,2,3,7,8,9-HxCDF; 1,2,3,4,6,7,8-HpCDF;1,2,3,4,7,8,9-HpCDF; OCDF

Return to table 1 footnote 3 referrer

Char Farmed 6 0.35 0.19, 0.63 0.08 0.53 0.51
Wild 5 0.31 0.16, 0.62 0.09 0.66 0.46
Oysters Farmed 12 1.2 0.79, 1.9 0.17 8.2 0.22
Wild 4 0.78 0.37, 1.7 0.22 0.96 0.12
Salmon Farmed 19 0.93 0.65, 1.3 0.21 2.8 1.1
Wild 3 0.52 0.21, 1.2 0.16 0.5 0.33
Shrimp Farmed 13 0.37 0.25, 0.57 <MDL 1.4 0.06
Wild 4 0.66 0.31, 1.4 <MDL 5.8 0.09
Tilapia Farmed 15 0.39 0.26, 0.57 <MDL 1.7 0.08
Wild 3 0.31 0.13, 0.74 <MDL 0.32 0.11

Mercury

Mercury is ubiquitous in the environment and can be found in soil, rocks and also in lakes, streams and oceans. It can also be released into the environment by human activities such as pulp and paper processing, mining operations, and through the burning of garbage and fossil fuels.

Inorganic mercury in the aquatic environment (e.g., lakes, streams and oceans) can be transformed by bacteria to organic mercury, such as methylmercury. Most human exposure to mercury is through the consumption of fish since methylmercury binds tightly to the proteins in fish tissue and tends to concentrate or bioaccumulate in the food chain. Therefore, predatory fish species that are higher in the food chain tend to have higher levels of methylmercury than fish species that are lower in the food chain. The levels of mercury and methylmercury in the food supply are monitored since children and developing fetuses are particularly sensitive to the known neurotoxic effects of methylmercury.

Mercury was found in all the tested fish and seafood products. Total mercury levels in crab, salmon, and oysters (Table 4) were well below the Canadian standard of 0.5 ppm for total mercury. One of the canned tuna samples contained 0.587 ppm total mercury, slightly above the Canadian standard for total mercury. Average total mercury levels in the predatory fish samples were all above 0.5 ppm, ranging from 0.930 ppm for fresh/frozen tuna to 1.820 ppm for swordfish. Average methyl mercury levels in the predatory fish ranged from 0.489 ppm for marlin to 1.080 ppm for swordfish.

It should be noted that at the time of conducting this survey (sample pick-up and analysis), three of the predatory fish (swordfish, shark, and fresh/frozen tuna) were exempted from the Canadian standard of 0.5 ppm for total mercury in commercial fish. For these exempted species, Health Canada had issued a consumer advisory aiming at enhanced protection of more susceptible consumers, namely children, pregnant women and women of child bearing age, from potential risks associated with food borne mercury.

The results stemming from this study, along with data generated through CFIA's inspection and monitoring programs supported the planned review and revision of the risk management strategy for mercury in fish. This study provided valuable data on levels of mercury in fish available at the retail in Canada. This study and its results have been peer reviewed and published in the scientific literature (Dabeka et al., 2004; Forsyth et al., 2004)

Table 4: Contaminants in Retail Fish and Seafood Products: Summary Statistics for Total Mercury and Methylmercury Levels (results expressed in ppm )
Species Total mercury concentration (ppm) Methylmercury concentration (ppm)
Number of samples tested Mean Minimum Maximum Number of samples tested Mean Standard Deviation Minimum Maximum
ppm is parts per million, and represents 0.000001 grams of mercury per gram of sample (edible portion)
Swordfish 10 1.820 0.400 3.845 10 1.080 0.576 0.300 2.346
Shark 13 1.260 0.870 2.729 12 0.849 0.399 0.285 1.538
Marlin 4 1.430 0.340 3.192 3 0.489 0.349 0.212 0.881
Tuna, canned 39 0.150 0.020 0.587 37 0.098 0.090 0.009 0.411
Tuna, fresh or frozen 13 0.930 0.077 2.121 13 0.662 0.436 0.061 1.319
Crab, Dungeness 8 0.059 0.036 0.090  
Salmon 22 0.040 0.016 0.063  
Oysters 27 0.011 0.004 0.028  
Shrimp 19 0.023 0.007 0.060  

References

Dabeka, R.; McKenzie, A.D.; Forsyth, D.S.; Conacher, H.B.S. (2004) Survey of total mercury in some edible fish and shellfish species collected in Canada in 2002. Food Additives and Contaminants, vol. 21, No. 5, pp. 434-440.

Forsyth, D.S.; Casey, C.; Dabeka, R.W.; McKenzie, A. (2004) Methylmercury levels in predatory fish species marketed in Canada. Food Additives and Contaminants, Vol. 21, No. 9, pp. 849-856.

Rawn, D.F.K.; Forsyth, D.S.; Ryan, J.J., Breakell, K.; Verigin, V.; Nicolidakis, H.; Hayward, S.; Laffey, P.; Conacher, H.B.S. (2006) PCB, PCDD and PCDF residues in fin and non-fin fish products from the Canadian retail market 2002. Science of the Total Environment, Vol. 359, pp. 101-110.

Tittlemier, S.A. ; Forsyth, D.; Breakell, K.; Vergin, V.; Ryan, J.J.; Hayward, S. (2004) Polybrominated diphenyl ethers in retail fish and shellfish samples purchased from Canadian markets. J. Agric. Food Chem., Vol. 52, pp. 7740-7745.