Hg levels in seafood
In the 518 seafood samples analyzed, the median and mean concentrations of THg were 24.6 ng/g and 37.2 ng/g, respectively (Table 3). The THg concentrations were highly variable among different seafood groups, ranging from 1.02 to 317 ng/g in fish, 0.11 to 55.4 ng/g in shrimp, 9.45 to 107 ng/g in crabs, and 1.84 to 53.3 ng/g in mollusks. The THg concentrations in fish (p < 0.001) and crabs (p < 0.001) were significantly higher than in mollusks, but no significant difference of THg concentrations was observed between shrimp and mollusks (p > 0.05). It is suggested that bioaccumulation of THg in seafood products is highly species-specific, probably due to their different ecological characteristics, such as feeding habits and habitats.
Table 3. Statistics summary of total mercury concentrations in different seafood types (ng/g)
The comparison of THg concentrations in different functional groups of fish is presented in Figure 2. In the present study, fish species were categorized by food preferences into three ecological functional groups (carnivorous, omnivorous, and herbivorous fish). The averages of THg concentrations in each functional group of fish were 67.6, 7.82, and 23.7 ng/g for carnivorous, omnivorous, and herbivorous fish, respectively. The THg concentrations in carnivorous (p < 0.001) and herbivorous fish (p < 0.01) were significantly higher than that in omnivorous fish. The results demonstrate Hg bioaccumulation in species with higher trophic levels 25, 26. But the two species of omnivorous fish (Tilapia and Mugil cephalus) had very low THg concentrations, with the means of 7.81 and 7.97 ng/g, respectively.
Figure 2. Comparison of total mercury (THg) concentrations in fish with different functional groups. Each box represents interquartile range (25th to 75th percentile). The horizontal band near the middle of the box is the 50th percentile (the median), and the whisker represents the 5th and 95th percentiles. *** p < 0.001, and ** p < 0.01, compared with omnivorous fish.
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According to the cultured styles, the fish species were divided into three groups (FWA, MC, and OF). The averages of THg concentrations were 34.7, 65.1, and 65.9 ng/g for FWA, MC, and OF fish, respectively. Two-way ANOVA (two factors: functional group and cultured style) was used to test the difference of THg concentrations in fish tissues. Interaction was not observed between the two factors. The THg concentrations in MC (p < 0.01) and OF fish (p < 0.01) were significantly higher than that from FWA fish. Compared with OF fish, significantly higher THg concentrations were also found in MC fish (p < 0.05; Fig. 3). The results indicated that the cultured style had significant effects on THg concentrations in the fish. The degradation of food supplies and fish excretion can produce anoxic conditions at the sediment/water interface, which favor Hg methylation; but these organic matters can potentially be complex Hg and reduce Hg bioavailability to the methylating bacteria 26. In fieldwork conducted in Hong Kong, Liang et al. 21 found that mariculture increased Hg loading from uneaten fish feed and fish excretions, whereas organic matter inhibited Hg methylation in surface sediments.
Figure 3. Comparison of total mercury (THg) concentrations in fish with different cultured styles. Each box represents interquartile range (25th to 75th percentile). The horizontal band near the middle of the box is the 50th percentile (the median), and the whisker represents 5th and 95th percentiles. a = p < 0.01, compared with freshwater aquaculture; b = p < 0.05, compared with ocean fishery.
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Regarding the regional difference, the averages of THg concentrations in the fish collected from the western area of the province (Zhanjiang, Maoming, and Yangjiang) were higher than that from Pearl River Delta and the eastern area (Fig. 4). The THg concentrations in the fish collected from Zhanjiang, Maoming, and Yangjiang were significantly higher (p < 0.05) than that in fish collected from Dongguan. Most fish from the western areas were carnivorous fish, which may explain the high THg concentrations in the fish tissues.
Figure 4. Comparison of total mercury (THg) concentrations in the seafood samples from different cities. Each box represents interquartile range (25th to 75th percentile). The horizontal band near the middle of the box is the 50th percentile (the median), the square near the middle of the box is the mean, and the whisker represents the 5th and 95th percentiles. ZJ = Zhanjiang; MM = Maoming; YJ = Yangjiang; DG = Dongguan; FS = Foshan; GZ = Guangzhou; JM = Jiangmen; ZH = Zhuhai; ZS = Zhongshan; SW = Shanwei; ST = Shantou.
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The relationship between THg concentrations and fish length is well established, with larger fish tending to accumulate substantial Hg concentrations 27, 28. However, this phenomenon was not observed in the present study. There were no significant correlations between THg concentrations in fish and the fish length (Fig. 5) and body weight (Fig. 6). Previous studies have confirmed that this relationship can be confounded by other variables, such as changes in growth rate, diet, and activity 29. Most of the fish in the present study were cultured fish, and the relatively high growth rate might have a dilution effect on THg concentration in the fish muscle 30. Moreover, the main Hg source for cultured fish is food ingestion, and fish do not have competitive trophic position in the same aquaculture pond 22.
The THg concentrations in the seafood samples in the present study were relatively low compared with those reported in United States and Canada (Table 4). No fish samples exceeded the recommended maximum THg limit for human consumption (0.5 µg/g wet wt) set by the World Health Organization and the Standardization Administration of China. The results in the present study were comparable to most studies in China and were lower than these from Zhoushan Island 23 and Hg-contaminated Ya-Er Lake 38. Although China is considered to be the largest source of anthropogenic Hg emission 10, and serious Hg pollution problems were found in local environments 43, 44, the THg concentrations in the fish samples showed rather low levels. This may be due to the specific aquaculture characteristics, young age, high growth rate, and substantially short and simple aquatic food chains 45 of the samples collected for the present study.
Table 4. Comparison of total Hg concentration in fish from China (ng/g wet wt)
|Study area||n||Mean (range)||Reference|
|12 provinces|| ||18.5 (4.77–46.0)||20|
|Pearl River Delta, Guangdong Province||122||7.43–76.7||21|
|Zhoushan Island, Zhejiang Province||148||260 (20–660)||23|
|Changchun, Jilin Province|| ||41 (10–126)||31|
|Hong Kong||280||63 (3–1370)||32|
|Three Gorges Reservoir, Central China||74||37.8–249||33|
|6 reservoirs, Guizhou Province||235||66 (2.0–445)||34|
|Hongjiadu Reservoir, Guizhou Province||65||44 (10–170)||35|
|Mercury-polluted Songhua River, Northeast China||111||93 (6.0–295)||36|
|Mercury-polluted Di'er Songhua River, Northeast China||186||90 (2.0–660)||37|
|Mercury-polluted Ya-Er Lake, Hubei Province||40||400 (23.6–1360)||38|
|Beijing City||32||18.9 (2.17–77.6)||39|
|Qingdao, Shandong Province||102||96.2 (49.0–150)||40|
|U.S. fish and shellfish with highest levels of Hg (mackerel king, shark, swordfish, and tilefish)|| ||730, 979, 995, 1450||41|
|U.S. fish and shellfish with lower levels of Hg (anchovies, butterfish, catfish, etc.)|| ||From 3 to 128 for each fish||41|
|U.S. other fish and shellfish (bass, carp, tuna, etc.)|| ||From 93 to 689 for each fish||41|
|Canada, regularly consumed fish (barracuda, cod, shark, tuna, etc.)|| ||From 50 to 1,820 for each fish||42|
|11 coastal cities, Guangdong Province||327||48.6 (1.02–317)||The present study|
Dietary intake and risk assessment
The average daily intake of seafood was 132 g/d for individuals in the present study (Table 2), which was about twice that for Guangdong urban residents (61.5 g/d) and about three times the national average for urban residents (42.3 g/d). The exceptionally large fish consumption may be related to the geographic location of the study area, because coastal residents generally eat a substantial amount of fish.
Because of the high variability of THg levels among individual seafood samples, the median concentrations in each group (fish, shrimp, crabs, and mollusks) were used for health risk assessment. The PDI50 of MeHg via seafood consumption ranged from 42.6 to 71.4 ng/kg/d for different age groups (Table 5). The daily intake of seafood was highest among male children and teenagers (6–17 years old). This may be due to the relative low body weight and large seafood consumption. Therefore, based on the results of the present study, the sensitive subgroup should avoid eating carnivorous fish with high THg concentrations.
Table 5. The median probable daily intake (PDI50) of MeHg via seafood consumption for different age groups (ng/kg/d)
|Age and gender||FWA||MC||OF||S&C||Mo||Total|
The PDIs of MeHg from seafood consumption (0.054 µg/kg/d) in the present study were higher than PDIs in previous reports on MeHg exposure from fish consumption in 12 provinces in China (0.006 µg/kg/d 20), on U.S. adult women (0.02 µg/kg/d 46), and on the Korean population (0.03 µg/kg/d 47), but were comparable with those in reports on Hong Kong residents (0.053 µg/kg/d 21), on Southeast Asia populations (0.027–0.097 µg/kg/d 48), and on a Norwegian group (0.058 µg/kg/d, with a range of 0.028–0.18 µg/kg/d; G. Mangerud, 2005, Master's thesis, Nordic School of Public Health, Goteborg, Sweden]). The PDIs were lower than those reported in Japanese populations with frequent fish consumption (0.14 µg/kg/d 49).
Regarding the relative contribution of each seafood group to the PDIs of MeHg (Fig. 7), fish contributed the majority (with an average of 84%), followed by shrimp and crab (with an average of 12%). The high contribution from fish mainly resulted from its relatively high consumption rate and high THg concentrations compared with other seafood groups.
Figure 7. The relative contribution of each seafood group to median probable daily intakes (PDI50) of MeHg in different age groups. Mo = mollusk; S&C = shrimp and crab; OF = ocean fishery; MC = mariculture; FWA = freshwater aquaculture; M = male; F = female. [Color figure can be seen in the online version of this article, available at wileyonlinelibrary.com.]
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For different age groups, the HQ was used to evaluate the exposure risks to MeHg through seafood consumption (Table 6). The HQ in all age groups (PDI50/PTWI and PDI95/PTWI) were less than 1, which indicates that MeHg exposure through seafood consumption may not lead to adverse health effects. However, taking the PTWI of 230 ng/kg/d as the boundary, approximately 2.9% of the population (about 1.58 million) in these 11 coastal cities in Guangdong Province, South China, had a PDI that exceeded the JECFA guideline. Thus, potential health risks are associated with seafood consumption for the general population in the coastal area of Guangdong, South China. The more sensitive populations, such as young children and pregnant women, should avoid eating carnivorous fish with high THg concentrations.
Table 6. Hazard quotients of MeHg exposure through seafood consumption for different age groups