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MERCURY CONCENTRATION IN SOME SPECIES OF FRESH WATER FISH OF BANGLADESH

INTRODUCATION

Mercury is one of the most toxic elements to man and other animals. In the last two decades, measurement and evaluation of mercury in environment has received much attention. The presence of high concentration of mercury in river and marine organisms is well–documented (Lee and Hultberg, 1990: Bjorklund et al., 1981: Akielazek and Haines, 1981 and Bacci, 1989). Investigatiions of heavy metals in aquatic ecosystems have been well documented in Europe, Australia and North America, but few studies are available about the levels of contamination, particularly that of Hg in the fishes from rivers, and of the Bay of Bengal. Sharif et al., (1991, 1993a, 1993b) reported level of few metals including Hg in some rivers and marine fishes of the Bay of Bengal.

In the rivers and sea, mercury comes mainly from weathering, sewage disposal and industrial wastes. From discharges of Chloro-alkali industries of Chittagong and Sylhet, through the rivers,the Karnaphuli

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and the Surma, mercury comes to the Bay of Bengal. Nearly, one hundred and eighty tons of Hg is introduced every year into the Indian environment alone (Chaudhury, 1980). Yearly 2.4 billion tons of sediments (Holeman, 1968; Coleman, 1969 and Khan, 1978) are carried by the river system of Bangladesh. Thus the high mercury level in the Bay of Bengal may be due to direct disposal of industrial wastes or solid wastes dumped into the rivers of the subcontinent.

An  estimated value of Hg content in daily diet varies from 5 to 20 mg and may reach up to 100- 300 mg in coastal areas (Sharif et al., 1993b). Fish can accumulate both organic and inorganic mercury in its metabolic system and they are the natural sources of Hg to human being (Chovjka and Williams, 1980). The total intake of Hg from other sources is negligible ( Clarkson, 1984; Turner  et al., 1980).

People living to the coastal areas are directly or indirectly dependent on the resources of the sea. The main objective of the present study is to know the level of mercury in the tropical fresh water fishes of rivers, and from other water sources of Bangladesh, and to compare whether it is safer or not to health.

MATERIALS AND METHODS

Samples of Collection

Fish samples - Tenualosa Ilisha,Pangasius pangasius, Septinna phasa, Neopropius atherinoides,Otolithoides pama , Puntius sarana,Ompok pabda, Sperata aor, Clarias batrachus, Heteropneustes fossil ,Labeo rohita, and Catla catla were collected from different fish markets of Dhaka, Commilla,Chandpur,Chittagong,Sylhet,and  Khulna. Special precautions were taken during their collection and preservation in containers so that these specimens were kept in good condition. Their weights and sizes were recorded. The samples were cleaned and washed with demineralized water. Only the flesh samples after removal internal organs, head, skin and tails, were taken out from the body. These samples were sun-dried .The dried samples were then chopped into pieces with the help of a stainless steel knife (steam cleaned). Only the edible muscle tissue samples were used for analysis.  The samples pieces were dried at 105-110oC in an oven until a constant weight was obtained (dry weight) .The dried samples were ground, sieved and thoroughly mixed in a stainless steel rotating drum for 100 hours to produce homogeneous powders .These powder samples were finally preserved in clean and dry analysis  polythene bottles prior to analysis.  All the chemicals and reagents were of analytical grade.

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Irradiation

Portions of the samples (200-300 mg each) were thermally sealed in polyethylene bags and  irradiated for 15 hours along with a known amount of MA-A-2™, the fish flesh homogenate standard of IAEA (International Atomic Energy Agency)  in the TRIGA MARK II  reactor at AERE, Savar, Dhaka at a flux of about (0.5 to 1.0) x1012 n.cm-2.s-1.

Radiochemical Separation

 

The process for dissolution and chemical separation was the same as the process described

elsewhere (Sharif et al .1993b).

Counting

 

The samples and the standard were counted on a 125cm3 HPGe detector connected to a 4096 pulse height analyzer  PCA II MCA card. The energies (in keV) chosen for the evaluation of the peak areas were                                                            [197Hg: t1/2 (65 h)] Eγ = 69 and 77.3 keV. The minimum detection limit was one nanogram for mercury

for the total sample volume. The counting errors at the detection limits were around 7% .The blank polyethylene bag did not show any measureable mercury content.

Accuracy and Precision

Experiments were initially carried out using a radioactive tracer and the corresponding carrier to evaluate the recovery. The accuracy of the method was evaluated by analyzing the homogenate fish flesh (IAEA)

Standard Reference Material, MA-A-2 (TM). The mercury concentration was 0.45± 0.06-µg g-1 and this is in good agreement with the IAEA certified value (0.47±0.02 µg g-1).

RESULTS AND DISCUSSION

The concentration of mercury found in the samples of  fish has been shown in Table 1. Mercury concentration was observed at the level 0.243  to 0.675 µg g-1 in these samples. The highest concentration (0.582-0.836±0.127 µ g g-1) is found in Setipinna phasa and the lowest (0.250-0.260±0.004 µg g-1) Neopropius atherinoides. It is observed that the level of Hg in fishes of fresh water origin is much lower than that in other countries. A. comparison of mercury concentrations in some fresh water fishes available in Bangladesh and West Bengal (India) (µg g-1 dry weight basis) is shown in Table 2. A relationship between the amount of mercury (µg g-1) and the number of fishes is shown in Figure 1.

 

 

Though the liver and kidney are usual organs for major concentrations of metals but mercury is accumulated at higher level only in the muscle of fish (Peterson et al., 1973) and in its case biomagnifications occurs through food chain.

The permissible limit varies a little bit from country to country. One proposed by FDA (Food and Drug Administration) of the United States is 500 ppb. The mercury level in the present investigation is much lower than this value. The trend of increasing concentration of Hg with increases in size of fish indicates long time of its conservation in fish.

Estimation of Dietary Intake of Hg in Bangladesh

Average concentration of Hg found in the fish samples of various sources of fresh water, is 0.362 µg g-1. Assuming Maximum average of fish consumption person-1 day -1 is 6-10 g dry fish for Chittagong and

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a coastal area of Bangladesh, the maximum Hg intake through fish is estimated to be 2.172-3.62 µg   person-1 day-1 . This value is well below the maximum acceptable level for human being (0.5 µg g-1 wet weight) (WHO, 1976: FAO/WHO, 1972-1987: Nauen, 1983: Denton et al. ,1996: Hakanson et al.,1988.

Similarly, 70 g dry per week would result in an intake of 25.34 µg of Hg which is also below the tolerable limit (7.0 µg  kg-1 body wt.day-1 ) of an about 70 kg man as suggested by WHO (1976).

CONCLUSION

The level of mercury reported here may be taken as a convenient baseline for determining future pollution trends. Low Hg content observed in this study indicates that water from different sources has pollution free and clean environment. So there is no danger to have these fishes for a Bangladeshi person.

Acknowledgement

The authors express thanks to Mr.K.R.Krishnamorti,  Analytical Chemistry Division, Bhava Atomic Research Center, Trombay, India, for using the method, the radiochemical separation of trace metals in biological samples, used in that Laboratory

 

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REFERENCES

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FAO/WHO (1972-1987), Joint FAO/WHO Expert Committee on Food Additives, Reports 505, 631, 696 and 751. World Health Organization, Geneva.

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