The Science of the Total Environment, 106 (1991 ) 71-82 Elsevier Science Publishers B.V., Amsterdam
71
Mercury in some commercial fish from Kuwait: a pilot study Amer H. A1-Hashimi" and Mazin A. Al-Zorba h ~'Environmental Health Department, College of Health Sciences, P.O. Box 33496. Rawdha 73455, Kuwait bEnvironmental Science Department, KISR. P.O. Box 24885, Sq[at, Kuwait
ABSTRACT We investigated the total mercury content in the muscles, liver and gonads of eight commercial fish from Kuwait. Mercury concentrations in the muscle of all the fish were below the action level of 1 pg g 1 (wet weight) set by the United States Food and Drug Administration. However, large specimens of Epinephelus tauvina (Hamoor) and Lutjanus Coccineus (Hamrah) showed concentrations which were close to the action level. The relationship between mercury concentration and fish age and length was in the form of an exponential correlation ( Y = aXh), in which concentration increases with an increase in fish age or length. Correlation coefficients obtained ranged between 0.65 and 0.86.
INTRODUCTION
Mercury reaches the marine environment from various sources, with most accumulating in the sediments (Kothny, 1973; F6rstner and Muller, 1974); there is a distinct gradient of accumulation with higher concentrations found towards the surface (Young et al., 1973). Plankton, the first link in the aquatic food chain, take up and concentrate both inorganic and alkylated mercury compounds by direct assimilation from the adjacent medium (Klein and Goldberg, 1970; Fujita and Hashizume, 1972). Higher trophic levels feed upon these organisms thereby contributing to biological magnification from algae (which have a mercury concentration of 0.001-0.18/~g g- I ) to predators such as pike, tuna and shark (which have a mercury concentration of 0.015.82/~g g- ~) (Hamilton, 1971). In fish, concentration factors of 5000 to 100000 have been reported (Johnels et al., 1967; Rucker and Amend, 1969); fish have such high levels because they take up mercury both by ingestion and from the adjacent water. The highest concentrations of mercury in fish are found in the liver, kidney and muscle (Hannerz, 1968; Rucker and Amend, 1969). The mechanism of
72
A . H A L - H A S H I M I A N D M.A. A L - Z O R B A
accumulation is not clear, but may be a function of the metabolic rate of individual fish, differences in selection of food as the fish matures, or the epithelial surface area of the fish (Hannerz, 1968; Wobeser et al., 1970). Major concern has been expressed regarding the concentration of mercury in fish (Peterson et al., 1973); however, it is difficult to give an average level for all fish, because the concentration of mercury varies significantly depending upon the species of fish and its diet, age, size, and location in its environment. The Joint FAO/WHO (1972) committee stated that 99% of the world's fish catch has a total mercury content not exceeding 0.5 # g g - I , and that 95% probably contains < 0.3ktgg -I. Swordfish is the only species of ocean fish which has been banned from US markets, now being sold only after individual fish certification (Anon., 1971). MATERIALS AND METHODS
Sampling Fish samples were obtained primarily from local fish markets (Kuwait fish market and Fahaheel fish market). However, some samples were collected by trawling from KISR's research vessel R.V. Oloum. Collections were made between April and August 1983 as part of KISR's Oil Slick Task Force/ Environmental Monitoring Group Activities. The following fish species were sampled: Species name
Common name
Local name
Lutjanus coccineus Epinephelus tauvina Pampus argenteus Otolithus argenteus Pomadasys argenteus Platycephalus indicus Lethrinus spp. Pseudorhombus arsius
Red snapper Brown spotted grouper Silvery pomfret Silvery croaker Silvery grant Indian flathead Emperor Large-toothed flounder
Hamrah Hamoor Zobaidy Newaiby Nakroor Wahar Sharee Khofa'ah
Biological observations and measurement were made, viz. total length, fresh weight and sex. The ages of the fish were assessed (except for Zobaidy and Khofa'ah) by measuring the otolith bones; fish were selected so that a range of sizes was available for each species. Muscle was sampled from fresh and frozen specimens (half-thawed before dissection). Liver and gonad samples were collected from fresh specimens only. Prior to dissection, samples were rinsed carefully with water to remove any adhering particles.
MERCURY IN COMMERCIAL FISH FROM KUWAIT
73
Sample preparation Guidelines for the F A O ( G F C M ) / U N E P Joint Co-ordinated Project on Pollution in the Mediterranean were published by Bernhard (1976). The procedure described for the preparation of tuna and swordfish was followed during sample preparation and dissection in this study. All samples were freeze-dried for 40-48 h. They were then ground to a fine powder using an automatic grinder fitted with agate mortar and pestle. Ground samples were stored in sealed plastic cups until required. Dried samples of between 0.5 and 0.8 g were digested in a 2 : 1 (v/v) mixture of sulphuric and nitric acids. To each sample, after 2-3 h digestion, a freshly prepared 5% potassium permanganate solution was added in excess until the purple colour persisted for at least 15 min. To complete the oxidation, 2 ml of 5% potassium persulfate solution was added to each sample. Reduction of the excess potassium permanganate was carried out by adding 10ml of 12% sodium chloride-hydroxyl-ammonium chloride solution. Final volumes were made up to 70 ml by adding distilled water. Aliquots (10ml) of the final solutions were analyzed for total mercury. A constant volume of 10% stannous chloride solution was added automatically to liberate the mercury from solution. The analysis was conducted following the cold vapour technique (measuring the absorbance) using the mercury hydride system MHS-20 attached to a Perkin-Elmer Model 5000 atomic absorption spectrophotometer. Concentrations were calculated by plotting absorbance against a calibration curve obtained by analysis of standard solutions (0, 0.01 and 0.02 ppm). The system was autozeroed on distilled water, and a reagent blank was measured with every batch of samples. Absorbance readings of the reagent blanks were subtracted from the absorbance reading of the samples. Standard reference material of known mercury concentration (NBS Oyster Tissue, SRM-1566) was prepared and analyzed for mercury with every batch of samples. The average concentration obtained for NBS Oyster Tissue was 0.061 _ 0.05 ppm compared with the certified value of 0.057 +_ 0.015 ppm. Recovery tests were carried out for two concentrations (0.01 and 0.02 ppm) on two samples: the NBS Oyster Tissue and fish muscle. The recovery was found to be 91-108% for the 0.01 p p m standard and 90-102% for the 0.02 ppm standard. By applying the standard addition method, the concentration of mercury in NBS Oyster Tissue was found to be 0.094 ppm, whereas the concentration of mercury in the fish muscle sample was found to be 0.25 #g g '. All mercury values were in terms of a weight-specific concentration (pg/g dry weight). Mean values ()?) and ranges of age and total length for each fish species were calculated. Data for mercury concentration in fish muscle were analyzed using the
74
A H . AL-HASHIMI AND M.A. Ak-ZORBA
family regression program [HP Statistical Library 1/General Statistics Vol. 1 (09825-15004)] to obtain the regression coefficients (a and b) and the correlation coefficient (r). The programme fits "exponential" curves by transforming the Xand/or Ydata to fit the simple linear model, Y = a + b X . Among the eight different models (curves) obtained from running the program, the model Y = a X h was selected since it constantly gave the best r values for all species and for both variables (i.e., age and total length). RESULTS
The mean values and ranges for age, total length and fresh weight for each fish species are given in Table 1. The average mercury concentrations in the various organs of the fish species studied are listed in Table 2. Attempts were made to correlate the mercury concentration in fish muscle with the age and/or length of fish. Mercury and fish age
When the correlations of mercury concentrations in fish muscle with age of
TABLE 1 Mean values ()() and ranges (R) of age, total length and fresh weight for the various fish species Fish
Red snapper Brown spotted grouper Silvery pomfret
Biological parameters
)? R )? R )(
Age (years)
Total length (cm)
Fresh weight (kg)
8 (2-19) 8 (4-18)
56 (36-72) 69 (47-90) 21 (16-28) 37 (26-48) 56 (52-61) 37 (30-53) 49 (30-60) 27 (23-34)
2.81 (0.65-5.43) 5.43 (1.26-13. I 0) 0.38 (0.15-0.77) 0.53 (0.17-1.06) 2.39 (1.74-3.28) 0.38 (0.12-1.18) 1.67 (0.40-2.97) 0.27 (0.14-0.40)
R
Silvery croaker Silvery grant Indian flathead Emperor Large toothed flounder
)(
2
R
(1-4)
)7 R )? R )~ R )(
7 (6-9) 3 (2-6) 7 (4-10)
R
Number
30 30 30 30 10 10 10 10
MI!RCUR~ IN COMMERCIAL FISH FROM KUWAIT
75
TABLE 2 Average mercury concentrations in tissues of various fish Irish
Mercury concentration (pgg ~ dry wt) Muscle
Red snapper
Brown spotted grouper
)7" _+ SD
1.20 + 1.60
R
0.10-6.30
N Ave. % DM )( ± SD R
Silvery pomfret
N Ave. % DM ± SD R
Silvery croaker
Silvery grant
N Ave. % DM )( _+ SD R N Ave. % DM )( _+ SD R
Indian flathead
Emperor
N Ave. % DM )( ± SD R N Ave. % DM )7 _+ SD R
Large toothed flounder
N Ave. % DM )? _+ SD R N Ave. % DM
30 21.3 1.80 ± 0.90 0.60-4.30 30 22.9 0.15 + 0.10 0.00-0.40 30 24.1 1.30 ± 0.75 0.40-3.60 30 21.6 0.83 _+ 0.04 0.78-0.88 10 20.2 1.60 ± 1.00 0.46-4.05 10 22.6 0.83 ± 0.04 0.76-0.88 10 16.5 0.68 ± 0.24 0.37-1.12 10 22.9
Liver
Gonad
3.80 +_ 8.40 0.10-38.20 23 27.8 1.70 + 0.10 0.1/)-4.80 30 34.7 0.20 _+ 0.17 0.00-0.50 10 26.2 1.40 +_ 0.63 0.64-2.60 7 22.9 0.67 _+ 0.39 0.20-1.40
0.90 _+ 1.40 0.00-4.60 9 21.96 0.30 _+ 0.50 0.00-2.30 20 27.4 0 0 25.4 0.14 ± 0.08 0.07-0.3 I l0 24.2 0.02 _+ 0.03 0.00-0.08
10
10
20.1
25.5
)7, mean; SD, standard deviation; R, range; N, number of samples: DM, dry matter.
the fish w e r e c a l c u l a t e d , the coefficients ( a f t e r r e g r e s s i o n a n a l y s i s a p p l y i n g the e q u a t i o n Y = a X b) w e r e 0.86, 0.68 a n d 0.65 f o r r e d s n a p p e r , b r o w n s p o t t e d g r o u p e r a n d silvery c r o a k e r , r e s p e c t i v e l y ( T a b l e 3). P l o t s o f m e r c u r y c o n c e n t r a t i o n in t h e m u s c l e v e r s u s age ( y e a r ) g a v e a n e x p o n e n t i a l c u r v e f o r e a c h o f the a b o v e - m e n t i o n e d species (Figs. 1-3) a n d
76
A H. AL-HASHIMIAND M.A. AL-ZORBA
TABLE 3 Association between mercury content of muscle and age of fish Fish
N
Conc. Hg (#g g- ~ dry wt) £
Red snapper Brown spotted grouper Silvery croaker
30 30 30
Correlation coefficient (r)
Regression coefficients (a)
(b)
0.06 0.75 0.50
1.36 0.90 1.10
SD
1.20 +__ 1.60 1.80 4- 0.90 1.30 _ 0.75
0.86 0.68 0.65
demonstrated increases in the accumulation rate of mercury in fish muscle with an increase in fish age. The accumulation of mercury was higher in red snapper than in brown spotted grouper and silvery croaker.
Mercury and fish length The correlation coefficients (r) of the mercury concentration in fish muscle versus length of the fish were 0.82, 0.68, 0.21 and 0.59 for Hamrah, Hamoor, Zobaidy and Nakroor, respectively (Table 4)
d
B~
5
5.
5
4.
5
4.
0
"a. 5
0. )-
2.
5
I.
5
I.
0
3 U E
~-~.
•
~
.
.
AG~"
.
t~ U hi
I
L~'NC;
I
TH
,c
I
~)
Fig. 6. Accumulation of mercury in Zobaidy muscle with increasing length (r = 0.21).
I
I
80
A.H. AL-HASHIMI
t,
.4..
IZl
~
5
AND
M.A. AL-ZORBA
d E O.
2
-
~
>-
0 OC b.l I
I.
'=;
1.
I~ -+-
I
~
-+-
.4-.
I
L E N G T H
(~
m)
Fig. 7. Accumulation of mercury in Nakroor muscle with increasing length (r = 0.59).
This finding is in agreement with the findings of other workers (Johnels et al., 1967a,b; Bache et al., 1971; Forrester et al., 1972; Shultz et al., 1976; Aydogdn et al., 1982). The mercury concentration of fish muscle was also correlated with fish length, and again this has been shown by other investigators (Hornung et al., 1980; Walker, 1980; Denton and Breck, 1981; Zorba et al., 1981; Aydogdn et al., 1982). The concentration of mercury in the muscle of fish was slightly lower than that in the liver, but both were considerably higher than those in the gonads. This is in agreement with other relevant works (e.g., Bryan, 1976; Zorba et al., 1981). The results of this investigation showed no significant differences in mercury concentration between females and males. Therefore, the combined mean concentration of mercury in muscles of males and females was presented. Walker (1980) stated that two factors affected the mercury concentration of fish, namely the species longevity and a carnivorous diet. Both red snapper and brown spotted grouper are known to live for 20 years or more. They also are known to be voracious and carnivorous, occupying high positions in the food chain. In spite of the long life span of silvery grant (average 7 years), the average concentration of mercury was much lower than in red snapper and brown spotted grouper (0.83~gg -~ dry weight = 0.17#gg ' weight). This may be attributed to the omnivorous feeding habit of this species (Kuronuma and
MERCURY IN COMMERCIAL FISH FROM KUWAIT
81
Abe, 1972). This is exactly the opposite in the cases of silvery croaker and Indian flathead, which showed higher mercury concentrations (0.28 and 0.36/~gg ~ wet weight, respectively), although they have shorter life spans; this finding may also be related to feeding habit since both are carnivorous fish. Among the fish species studied, silvery pomfret contained the lowest mercury concentration, probably due to its short life span (aging was not possible) and its habit of feeding on small animals (Kuronuma and Abe, 1972), so that preconcentration through the food chain is small compared with that of fish at the top of the food chain. The United States Food and Drug Administration (USFDA) set the action level of 0.05 pg g- 1wet weight mercury as the upper tolerance limit for human consumption. This is exactly the same level set by the Australian National Health and Medical Research Council (NHMRC, 1972) and the Commonwealth and Victorian Statutory limit. However, the USFDA (1979) changed the action level for mercury from 0.05 to 1.0~gg -t wet weight. The mercury concentrations in the muscle of all the fish species studied did not exceed the 1.0 ~g g-I limit set by the USFDA, although levels were close in a few cases (e.g., 0.91/~gg-1 wet weight in brown spotted grouper and 0.87/tgg -t wet weight in red snapper). The findings of the present investigation, whilst preliminary in nature, suggest that the fish species studied are only lightly contaminated with mercury. However, the total number of samples per individual species was limited. In their annual statistical book on the fisheries industry in Kuwait, the Ministry of Planning (Arabic reference) listed the silvery pomfret, brown spotted grouper, silvery grant, silvery croaker, red snapper and emperor as the most favourable fish on the market in terms of taste and price. Therefore, a detailed study on accumulation of mercury and probably other heavy metals (e.g., cadmium, lead, copper, chromium, zinc) is of utmost importance.
REFERENCES Anon., 1971. Swordfish banned as U.S. Food. F D A Food Process., 32(6): 14. Aydogdn, T., T.I. Balkas, F. Bingel, I. Salihoglu and S. Tugrul, 1982. Mercury in some benthic fish of the north Levantine (eastern Mediterranean). ICSEM/IOC/UNEP VIth Workshop Marine Pollution of the Mediterranean, Cannes, 2-4 December 1982. Bache, C.A., W.H. Gutenmann and D.J. Lisk, 1971. Residue of total mercury and methylmercuric salts in lake trout as a function of age. Science, 172: 951. Bernhard, M., 1976. Manual of Methods in Aquatic Environment Research, Part 3: Sampling and Analyses of Biological Material. F A O of the United Nations, Rome, 1976. Bryan, G.W., 1976. Heavy metal contamination in the sea. In: Johnston (Ed.), Marine Pollution. Academic Press, London, pp. 185-302.
82
A.H. A L - H A S H I M I A N D M.A. A L - Z O R B A
Denton, G.R.W. and W.G. Breck, 1981. Mercury in tropical marine organisms from North Queensland. Mar. Pollut. Bull., 12(4): 116. FAO/WHO, Joint Expert Committee on Food Additives, 1972. Evaluation of mercury, lead, cadmium, and the food additives, amaranth, dethylphrocarbonate, and octylgallate. WHO Food Addit. Ser., 4: 1. Forrester, C.R, Ketchen and C.C. Wong, 1972. Mercury content of spiny dogfish (Squalus acanthias) in the Strait of Georgia, British Columbia. J. Fish. Res. Board Can., 29: 1487. F6rstner, U. and G. Muller, 1974. Schwermetalle in flussen and seen. Springer-Verlag, Berlin. Fujita, M. and K. Hashizume, 1972. The accumulation of mercury by a freshwater planktonic diatom. Chemosphere, 1: 203. Hamilton, A., 1971. International Symposium on Mercury in Man's Environment. Royal Society of Canada, Ottawa, Canada, 15-16 Feb. 1971, p. 87. Hannerz, L., 1968. Experimental investigations on the accumulation of mercury compounds in water organisms. Rep. Inst. Freshwater Res., Sweden, 48: 120. Hornung, H., L. Zismann and O.H. Oren, 1980. Mercury in twelve Mediterranean travel fishes of Israel. Environ. Int., 3: 243. Johnels, A.G., M. Olsson and T. Westemark, 1967a. Mercury in fish: investigation on mercury levels in Swedish fish. Var Foeda, 7: 67. Johnels, A.G., T. Westermark, W. Berg, P.I. Persson and B. Sjostrand, 1967b. Pike (Esox lucius) and some other aquatic organisms in Sweden as indicators of mercury contamination in the environment. Oikos, 18: 323. Klein, D.H. and E.D. Goldberg, 1970. Mercury in the marine environment. Environ. Sci. Technol., 4: 765. Kothny, E.L., 1973. Trace Elements in the Environment. Adv. Chem. Ser., 123. American Chemical Society. Kuronuma, K. and Y. Abe, 1972. Fishes of Kuwait. Kuwait Institute for Scientific Research, State of Kuwait. National Health and Medical Research Council (NHMRC), 1972. Methyl mercury in fish: effect on human health. Australian Government Publishing Service, Canberra. Peterson, C.L., W.L. Klawe and G.D. Sharp, 1973. Mercury in tunas. A review. Fish Bull., 71: 603. Rucker, R.R. and D.F. Amend, 1969. Absorption and retention of organic mercurials by rainbow trout and chinook and sockeye salmon. Prog. Fish-Cult., 31: 197. Shultz, C.D., D. Crear, J.E. Pearson, J.B. Rivews and J.W. Hylin, 1976. Total and organic mercury in the Pacific blue marlin. Bull. Environ. Contam. Toxicol., 15: 230. US Food and Drug Administration (USFDA), 1979. Action level for mercury in fish, shellfish, crustaceans and other aquatic animals. Fed. Reg., 44(14): 3990. Walker, T.I., 1980. Mercury content of edible flesh from snapper, Chrysophrys auratus (Bloch and Schneider), in the Victorian commerical catch. Aust. J. Mar. Freshwater Res., 32: 75. Wobeser, B., N.O. Nielson, R.H. Dunlop and F.M. Atton, 1970. Mercury concentration in tissues of fish from the Sakatchewan River. J. Fish. Res. Board Can. 27(4): 830. Young, D.R., J.N. Johnston, A. Soutar and J.D. Isaacs, 1973. Mercury concentration in dated varved marine sediments collected off southern California Nature, 244: 273. Zorba, M.A., V. Anderlini and C.P. Mathews, 1981. Effect of size and weight on the trace metal content of Hamrah, Lutjanus conccineus. Annu. Res. Rep. 1981, Kuwait Institute for Scientific Research, Kuwait. Zief, M. and J.W. Mitchell, 1976. Contamination Control in Trace Analysis. Wiley, New York, Vol. 47.
71
Mercury in some commercial fish from Kuwait: a pilot study Amer H. A1-Hashimi" and Mazin A. Al-Zorba h ~'Environmental Health Department, College of Health Sciences, P.O. Box 33496. Rawdha 73455, Kuwait bEnvironmental Science Department, KISR. P.O. Box 24885, Sq[at, Kuwait
ABSTRACT We investigated the total mercury content in the muscles, liver and gonads of eight commercial fish from Kuwait. Mercury concentrations in the muscle of all the fish were below the action level of 1 pg g 1 (wet weight) set by the United States Food and Drug Administration. However, large specimens of Epinephelus tauvina (Hamoor) and Lutjanus Coccineus (Hamrah) showed concentrations which were close to the action level. The relationship between mercury concentration and fish age and length was in the form of an exponential correlation ( Y = aXh), in which concentration increases with an increase in fish age or length. Correlation coefficients obtained ranged between 0.65 and 0.86.
INTRODUCTION
Mercury reaches the marine environment from various sources, with most accumulating in the sediments (Kothny, 1973; F6rstner and Muller, 1974); there is a distinct gradient of accumulation with higher concentrations found towards the surface (Young et al., 1973). Plankton, the first link in the aquatic food chain, take up and concentrate both inorganic and alkylated mercury compounds by direct assimilation from the adjacent medium (Klein and Goldberg, 1970; Fujita and Hashizume, 1972). Higher trophic levels feed upon these organisms thereby contributing to biological magnification from algae (which have a mercury concentration of 0.001-0.18/~g g- I ) to predators such as pike, tuna and shark (which have a mercury concentration of 0.015.82/~g g- ~) (Hamilton, 1971). In fish, concentration factors of 5000 to 100000 have been reported (Johnels et al., 1967; Rucker and Amend, 1969); fish have such high levels because they take up mercury both by ingestion and from the adjacent water. The highest concentrations of mercury in fish are found in the liver, kidney and muscle (Hannerz, 1968; Rucker and Amend, 1969). The mechanism of
72
A . H A L - H A S H I M I A N D M.A. A L - Z O R B A
accumulation is not clear, but may be a function of the metabolic rate of individual fish, differences in selection of food as the fish matures, or the epithelial surface area of the fish (Hannerz, 1968; Wobeser et al., 1970). Major concern has been expressed regarding the concentration of mercury in fish (Peterson et al., 1973); however, it is difficult to give an average level for all fish, because the concentration of mercury varies significantly depending upon the species of fish and its diet, age, size, and location in its environment. The Joint FAO/WHO (1972) committee stated that 99% of the world's fish catch has a total mercury content not exceeding 0.5 # g g - I , and that 95% probably contains < 0.3ktgg -I. Swordfish is the only species of ocean fish which has been banned from US markets, now being sold only after individual fish certification (Anon., 1971). MATERIALS AND METHODS
Sampling Fish samples were obtained primarily from local fish markets (Kuwait fish market and Fahaheel fish market). However, some samples were collected by trawling from KISR's research vessel R.V. Oloum. Collections were made between April and August 1983 as part of KISR's Oil Slick Task Force/ Environmental Monitoring Group Activities. The following fish species were sampled: Species name
Common name
Local name
Lutjanus coccineus Epinephelus tauvina Pampus argenteus Otolithus argenteus Pomadasys argenteus Platycephalus indicus Lethrinus spp. Pseudorhombus arsius
Red snapper Brown spotted grouper Silvery pomfret Silvery croaker Silvery grant Indian flathead Emperor Large-toothed flounder
Hamrah Hamoor Zobaidy Newaiby Nakroor Wahar Sharee Khofa'ah
Biological observations and measurement were made, viz. total length, fresh weight and sex. The ages of the fish were assessed (except for Zobaidy and Khofa'ah) by measuring the otolith bones; fish were selected so that a range of sizes was available for each species. Muscle was sampled from fresh and frozen specimens (half-thawed before dissection). Liver and gonad samples were collected from fresh specimens only. Prior to dissection, samples were rinsed carefully with water to remove any adhering particles.
MERCURY IN COMMERCIAL FISH FROM KUWAIT
73
Sample preparation Guidelines for the F A O ( G F C M ) / U N E P Joint Co-ordinated Project on Pollution in the Mediterranean were published by Bernhard (1976). The procedure described for the preparation of tuna and swordfish was followed during sample preparation and dissection in this study. All samples were freeze-dried for 40-48 h. They were then ground to a fine powder using an automatic grinder fitted with agate mortar and pestle. Ground samples were stored in sealed plastic cups until required. Dried samples of between 0.5 and 0.8 g were digested in a 2 : 1 (v/v) mixture of sulphuric and nitric acids. To each sample, after 2-3 h digestion, a freshly prepared 5% potassium permanganate solution was added in excess until the purple colour persisted for at least 15 min. To complete the oxidation, 2 ml of 5% potassium persulfate solution was added to each sample. Reduction of the excess potassium permanganate was carried out by adding 10ml of 12% sodium chloride-hydroxyl-ammonium chloride solution. Final volumes were made up to 70 ml by adding distilled water. Aliquots (10ml) of the final solutions were analyzed for total mercury. A constant volume of 10% stannous chloride solution was added automatically to liberate the mercury from solution. The analysis was conducted following the cold vapour technique (measuring the absorbance) using the mercury hydride system MHS-20 attached to a Perkin-Elmer Model 5000 atomic absorption spectrophotometer. Concentrations were calculated by plotting absorbance against a calibration curve obtained by analysis of standard solutions (0, 0.01 and 0.02 ppm). The system was autozeroed on distilled water, and a reagent blank was measured with every batch of samples. Absorbance readings of the reagent blanks were subtracted from the absorbance reading of the samples. Standard reference material of known mercury concentration (NBS Oyster Tissue, SRM-1566) was prepared and analyzed for mercury with every batch of samples. The average concentration obtained for NBS Oyster Tissue was 0.061 _ 0.05 ppm compared with the certified value of 0.057 +_ 0.015 ppm. Recovery tests were carried out for two concentrations (0.01 and 0.02 ppm) on two samples: the NBS Oyster Tissue and fish muscle. The recovery was found to be 91-108% for the 0.01 p p m standard and 90-102% for the 0.02 ppm standard. By applying the standard addition method, the concentration of mercury in NBS Oyster Tissue was found to be 0.094 ppm, whereas the concentration of mercury in the fish muscle sample was found to be 0.25 #g g '. All mercury values were in terms of a weight-specific concentration (pg/g dry weight). Mean values ()?) and ranges of age and total length for each fish species were calculated. Data for mercury concentration in fish muscle were analyzed using the
74
A H . AL-HASHIMI AND M.A. Ak-ZORBA
family regression program [HP Statistical Library 1/General Statistics Vol. 1 (09825-15004)] to obtain the regression coefficients (a and b) and the correlation coefficient (r). The programme fits "exponential" curves by transforming the Xand/or Ydata to fit the simple linear model, Y = a + b X . Among the eight different models (curves) obtained from running the program, the model Y = a X h was selected since it constantly gave the best r values for all species and for both variables (i.e., age and total length). RESULTS
The mean values and ranges for age, total length and fresh weight for each fish species are given in Table 1. The average mercury concentrations in the various organs of the fish species studied are listed in Table 2. Attempts were made to correlate the mercury concentration in fish muscle with the age and/or length of fish. Mercury and fish age
When the correlations of mercury concentrations in fish muscle with age of
TABLE 1 Mean values ()() and ranges (R) of age, total length and fresh weight for the various fish species Fish
Red snapper Brown spotted grouper Silvery pomfret
Biological parameters
)? R )? R )(
Age (years)
Total length (cm)
Fresh weight (kg)
8 (2-19) 8 (4-18)
56 (36-72) 69 (47-90) 21 (16-28) 37 (26-48) 56 (52-61) 37 (30-53) 49 (30-60) 27 (23-34)
2.81 (0.65-5.43) 5.43 (1.26-13. I 0) 0.38 (0.15-0.77) 0.53 (0.17-1.06) 2.39 (1.74-3.28) 0.38 (0.12-1.18) 1.67 (0.40-2.97) 0.27 (0.14-0.40)
R
Silvery croaker Silvery grant Indian flathead Emperor Large toothed flounder
)(
2
R
(1-4)
)7 R )? R )~ R )(
7 (6-9) 3 (2-6) 7 (4-10)
R
Number
30 30 30 30 10 10 10 10
MI!RCUR~ IN COMMERCIAL FISH FROM KUWAIT
75
TABLE 2 Average mercury concentrations in tissues of various fish Irish
Mercury concentration (pgg ~ dry wt) Muscle
Red snapper
Brown spotted grouper
)7" _+ SD
1.20 + 1.60
R
0.10-6.30
N Ave. % DM )( ± SD R
Silvery pomfret
N Ave. % DM ± SD R
Silvery croaker
Silvery grant
N Ave. % DM )( _+ SD R N Ave. % DM )( _+ SD R
Indian flathead
Emperor
N Ave. % DM )( ± SD R N Ave. % DM )7 _+ SD R
Large toothed flounder
N Ave. % DM )? _+ SD R N Ave. % DM
30 21.3 1.80 ± 0.90 0.60-4.30 30 22.9 0.15 + 0.10 0.00-0.40 30 24.1 1.30 ± 0.75 0.40-3.60 30 21.6 0.83 _+ 0.04 0.78-0.88 10 20.2 1.60 ± 1.00 0.46-4.05 10 22.6 0.83 ± 0.04 0.76-0.88 10 16.5 0.68 ± 0.24 0.37-1.12 10 22.9
Liver
Gonad
3.80 +_ 8.40 0.10-38.20 23 27.8 1.70 + 0.10 0.1/)-4.80 30 34.7 0.20 _+ 0.17 0.00-0.50 10 26.2 1.40 +_ 0.63 0.64-2.60 7 22.9 0.67 _+ 0.39 0.20-1.40
0.90 _+ 1.40 0.00-4.60 9 21.96 0.30 _+ 0.50 0.00-2.30 20 27.4 0 0 25.4 0.14 ± 0.08 0.07-0.3 I l0 24.2 0.02 _+ 0.03 0.00-0.08
10
10
20.1
25.5
)7, mean; SD, standard deviation; R, range; N, number of samples: DM, dry matter.
the fish w e r e c a l c u l a t e d , the coefficients ( a f t e r r e g r e s s i o n a n a l y s i s a p p l y i n g the e q u a t i o n Y = a X b) w e r e 0.86, 0.68 a n d 0.65 f o r r e d s n a p p e r , b r o w n s p o t t e d g r o u p e r a n d silvery c r o a k e r , r e s p e c t i v e l y ( T a b l e 3). P l o t s o f m e r c u r y c o n c e n t r a t i o n in t h e m u s c l e v e r s u s age ( y e a r ) g a v e a n e x p o n e n t i a l c u r v e f o r e a c h o f the a b o v e - m e n t i o n e d species (Figs. 1-3) a n d
76
A H. AL-HASHIMIAND M.A. AL-ZORBA
TABLE 3 Association between mercury content of muscle and age of fish Fish
N
Conc. Hg (#g g- ~ dry wt) £
Red snapper Brown spotted grouper Silvery croaker
30 30 30
Correlation coefficient (r)
Regression coefficients (a)
(b)
0.06 0.75 0.50
1.36 0.90 1.10
SD
1.20 +__ 1.60 1.80 4- 0.90 1.30 _ 0.75
0.86 0.68 0.65
demonstrated increases in the accumulation rate of mercury in fish muscle with an increase in fish age. The accumulation of mercury was higher in red snapper than in brown spotted grouper and silvery croaker.
Mercury and fish length The correlation coefficients (r) of the mercury concentration in fish muscle versus length of the fish were 0.82, 0.68, 0.21 and 0.59 for Hamrah, Hamoor, Zobaidy and Nakroor, respectively (Table 4)
d
B~
5
5.
5
4.
5
4.
0
"a. 5
0. )-
2.
5
I.
5
I.
0
3 U E
~-~.
•
~
.
.
AG~"
.
t~ U hi
I
L~'NC;
I
TH
,c
I
~)
Fig. 6. Accumulation of mercury in Zobaidy muscle with increasing length (r = 0.21).
I
I
80
A.H. AL-HASHIMI
t,
.4..
IZl
~
5
AND
M.A. AL-ZORBA
d E O.
2
-
~
>-
0 OC b.l I
I.
'=;
1.
I~ -+-
I
~
-+-
.4-.
I
L E N G T H
(~
m)
Fig. 7. Accumulation of mercury in Nakroor muscle with increasing length (r = 0.59).
This finding is in agreement with the findings of other workers (Johnels et al., 1967a,b; Bache et al., 1971; Forrester et al., 1972; Shultz et al., 1976; Aydogdn et al., 1982). The mercury concentration of fish muscle was also correlated with fish length, and again this has been shown by other investigators (Hornung et al., 1980; Walker, 1980; Denton and Breck, 1981; Zorba et al., 1981; Aydogdn et al., 1982). The concentration of mercury in the muscle of fish was slightly lower than that in the liver, but both were considerably higher than those in the gonads. This is in agreement with other relevant works (e.g., Bryan, 1976; Zorba et al., 1981). The results of this investigation showed no significant differences in mercury concentration between females and males. Therefore, the combined mean concentration of mercury in muscles of males and females was presented. Walker (1980) stated that two factors affected the mercury concentration of fish, namely the species longevity and a carnivorous diet. Both red snapper and brown spotted grouper are known to live for 20 years or more. They also are known to be voracious and carnivorous, occupying high positions in the food chain. In spite of the long life span of silvery grant (average 7 years), the average concentration of mercury was much lower than in red snapper and brown spotted grouper (0.83~gg -~ dry weight = 0.17#gg ' weight). This may be attributed to the omnivorous feeding habit of this species (Kuronuma and
MERCURY IN COMMERCIAL FISH FROM KUWAIT
81
Abe, 1972). This is exactly the opposite in the cases of silvery croaker and Indian flathead, which showed higher mercury concentrations (0.28 and 0.36/~gg ~ wet weight, respectively), although they have shorter life spans; this finding may also be related to feeding habit since both are carnivorous fish. Among the fish species studied, silvery pomfret contained the lowest mercury concentration, probably due to its short life span (aging was not possible) and its habit of feeding on small animals (Kuronuma and Abe, 1972), so that preconcentration through the food chain is small compared with that of fish at the top of the food chain. The United States Food and Drug Administration (USFDA) set the action level of 0.05 pg g- 1wet weight mercury as the upper tolerance limit for human consumption. This is exactly the same level set by the Australian National Health and Medical Research Council (NHMRC, 1972) and the Commonwealth and Victorian Statutory limit. However, the USFDA (1979) changed the action level for mercury from 0.05 to 1.0~gg -t wet weight. The mercury concentrations in the muscle of all the fish species studied did not exceed the 1.0 ~g g-I limit set by the USFDA, although levels were close in a few cases (e.g., 0.91/~gg-1 wet weight in brown spotted grouper and 0.87/tgg -t wet weight in red snapper). The findings of the present investigation, whilst preliminary in nature, suggest that the fish species studied are only lightly contaminated with mercury. However, the total number of samples per individual species was limited. In their annual statistical book on the fisheries industry in Kuwait, the Ministry of Planning (Arabic reference) listed the silvery pomfret, brown spotted grouper, silvery grant, silvery croaker, red snapper and emperor as the most favourable fish on the market in terms of taste and price. Therefore, a detailed study on accumulation of mercury and probably other heavy metals (e.g., cadmium, lead, copper, chromium, zinc) is of utmost importance.
REFERENCES Anon., 1971. Swordfish banned as U.S. Food. F D A Food Process., 32(6): 14. Aydogdn, T., T.I. Balkas, F. Bingel, I. Salihoglu and S. Tugrul, 1982. Mercury in some benthic fish of the north Levantine (eastern Mediterranean). ICSEM/IOC/UNEP VIth Workshop Marine Pollution of the Mediterranean, Cannes, 2-4 December 1982. Bache, C.A., W.H. Gutenmann and D.J. Lisk, 1971. Residue of total mercury and methylmercuric salts in lake trout as a function of age. Science, 172: 951. Bernhard, M., 1976. Manual of Methods in Aquatic Environment Research, Part 3: Sampling and Analyses of Biological Material. F A O of the United Nations, Rome, 1976. Bryan, G.W., 1976. Heavy metal contamination in the sea. In: Johnston (Ed.), Marine Pollution. Academic Press, London, pp. 185-302.
82
A.H. A L - H A S H I M I A N D M.A. A L - Z O R B A
Denton, G.R.W. and W.G. Breck, 1981. Mercury in tropical marine organisms from North Queensland. Mar. Pollut. Bull., 12(4): 116. FAO/WHO, Joint Expert Committee on Food Additives, 1972. Evaluation of mercury, lead, cadmium, and the food additives, amaranth, dethylphrocarbonate, and octylgallate. WHO Food Addit. Ser., 4: 1. Forrester, C.R, Ketchen and C.C. Wong, 1972. Mercury content of spiny dogfish (Squalus acanthias) in the Strait of Georgia, British Columbia. J. Fish. Res. Board Can., 29: 1487. F6rstner, U. and G. Muller, 1974. Schwermetalle in flussen and seen. Springer-Verlag, Berlin. Fujita, M. and K. Hashizume, 1972. The accumulation of mercury by a freshwater planktonic diatom. Chemosphere, 1: 203. Hamilton, A., 1971. International Symposium on Mercury in Man's Environment. Royal Society of Canada, Ottawa, Canada, 15-16 Feb. 1971, p. 87. Hannerz, L., 1968. Experimental investigations on the accumulation of mercury compounds in water organisms. Rep. Inst. Freshwater Res., Sweden, 48: 120. Hornung, H., L. Zismann and O.H. Oren, 1980. Mercury in twelve Mediterranean travel fishes of Israel. Environ. Int., 3: 243. Johnels, A.G., M. Olsson and T. Westemark, 1967a. Mercury in fish: investigation on mercury levels in Swedish fish. Var Foeda, 7: 67. Johnels, A.G., T. Westermark, W. Berg, P.I. Persson and B. Sjostrand, 1967b. Pike (Esox lucius) and some other aquatic organisms in Sweden as indicators of mercury contamination in the environment. Oikos, 18: 323. Klein, D.H. and E.D. Goldberg, 1970. Mercury in the marine environment. Environ. Sci. Technol., 4: 765. Kothny, E.L., 1973. Trace Elements in the Environment. Adv. Chem. Ser., 123. American Chemical Society. Kuronuma, K. and Y. Abe, 1972. Fishes of Kuwait. Kuwait Institute for Scientific Research, State of Kuwait. National Health and Medical Research Council (NHMRC), 1972. Methyl mercury in fish: effect on human health. Australian Government Publishing Service, Canberra. Peterson, C.L., W.L. Klawe and G.D. Sharp, 1973. Mercury in tunas. A review. Fish Bull., 71: 603. Rucker, R.R. and D.F. Amend, 1969. Absorption and retention of organic mercurials by rainbow trout and chinook and sockeye salmon. Prog. Fish-Cult., 31: 197. Shultz, C.D., D. Crear, J.E. Pearson, J.B. Rivews and J.W. Hylin, 1976. Total and organic mercury in the Pacific blue marlin. Bull. Environ. Contam. Toxicol., 15: 230. US Food and Drug Administration (USFDA), 1979. Action level for mercury in fish, shellfish, crustaceans and other aquatic animals. Fed. Reg., 44(14): 3990. Walker, T.I., 1980. Mercury content of edible flesh from snapper, Chrysophrys auratus (Bloch and Schneider), in the Victorian commerical catch. Aust. J. Mar. Freshwater Res., 32: 75. Wobeser, B., N.O. Nielson, R.H. Dunlop and F.M. Atton, 1970. Mercury concentration in tissues of fish from the Sakatchewan River. J. Fish. Res. Board Can. 27(4): 830. Young, D.R., J.N. Johnston, A. Soutar and J.D. Isaacs, 1973. Mercury concentration in dated varved marine sediments collected off southern California Nature, 244: 273. Zorba, M.A., V. Anderlini and C.P. Mathews, 1981. Effect of size and weight on the trace metal content of Hamrah, Lutjanus conccineus. Annu. Res. Rep. 1981, Kuwait Institute for Scientific Research, Kuwait. Zief, M. and J.W. Mitchell, 1976. Contamination Control in Trace Analysis. Wiley, New York, Vol. 47.
