HEAVY METAL BIOACCUMULATION IN DIFFERENT TISSUES OF T W O FISH SPECIES WITH REGARDS TO THEIR FEEDING HABITS AND T R O P H I C LEVELS N. POURANG Iranian Fisheries Research and Training Organization (Received: June 1994; revised: December 1994)
Abstract. Heavy metals residues (i.e. Cu, Zn, Mn, and Pb) were determined in seven chosen tissues of two fish species (Esox lucius and Carassius auratus) from Anzali wetland. The impact of feeding habit on metal accumulations in different tissues as well as the respective contribution of water and food to matel uptake by fishes were considered. No tendency for bioamplificationwas observed for the metals along the wetland trophic levels. Generally, there was no statistically significant relationship between the metal content of the tissues and the fish length for both species. In all cases, except for lead, the mean amounts of the metals in the flesh of the species were higher than those in commercially important fishes from the Caspian sea. However, they were below the recommended levels for human consumption.
1. Introduction
Over the past several decades the increasing use of metals in industry is causing serious environmental pollution through effluents and emanations. Among the myriad of organic and inorganic substances released into aquatic ecosystems, heavy metals have received considerable attention due to their toxicity and potential bioaccumulation in many aquatic species (Blevins, 1985; Gupta and Mathur, 1983). A number of these elements are biologically essential and natural constituents of the aquatic ecosystems. Same other metals (such as lead, mercury, etc.) are generally toxic to living organisms at quite low concentrations (Kindt, 1986; Kinne, 1984). In general, studies on heavy metals can be important in two main aspects. First, from the public health point of view. In this respect, attention has been drawn to the necessity of measuring the accumulation of havy metals; particularly certain metals which pose an imminent health hazard to humans and are included in the 'black list' of metals (Cd, Pb, Hg, etc.). Second, from the aquatic environment viewpoint the main problem has been to prevent biological deterioration and to identify the sources which threaten ecological equilibrium. In this regard, the more abundant metals such as copper, zinc and manganese may sometimes be of greater hazard than lead, mercury and cadmium (Kinne, 1984). In the present study both the above mentioned aspects have been taken into consideration. Bioaccumulation of heavy metals by fish is influenced by various factors, the more important factors of which are: feeding behaviour, growth rate, temperature, Environmental Monitoring and Assessment 35: 207-219, 1995. (~) 1995 Kluwer Academic Publishers. Printed in the Netherlands.
208
N. POURANG
hardness, salinity, age, sex and metals interactions (McCarty, 1978; Hakanson, 1980; Czarnezki, 1985; Bendell-Young and Harvey, 1989; Mance, 1990). There are three possible ways by which metals may enter fish bodies: the body surface, the gills and the digestive tract. Results of different investigations have shown that the body surface of fish does not play a dominant role in the uptake of heavy metals. Laboratory experiments have indicated that in fishes which take up heavy metals from water the gills generally show higher concentration than in the digestive tract. On the other hand, fish accumulating heavy metals from food show elevated metal levels in the digestive tract compared to the gills (Ney and Van Hassel, 1983; Dallinger et al., 1987; Heath, 1990). In summary, when concentrations of heavy metals in water are high, the contribution of food to total body burdens in fish will be relatively insignificant because of the greater rate and efficiency of transport across the gills. When concentrations in water are low, the food chain transfer of metals may be the primary route of exposure (Clements, 1991). The primary objective of the study was to evaluate bioaccumulation of heavy metals (Cu, Zn, Mn and Pb) in different tissues of two carnivorous and detrivorous fish species (Esox lucius and Carassius auratus respectively; Coad and Abdoli, 1993) in the Anzali wetland and to compare them with the maximum acceptable standards for human health. Furthermore the relative contribution of food and water to the total body burden of metals in the two fish species is discussed. The other purpose of this investigation was to assess the transfer of metals through different trophic levels of aquatic foodchains in the wetland. This study was carried out within the framework of a larger project assessing the relationships between heavy metal contents in sediments, benthic invertebrates, macrophytes and fishes from the Anzali wetland. In this paper some of the results obtained from other parts of the project have also been used.
2. Materials and Methods
2.1. STUDY AREA This study was conducted between December 1992 and November 1993 in the Anzali wetland, one of the most important water bodies in northern Iran (37°281N, 49°251W) connected to the Caspian sea (Figure 1). The wetland is known as an important spawning and nursery ground for the economically valuable anadromous fish. The wetland also represents an internationally important wildlife reserve and sanctuary which is listed under the Ramsar Convention (Olah, 1990; Holcik, 1993). With 31 species of fish, the Anzali wetland is rich in species (Holcik and Olah, 1992). Among fish species occurring in the wetland, two investigated species deserve special attention because of their relative dominance and commercial value.
209
HEAVY METAL BIOACCUMULATION IN FISH
S/tI.IpL:UtO S Z T Z $
:
gc^~ o r,
1 i
= I
3 r
4
Fig. 1. Map of the Anzali wetland showing the location of the sampling sites.
2.2. SAMPLING
Fish were caught from six chosen sampling sites in various parts of the wetland with an electroshocker (the locations of sampling sites are depicted in Figure 1).
210
N. POURANG
About 20 samples were collected each season and a total of 80 speciemen were included. The total length of each fish sample was recorded. The samples were rinsed with distilled water and then sealed in sterile polyethylene bags, and frozen at -20 °C until the tissues could be dissected.
2.3. DISSECTION
Muscles, gill filaments, digestive tracts, liver, kidney, heart and swim bladder of thawed fish were taken using stainless steel instruments on a clean glass working surface. Muscle samples were separated using the methods recommended by UNEP (1984). The digestive tracts were opened to remove their contents.
2.4. ANALYTICALPROCEDURES
Dissected organs and tissues were placed in cleaned labelled petri dishes and oven dried overnight at 105 °C. The samples were precisely weighted using an analytical balance (with a precision of 0.1 mg). To minimize trace element contamination, all labware was soaked in 15% nitric acid for 24 h and rinsed repeatedly in deionized water prior to use. Each of the weighted samples was transferred to a 25 ml conical flask and 1-2 ml of deionized water was added to wet the sample and then 5 ml high purity nitric acid was added slowly to the digestion container. Conical flasks were covered with watchglasses and allowed to stand for 2 h at room temperature. The other processes of the digestion procedure followed the ROPME instructions (1989), except that cooled digested samples were diluted with deionized double distilled water to exactly 10 ml (in acid washed volumetric flasks). Diluted samples were transferred to tightly sealed linear polyethylene (Nalgene) containers to avoid the adsorption of metals from digested solution (Betti and Papoff, 1988; Sansoni and Iyengar, 1978) and kept at 4 °C. The trace element concentrations were measured as soon as possible. For the analysis of the metals both flame- and tameless atomic absorption spectroscopy (AAS) were applied. Cu, Mn, and Zn contents in samples were determined using FAAS (Pye Unicam: sp9) and for lead concentration determination GFAAS (Perkin Elmer: 503) was used. All metal analyses of digested fish tissues were undertaken at least in triplicate and the mean values calculated. Standard reference materials (NBS 1577, Bovine liver) were run along with samples to ensure that the sample preparation procedures and analytical measurements produced acceptable values. In cases where results were outside the acceptable limits for each metal, the samples were reanalysed. The record of the laboratory shows more than 95 % of the analyses of standard reference materials to be inside the limits.
HEAVY METAL BIOACCUMULATION IN FISH
2.5.
211
DATA ANALYSIS
The sample size (number of fish specimens from each of the two investigated species) was 40; therefore, on the basis of a central limit theorem, it could be assumed that the sampling distribution of the sample mean is approximately normal. In spite of this, a goodness of fit test was employed to test the null hypothesis that the samples had come from a normal population (Daniel, 1977; Rees, 1991; Zar, 1984). Since this null hypothesis was acceptable, parametric statistical methods could be applied. To detect homogeneity (homoscedasticity) of the variances, Bartlett's test (Sokal and Rohlf, 1981; Zar, 1984) was used and because the variances were heterogeneous, Tylor's power law (Green, 1979) was applied to determine appropriate data transformation. As a result all data were In (n+l) transformed. Two-way ANOVA was used to examine whether all the four metals had been accumulated significantly different in the investigated tissues of each fish species. A total of eight one-way ANOVAs were also conducted to determine whether the differences among concentrations of each heavy metal in the different tissues of the two species could be significant. Since highly significant Fs resulted from all analysis of variances, Duncan's new multiple range test (Zar, 1984) was employed to determine which group means did not differ from one another. In all these cases statistical significance was evaluated at p < 0.05. Relationships between the total length of fish and heavy metal concentrations in chosen tissues were assessed by computing correlation coefficients and significance probabilities. The correlations were also calculated using natural log transformed data. Most of the statistical analyses were performed using packages, SPSS-X (Version 3.0 1988) and STATGRAPHICS (version 5.0 1991). Bioconcentrations factors (Bodou and Ribeyre, 1989; Heath, 1990) were calculated for the tissues which contained the highest concentration of each of the four metals in relation to ambient water (average concentrations of metals in different sampling sites of water measured by the Iranian Department of the Environment, unpublished data).
3. Results and Discussion
Since the F values of the last mentioned two-way ANOVAs were highly significant (t7 < 0.00001), the amount of each metal in the chosen tissues will be discussed separately. 3.1. COPPER Comparison between the mean concentrations of copper in the muscles of freshwater commercial fishes from the Caspian sea (0.6 + 0.2 ppm; Patin, 1982) and the levels obtained from the present study (1.3 -4- 0.5 ppm in C. auratus and 2.8 + 1.8 ppm in E. lucius) clearly indicates that the concentrations are markedly
212
N. POURANG
,o, ~
~.
i ~ ~:~
CU 7.0OO ,ODO I~&)O N~
~.x~6~~
~
~ ,,
%
, _..
%
Mnl
Pb
Ce
J Fig. 2. Comparisonof mean heavy metallevels amongseventissues of the two fish species
higher, nevertheless Cu content in fish from the study area is considerably below permissible amounts (70-100 ppm; Clark, 1992; Hadjmohammadi, 1988) and thus does not present any danger to human health. Figure 2 shows that in both species liver has accumulated the highest amount of Cu and muscle have the lowest concentration. Similar results have been obtained by other investigators (Khalaf, 1985; Malyarevskaya, 1991; Wachs, 1982). Bioconcentration Factors (B.F.) calculated for liver revealed that this tissue in E. lucius and C. auratus bioaccumulate Cu at levels about 12 327 and 4904 times greater than that in ambient water respectively. The higher bioaccumulation in E. lucius compared to C. auratus (about 2.5 times) may be closely related to the difference in their feeding habits. There is no data
HEAVY METAL BIOACCUMULATION IN FISH
213
available for the content of Cu in livers of other fish species in the Anzali wetland, hence it is difficult to conclude whether the observed considerably higher levels of Cu in E. lucius is only due to its diet quality. As shown in Table I, there were highly significant differences (p < 0.001) among levels of Cu in different tissues. Duncan's test indicated that none of the tissues was homogeneous in C. auratus, whereas in E. lucius no difference were found between mean amounts of Cu in the kidney and digestive tract, and the latter tissues were significantly different from others. Generally, it is to be expected that the contributions of water and food to metal uptake in whole fish are reflected by high concentrations in gills and digestive tracts tissues, respectively (Dallinger et al., 1987). Therefore in this study, it seems reasonable to assume that food is the primary pathway for the uptake of copper in both fish species, because the digestive tract Cu content was notably higher than the concentration in gills (see Figure 2 and Table I). These results are in accordance with the findings of some other workers (Dallinger and Kautzky, 1985; Khalaf, 1985). All correlation coefficients calculated for Cu concentrations in chosen tissues of E. lucius and the fish length were non-significant, while a slight positive relationship could be noted for swim bladder (r = 0.55, p = 0.0018) and the digestive tract (r = 0.51, p = 0.0032) in C. auratus. Lack of correlation between essential elements accumulated in different tissues and fish length have also been reported from other researchers (e.g. Khalaf, 1985). However, there are few exceptions in this case (Ewans et al., 1993). 3.2. ZINC The mean zinc contents in muscles of E. lucius (25.4 ± 8.3 ppm) was only slightly higher than that in freshwater commercial fishes from the Caspian sea (20.6 6.8 ppm), while amounts of this element in the muscles of C. auratus (55.4 ± 13.5 ppm) were quite high compared to fishes from the Caspian sea. However, it seems that the relatively high concentration of Zn in the flesh of this species cannot be dangerous for human health because zinc is an essential element for most living organisms and only it becomes toxic at very high concentrations (Dulka, 1976; Heckman, 1990; Kindt, 1986). The kidney appears to be the most important storage tissue of Zn in E. lucius (Figure 2). Whereas C. auratus the level of Zn in the digestive tract was markedly higher than any other tissues (about seven times more than that in gills which contain the second highest concentration) and also compared with the same tissue in E. lucius (about two times). The surprisingly elevated concentrations of Zn in the C. auratus digestive tract probably arose from the species specific feeding habits (i.e. detrivorous) and also the slow rate of excretion of this element (Heath, 1990), especially in the latter tissue. Moreover, the possibility of active uptake must also be considered as another
214
N. POURANG
&
g 0
d ~ ~
~
o .
.
•
.
.
"r.
~
M
o
~
F
~
I
•
',-4
N
o
o
#
&
©
~-~
#
Abstract. Heavy metals residues (i.e. Cu, Zn, Mn, and Pb) were determined in seven chosen tissues of two fish species (Esox lucius and Carassius auratus) from Anzali wetland. The impact of feeding habit on metal accumulations in different tissues as well as the respective contribution of water and food to matel uptake by fishes were considered. No tendency for bioamplificationwas observed for the metals along the wetland trophic levels. Generally, there was no statistically significant relationship between the metal content of the tissues and the fish length for both species. In all cases, except for lead, the mean amounts of the metals in the flesh of the species were higher than those in commercially important fishes from the Caspian sea. However, they were below the recommended levels for human consumption.
1. Introduction
Over the past several decades the increasing use of metals in industry is causing serious environmental pollution through effluents and emanations. Among the myriad of organic and inorganic substances released into aquatic ecosystems, heavy metals have received considerable attention due to their toxicity and potential bioaccumulation in many aquatic species (Blevins, 1985; Gupta and Mathur, 1983). A number of these elements are biologically essential and natural constituents of the aquatic ecosystems. Same other metals (such as lead, mercury, etc.) are generally toxic to living organisms at quite low concentrations (Kindt, 1986; Kinne, 1984). In general, studies on heavy metals can be important in two main aspects. First, from the public health point of view. In this respect, attention has been drawn to the necessity of measuring the accumulation of havy metals; particularly certain metals which pose an imminent health hazard to humans and are included in the 'black list' of metals (Cd, Pb, Hg, etc.). Second, from the aquatic environment viewpoint the main problem has been to prevent biological deterioration and to identify the sources which threaten ecological equilibrium. In this regard, the more abundant metals such as copper, zinc and manganese may sometimes be of greater hazard than lead, mercury and cadmium (Kinne, 1984). In the present study both the above mentioned aspects have been taken into consideration. Bioaccumulation of heavy metals by fish is influenced by various factors, the more important factors of which are: feeding behaviour, growth rate, temperature, Environmental Monitoring and Assessment 35: 207-219, 1995. (~) 1995 Kluwer Academic Publishers. Printed in the Netherlands.
208
N. POURANG
hardness, salinity, age, sex and metals interactions (McCarty, 1978; Hakanson, 1980; Czarnezki, 1985; Bendell-Young and Harvey, 1989; Mance, 1990). There are three possible ways by which metals may enter fish bodies: the body surface, the gills and the digestive tract. Results of different investigations have shown that the body surface of fish does not play a dominant role in the uptake of heavy metals. Laboratory experiments have indicated that in fishes which take up heavy metals from water the gills generally show higher concentration than in the digestive tract. On the other hand, fish accumulating heavy metals from food show elevated metal levels in the digestive tract compared to the gills (Ney and Van Hassel, 1983; Dallinger et al., 1987; Heath, 1990). In summary, when concentrations of heavy metals in water are high, the contribution of food to total body burdens in fish will be relatively insignificant because of the greater rate and efficiency of transport across the gills. When concentrations in water are low, the food chain transfer of metals may be the primary route of exposure (Clements, 1991). The primary objective of the study was to evaluate bioaccumulation of heavy metals (Cu, Zn, Mn and Pb) in different tissues of two carnivorous and detrivorous fish species (Esox lucius and Carassius auratus respectively; Coad and Abdoli, 1993) in the Anzali wetland and to compare them with the maximum acceptable standards for human health. Furthermore the relative contribution of food and water to the total body burden of metals in the two fish species is discussed. The other purpose of this investigation was to assess the transfer of metals through different trophic levels of aquatic foodchains in the wetland. This study was carried out within the framework of a larger project assessing the relationships between heavy metal contents in sediments, benthic invertebrates, macrophytes and fishes from the Anzali wetland. In this paper some of the results obtained from other parts of the project have also been used.
2. Materials and Methods
2.1. STUDY AREA This study was conducted between December 1992 and November 1993 in the Anzali wetland, one of the most important water bodies in northern Iran (37°281N, 49°251W) connected to the Caspian sea (Figure 1). The wetland is known as an important spawning and nursery ground for the economically valuable anadromous fish. The wetland also represents an internationally important wildlife reserve and sanctuary which is listed under the Ramsar Convention (Olah, 1990; Holcik, 1993). With 31 species of fish, the Anzali wetland is rich in species (Holcik and Olah, 1992). Among fish species occurring in the wetland, two investigated species deserve special attention because of their relative dominance and commercial value.
209
HEAVY METAL BIOACCUMULATION IN FISH
S/tI.IpL:UtO S Z T Z $
:
gc^~ o r,
1 i
= I
3 r
4
Fig. 1. Map of the Anzali wetland showing the location of the sampling sites.
2.2. SAMPLING
Fish were caught from six chosen sampling sites in various parts of the wetland with an electroshocker (the locations of sampling sites are depicted in Figure 1).
210
N. POURANG
About 20 samples were collected each season and a total of 80 speciemen were included. The total length of each fish sample was recorded. The samples were rinsed with distilled water and then sealed in sterile polyethylene bags, and frozen at -20 °C until the tissues could be dissected.
2.3. DISSECTION
Muscles, gill filaments, digestive tracts, liver, kidney, heart and swim bladder of thawed fish were taken using stainless steel instruments on a clean glass working surface. Muscle samples were separated using the methods recommended by UNEP (1984). The digestive tracts were opened to remove their contents.
2.4. ANALYTICALPROCEDURES
Dissected organs and tissues were placed in cleaned labelled petri dishes and oven dried overnight at 105 °C. The samples were precisely weighted using an analytical balance (with a precision of 0.1 mg). To minimize trace element contamination, all labware was soaked in 15% nitric acid for 24 h and rinsed repeatedly in deionized water prior to use. Each of the weighted samples was transferred to a 25 ml conical flask and 1-2 ml of deionized water was added to wet the sample and then 5 ml high purity nitric acid was added slowly to the digestion container. Conical flasks were covered with watchglasses and allowed to stand for 2 h at room temperature. The other processes of the digestion procedure followed the ROPME instructions (1989), except that cooled digested samples were diluted with deionized double distilled water to exactly 10 ml (in acid washed volumetric flasks). Diluted samples were transferred to tightly sealed linear polyethylene (Nalgene) containers to avoid the adsorption of metals from digested solution (Betti and Papoff, 1988; Sansoni and Iyengar, 1978) and kept at 4 °C. The trace element concentrations were measured as soon as possible. For the analysis of the metals both flame- and tameless atomic absorption spectroscopy (AAS) were applied. Cu, Mn, and Zn contents in samples were determined using FAAS (Pye Unicam: sp9) and for lead concentration determination GFAAS (Perkin Elmer: 503) was used. All metal analyses of digested fish tissues were undertaken at least in triplicate and the mean values calculated. Standard reference materials (NBS 1577, Bovine liver) were run along with samples to ensure that the sample preparation procedures and analytical measurements produced acceptable values. In cases where results were outside the acceptable limits for each metal, the samples were reanalysed. The record of the laboratory shows more than 95 % of the analyses of standard reference materials to be inside the limits.
HEAVY METAL BIOACCUMULATION IN FISH
2.5.
211
DATA ANALYSIS
The sample size (number of fish specimens from each of the two investigated species) was 40; therefore, on the basis of a central limit theorem, it could be assumed that the sampling distribution of the sample mean is approximately normal. In spite of this, a goodness of fit test was employed to test the null hypothesis that the samples had come from a normal population (Daniel, 1977; Rees, 1991; Zar, 1984). Since this null hypothesis was acceptable, parametric statistical methods could be applied. To detect homogeneity (homoscedasticity) of the variances, Bartlett's test (Sokal and Rohlf, 1981; Zar, 1984) was used and because the variances were heterogeneous, Tylor's power law (Green, 1979) was applied to determine appropriate data transformation. As a result all data were In (n+l) transformed. Two-way ANOVA was used to examine whether all the four metals had been accumulated significantly different in the investigated tissues of each fish species. A total of eight one-way ANOVAs were also conducted to determine whether the differences among concentrations of each heavy metal in the different tissues of the two species could be significant. Since highly significant Fs resulted from all analysis of variances, Duncan's new multiple range test (Zar, 1984) was employed to determine which group means did not differ from one another. In all these cases statistical significance was evaluated at p < 0.05. Relationships between the total length of fish and heavy metal concentrations in chosen tissues were assessed by computing correlation coefficients and significance probabilities. The correlations were also calculated using natural log transformed data. Most of the statistical analyses were performed using packages, SPSS-X (Version 3.0 1988) and STATGRAPHICS (version 5.0 1991). Bioconcentrations factors (Bodou and Ribeyre, 1989; Heath, 1990) were calculated for the tissues which contained the highest concentration of each of the four metals in relation to ambient water (average concentrations of metals in different sampling sites of water measured by the Iranian Department of the Environment, unpublished data).
3. Results and Discussion
Since the F values of the last mentioned two-way ANOVAs were highly significant (t7 < 0.00001), the amount of each metal in the chosen tissues will be discussed separately. 3.1. COPPER Comparison between the mean concentrations of copper in the muscles of freshwater commercial fishes from the Caspian sea (0.6 + 0.2 ppm; Patin, 1982) and the levels obtained from the present study (1.3 -4- 0.5 ppm in C. auratus and 2.8 + 1.8 ppm in E. lucius) clearly indicates that the concentrations are markedly
212
N. POURANG
,o, ~
~.
i ~ ~:~
CU 7.0OO ,ODO I~&)O N~
~.x~6~~
~
~ ,,
%
, _..
%
Mnl
Pb
Ce
J Fig. 2. Comparisonof mean heavy metallevels amongseventissues of the two fish species
higher, nevertheless Cu content in fish from the study area is considerably below permissible amounts (70-100 ppm; Clark, 1992; Hadjmohammadi, 1988) and thus does not present any danger to human health. Figure 2 shows that in both species liver has accumulated the highest amount of Cu and muscle have the lowest concentration. Similar results have been obtained by other investigators (Khalaf, 1985; Malyarevskaya, 1991; Wachs, 1982). Bioconcentration Factors (B.F.) calculated for liver revealed that this tissue in E. lucius and C. auratus bioaccumulate Cu at levels about 12 327 and 4904 times greater than that in ambient water respectively. The higher bioaccumulation in E. lucius compared to C. auratus (about 2.5 times) may be closely related to the difference in their feeding habits. There is no data
HEAVY METAL BIOACCUMULATION IN FISH
213
available for the content of Cu in livers of other fish species in the Anzali wetland, hence it is difficult to conclude whether the observed considerably higher levels of Cu in E. lucius is only due to its diet quality. As shown in Table I, there were highly significant differences (p < 0.001) among levels of Cu in different tissues. Duncan's test indicated that none of the tissues was homogeneous in C. auratus, whereas in E. lucius no difference were found between mean amounts of Cu in the kidney and digestive tract, and the latter tissues were significantly different from others. Generally, it is to be expected that the contributions of water and food to metal uptake in whole fish are reflected by high concentrations in gills and digestive tracts tissues, respectively (Dallinger et al., 1987). Therefore in this study, it seems reasonable to assume that food is the primary pathway for the uptake of copper in both fish species, because the digestive tract Cu content was notably higher than the concentration in gills (see Figure 2 and Table I). These results are in accordance with the findings of some other workers (Dallinger and Kautzky, 1985; Khalaf, 1985). All correlation coefficients calculated for Cu concentrations in chosen tissues of E. lucius and the fish length were non-significant, while a slight positive relationship could be noted for swim bladder (r = 0.55, p = 0.0018) and the digestive tract (r = 0.51, p = 0.0032) in C. auratus. Lack of correlation between essential elements accumulated in different tissues and fish length have also been reported from other researchers (e.g. Khalaf, 1985). However, there are few exceptions in this case (Ewans et al., 1993). 3.2. ZINC The mean zinc contents in muscles of E. lucius (25.4 ± 8.3 ppm) was only slightly higher than that in freshwater commercial fishes from the Caspian sea (20.6 6.8 ppm), while amounts of this element in the muscles of C. auratus (55.4 ± 13.5 ppm) were quite high compared to fishes from the Caspian sea. However, it seems that the relatively high concentration of Zn in the flesh of this species cannot be dangerous for human health because zinc is an essential element for most living organisms and only it becomes toxic at very high concentrations (Dulka, 1976; Heckman, 1990; Kindt, 1986). The kidney appears to be the most important storage tissue of Zn in E. lucius (Figure 2). Whereas C. auratus the level of Zn in the digestive tract was markedly higher than any other tissues (about seven times more than that in gills which contain the second highest concentration) and also compared with the same tissue in E. lucius (about two times). The surprisingly elevated concentrations of Zn in the C. auratus digestive tract probably arose from the species specific feeding habits (i.e. detrivorous) and also the slow rate of excretion of this element (Heath, 1990), especially in the latter tissue. Moreover, the possibility of active uptake must also be considered as another
214
N. POURANG
&
g 0
d ~ ~
~
o .
.
•
.
.
"r.
~
M
o
~
F
~
I
•
',-4
N
o
o
#
&
©
~-~
#
