Aquatic ecosystems contamination by metals is of worldwide concern due to their toxicity, persistence and bioaccumulation in the food web and the potential for human exposure (Baramaki et al. 2012). Metals can be divided into two categories; essential and nonessential. The Cu, Co, Ni and Fe are essential metals necessary for the optimal functioning of biological and biochemical processes in organisms. In contrast, Cd, Pb and Cr have no known biological functions and exert their toxicity by competing with essential metals for active enzyme or membrane protein sites (Stankovic et al. 2013). Thus, gaining information of metal levels in organs of fish is essential both respecting nature management and human consumption of fish.

It is well known that metals accumulated in organs of an aquatic organism can be used as good indication of ingested toxin by that organism (Mansouri et al. 2012a). Fish can accumulate a considerable amount of metals from the environment into its organs, especially its muscles which is a major dietary source for humans (Dural et al. 2006). The concentration of metals can be several times higher than the fish’s ambient level. Studies show that much of the variation in the level of metals in fish organs is associated with the fishes’ length, weight, age (Mansouri et al. 2011; Mohammadnabizadeh et al. 2013; Majnoni et al. 2013), sex (Norouzi et al. 2012; Majnoni et al. 2013), and feeding habits (Yilmaz 2005; Baramaki et al. 2012; Mohammadnabizadeh et al. 2013). Thus, it is believed that the level of metals in fish can be used as a good bioindicator to trace the environmental and ecological changes caused by metal contaminations in aquatic ecosystems (Vaseem and Banerjee 2013; Mansouri et al. 2013).

In Iran, the level of metals in fish organs captured from the north and south coasts of the country (Caspian Sea and Persian Gulf) has been well studied, but little research has been done on metal concentrations of fish in Sistan and Baluchistan province (in the south-east of the country). The Chahnimeh reservoirs in Zabol city are the only water resources in Sistan. The main water supply for Chahnimeh reservoirs is Helmand River originated from the Afghanistan Mountains with no reliable information on its safety in terms of metal contamination. Thus, studying the concentration of metals in Chahnimeh reservoirs is of great importance to know the potential of these pollutants for administering purposes. In view of the above, this study aims to measure the concentration of metals; Cd, Pb, Cu, Zn, Cr, Co, Ni, and Fe in the gill, skin, and muscle organs of common carp, Cyprinus carpio, and silver carp, Hypophthalmichthys molitrix in Zabol Chahnimeh reservoirs, which are frequently consumed in Zabol city, Iran.

Materials and Methods

The Zabol Chahnimeh reservoirs (30º45′ to 30º50′N; 61º38′ to 61º45′E) are located 5 km away from Zabol city to the north (Fig. 1). The Chahnimeh reservoirs are three big natural cavities in the south of Sistan Plain, spanning an area of 50 million square meters. The water stored in these cavities is used to irrigate the Sistan Plain and serves as one of the main water resources for household use in Zabol and Zahedan cities. The climate of Zabol is semi-arid, with a cold winter and an approximately eight-month dry season (from middle of April to December). The mean annual precipitation of Zabol is 94 mm, unevenly distributed throughout the year. The average annual temperature of the city is 22.7°C with a hot summer (average 35.2°C) and a cold winter (average 8.9°C).

Fig. 1
figure 1

Location of Chahnimeh reservoirs of south-eastern Iran

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Fish samples were caught from random catches in the Zabol Chahnimeh reservoirs during April to June, 2012. Fish species were carried to the laboratory by a thermos flask with ice. The length and weight average of sample fish were measured 75.5 (±6.8) cm and 425.0 (±127.8) g for C. carpio (n = 10); and 74.9 (±4.5) cm and 345.0 (±35.3) g for H. molitrix (n = 12), respectively. Approximately 1 g wet weight gill, skin, and muscle from each sample were dissected, washed with distilled water, and accurately weighed into 150-mL Erlenmeyer flasks. To each sample, 10 mL nitric acid (65 %) was added. Samples were left overnight in order to digest slowly. Afterward, 5 mL perchloric acid (70 %) added to each sample (Mansouri et al. 2012b). Digestion was performed on a hot plate (sand bath) at 150°C before diluting with 25 mL deionized water. In the end, the metal concentrations were measured using a Varian 220 atomic absorption spectrophotometer. The detection limits for each metal obtained to be 0.01 (Cd), 0.02 (Pb), 0.03 (Cu), 0.75 (Zn), 0.07 (Cr), 0.05 (Co), 0.05 (Ni), and 0.6 µg/g (Fe). Moreover, the mean recovery for Cd, Pb, Cu, Zn, Cr, Co, Ni, and Fe were 98 %, 97 %, 98 %, 99 %, 94 %, 93 %, 96 %, and 88 % respectively.

Statistical analysis was performed using SPSS statistical package (version 16; SPSS, Chicago, IL). The one-way analysis of variance (ANOVA) was used to verify significant differences in organ metal concentrations between two fish species. Moreover, metal concentrations in organs were compared between two fish species using statistical Student’s t test. Where required, to check the normality assumption, the non-parametric Kolmogorov–Smirnov test was used. Pearson correlation (r) was used to test correlations. The metal concentrations in organs were expressed as microgram per gram wet weight (ww). Values are given in mean ± standard deviation (SD).

Daily consumption limits were calculated according to the following equation. It shows allowable daily consumption of metal contaminated fish based on a contaminant’s carcinogenicity, expressed in kilograms of fish consumed per day (Raissy and Ansari 2014; EPA 1999):

$${\text{CR}}_{ \lim } = \frac{{{\text{RfD}} \times {\text{BW}}}}{{{\text{C}}_{\text{m}} }}$$

where CRlim is maximum allowable fish consumption rate (kg/day); RfD is reference dose (10 for Cd, none set for Pb, 3 for Cr, 300 for Zn, 20 for Cu, and 360 for Fe µg/kg/day); BW is consumer body weight (kg); and Cm is measured concentration of chemical contaminant m in a given species of fish (mg/kg).

The consumption limit is determined in part by the size of the meal consumed. A 0.227 kg average fish meal size was assumed. The following equation can be used to convert daily consumption limits to the number of allowable meals per month:

$${\text{CR}}_{\text{mm}} = \frac{{{\text{CR}}_{ \lim } \times {\text{T}}_{\text{ap}} }}{\text{MS}}$$

CRmm is maximum allowable fish consumption rate (meals per month); CRlim is maximum allowable fish consumption rate (kg/day); MS is average fish meal size (0.227 kg fish/meal); and Tap is time averaging period (365.25 days/12 months = 30.44 days/month).

Results and Discussion

The mean concentration of metals (Cd, Pb, Cu, Zn, Cr, Co, Ni, and Fe) in the gill, skin, and muscle tissue of two fish species (Cyprinus carpio and Hypophthalmichthys molitrix) are presented in Table 1. The mean concentrations of Cd, Pb, Cu, Zn, Cr, Co, Ni, and Fe were 0.11, 0.70, 3.7, 20.2, 0.3, 0.7, 0.8, 31.2 µg/g for gill, 0.07, 0.47, 1.2, 14.4, 0.2, 0.4, 0.3, 19.6 µg/g for skin, and 0.08, 0.57, 2.1, 18.1, 0.4, 0.4, 0.2, 30.3 µg/g for muscle of C. carpio, respectively. For H. molitrix, the mean concentrations of Cd, Pb, Cu, Zn, Cr, Co, Ni, and Fe were 0.13, 0.45, 2.3, 22.5, 0.5, 0.5, 0.4, and 24.5 µg/g for gill; and 0.08, 0.27, 0.6, 13.5, 0.1, 0.1, 0.1, and 14.5 µg/g for skin; and 0.07, 0.58, 2.3, 20.1, 0.4, 0.4, 0.4, and 27.7 µg/g for muscle, respectively. From Table 1, it can be seen that for both species, the highest level of metals has been accumulated in the gill tissue. The highest mean concentration of metals belongs to Fe in gill whereas the lowest relates to Cd in skin of C. carpio.

Table 1 Metal concentrations (Overall mean ± SD) in organs of C. carpio and H. molitrix
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A multivariate analyses of variance (MANOVA) that included all eight metal concentrations showed significant effects of tissue (Hotelling-Lawley F = 2.16; df = 16; p = 0.039) but no significant effects of species (Hotelling-Lawley F = 1.36; df = 8; p = 0.293). To determine which heavy metal contributed in the statistically mean differences between tissues a univariate ANOVA was performed for each metal (Table 2). It can be seen that the results of the Fe, Zn, and Cu are highly significant (p < 0.005). The results also suggest a significant difference between Ni levels among tissues (p = 0.021). Furthermore, the analysis of Pearson correlation coefficients of length, weight and metal concentrations in two fish species showed no significant association between total length, weight and metal concentrations (p > 0.05). However, a significant difference was observed in Cu, Ni, and Fe concentrations between organs of C. carpio (one-way ANOVA, p < 0.05, p < 0.01), as well as between Zn and Fe concentrations for H. molitrix (one-way ANOVA, p < 0.05) (Table 3).

Table 2 Dependence of metal concentrations on tissue and fish species based on multivariate analyse of variance (MANOVA)
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Table 3 Statistical Analysis of metal concentrations in the gill, skin and muscle of C. carpio and H. molitrix
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Assessments of the human health risks associated with consumption of Cd, Cu, Cr, Zn, and Fe content contaminated fish are given according to daily (kg/day) and monthly (meals per month) limits for the 3–75-year-old population demographic in Table 4. The results of this study showed that the maximum allowable fish consumption rate for an adult person with mean 71.5 kg body weight were 10.64, 0.677, 0.558, 1.17, and 0.92 kg/day based on Cd, Cu, Cr, Zn, and Fe levels respectively.

Table 4 Maximum allowable fish consumption rate according to the metals content
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In this study, we found the concentration of Pb, Cu, Co, Ni, and Fe was higher in the gills of both fish species rather than their muscle and skin. The metal concentrations in the gill were 1.5–3.1-fold higher than that of the skin and muscle organs. Agreement can be seen with Tekin-Ozan and Kir (2008) and Norouzi et al. (2012) findings who claimed muscles are not an active organ for binding metals in fish species. However, Baramaki et al. (2012) findings confirms that the metal concentrations of Ctenopharyngodon idella and Perca fluviatilis, captured from Anzali wetland, were also higher in gill. Dsikowitzky et al. (2013) surveyed several freshwater fish species from Awassa and Koka Rift Valley Lakes, Ethiopia, and reported that metal concentrations were generally higher in their gills compare to muscles. One reason for high level of metal concentrations in the gill organs of fish samples can be due to absorption and adsorption (Kargin 1998), as the main sites of metallothionein (MT) production and metal retention, after making direct contact with the surrounding waters. Mansouri et al. (2012b) reported a high tendency of the gill organ for accumulating metals by induction of metal-binding protein MT. Moreover, the extent of the gill’s damage depends on the concentration of toxicants and duration of exposure.

The results of this study showed that the overall mean concentration difference of Fe (and Cu) between the gill and muscle organs were statistically significant at 5 % level (one-way ANOVA, p < 0.05). However, we were not able to establish a statistically significant difference between two fish species, mainly due to our small sample sizes (C. carpio and four H. molitrixr) leading to low statistical power to detect true differences. A study with larger sample size is recommended for detecting the difference in concentration of metals between two species. We studied two fish species of the Cyprinid family. Based on our findings, we may conclude that different species of this family may follow similar foraging strategy; C. carpio has an herbivorous diet of water plants with preference for zooplankton as insects, crustaceans, and benthic worms and H. molitrix feeds from phytoplankton and zooplankton.

It should be noted that the main edible part of each fish is its muscle. Hence, monitoring the concentration of metals in fish organs is of great importance for human health (Mohammadnabizadeh et al. 2013). According to Food and Agriculture Organization (FAO 1983) standards, the concentrations of Cd, Cu, Zn, and Pb in the muscle of C. carpio and H. molitrix from Zabol Chahnimeh reservoirs are below levels of concern for human use. Due to lack of information, the Cr ion is not usually an analytical target within routine surveillance of pollutants in fish (Storelli et al. 2005; Baramaki et al. 2012).

The comparison of metal concentrations with other regions of the world, shows that Cd and Pb concentrations in the muscle of C. carpio and H. molitrix from the present study were higher than those in (1) Hypophthalmichthys molitrix, Chondrostoma nasus, and Alburnus alburnus from Zagreb, Croatia (Zrnčić et al. 2013), (2) Hypophthalmichthys molitrix, Ctenopharyngodon idellus, and Megalobrama amblycephala from China (Zhuang et al., 2013), and (3) Labeo calbasu, Cirrhinus reba, and Rita rita from Chenab River, Pakistan (Waheed et al. 2013). In contrast, Cd and Pb concentrations in the muscle of two fish species were lower than those in Barbus grypus, and Barbus xanthopterus from Karoon and Dez Rivers, Iran (Mohammadi et al. 2011) and Oreochromis aureus, Mugil cephalus, and Diapterus sp. from NW Mexico (Frıas-Espericueta et al. 2010).

The metal concentrations have been expressed mostly on a wet weight basis. A wet weight-dry weight conversion factor of 0.2 can be assumed (Pourang et al. 2005). According to Food and Agriculture Organization maximum permissible level (MPL) are for: Cd 0.5 µg/g (in this study 0.16 µg/g for C. carpio and 0.14 µg/g for H. molitrix), Pb: 0.5 µg/g (in this study 0.1 µg/g for C. carpio and 0.1 µg/g for H. molitrix), Cu: 30 µg/g (in this study 0.42 µg/g for C. carpio and 0.46 µg/g for H. molitrix), and Zn: 30 µg/g (in this study 3.6 µg/g for C. carpio and 4.1 µg/g for H. molitrix). The permitted Ni level for fish is 5.5 μg/g of fish, set by the Western Australian Food and Drug Regulations, in seafood (Waheed et al. 2013). Results of this study showed that Ni mean values in muscle (0.2–0.4 μg/g) are below the Western Australian Food and Drug Regulations.

The muscle organ is the main edible fish part that can directly affect human health if carries heavy metals beyond standards. For this reason most rules have established allowable concentration limits for toxic elements in seafood. The maximum allowable fish consumption rate by person (3–75 years), to estimate the human health risk, was calculated on basis of an average 0.227 kg of fresh fish muscle consumption per day (EPA 1999). According to the results, depending on the consumer’s age, the maximum allowable consumption rate varies between 1.65–10.64, 0.10–0.67, 0.08–0.55, 0.18–1.17, and 0.14–0.92 g/day base on Cd, Cu, Cr, Zn, and Fe content respectively. The maximum allowable consumption rate has been reported equal to 8–56 g/day for cultured fish from Persian Gulf in Iran base on the Hg content (Raissy and Ansari 2014). Kannan et al. (1998) found that consuming fish from South Florida Estuaries at rates >70 g/day was estimated to be hazardous to human health. Moreover, reported the maximum allowable consumption rate equal to 2.128, 0.136, 0.638, 1.280, and 1.124 kg/day for cultured fish from Zarivar Wetland in Iran base on the Cd, Cu, Cr, Zn, and Fe respectively.