Evaluation of the heavy metal content in the muscle tissue of common carp (Cyprinus carpio L.) reared in groundwater in Basrah province, Iraq

Introduction

The Shatt al-Arab river is the only source of freshwater in the Basrah governorate, south of Iraq. The Shatt al-Arab river faces major challenges, including a decrease in the amount of freshwater received from the Tigris and Euphrates rivers (1), an increase in the quantity of pollutants that reach it owing to agricultural, industrial, and domestic activities (2), and, the most difficult challenge, the phenomenon of frequent saltwater intrusion (SWI) from the Arabian Gulf (3). The SWI problem has caused the elimination of the vast majority of fish farms along the Shatt al-Arab River (in earthen pond and cage systems) (2).

Because of the availability of groundwater, a limited number of fish farms have been established for raising fish in earthen ponds or plastic tanks to avoid the economic losses caused by the exposure of fish to the direct environmental changes in the Shatt al-Arab river and sudden fluctuations in water quality resulting from SWI (2). The western part of Basrah, including Khour al-Zubair, is the industrial zone of the province, in which many of the industrial facilities are located such as oil refining sites, the south gas company, the Khor Al-Zubair gas power station, a petrochemical plant, a fertilizers plant, cement factories, iron and steel plants, which emit a large amount of pollutants (4).

Fish take heavy metals from the water, which accumulate in the tissue to a hazardous level without any visible signs (5). Fish are good indicators of metal contamination and are usually used to evaluate the risk of human ingestion of fish contaminated by metals and other potentially toxic elements (6). Therefore, the concentration of heavy metals in fish meat should be measured and compared with the maximum limits established by the international legislation such as FAO/WHO, European Commission and Food and Drug Administration to ensure its suitability for human consumption (7).

X-Ray fluorescence (XRF) is a fast, economic, and nondestructive analytical method for the analysis of a variety of hazardous materials and environmental samples. It can be used to identify up to 30 elements at the same time (8). XRF is usually used in archaeological studies to investigate the content of metals in human and animal fossil bones, in which the samples are precious and their destruction should be avoided (9). Total reflection XRF (TXRF) spectroscopy is a powerful analytical technique for elemental analysis of small masses. In this context, Bilo et al. (10) used TXRF to study Pb and Zn accumulation in zebrafish embryos. Similarly, Limburg et al. (11) used the X-ray fluorescence technique to investigate fish otolith trace element maps.

The aim of this study was to evaluate the heavy metals concentration in the edible part of common carp (Cyprinus carpio) reared in groundwater in Khor al-Zubair, Basrah province to determine their suitability for human consumption using the XRF technique.

Materials and methods

The fresh fish were purchased in January 2019 from one of the fish farms located in Khor al-Zubair, Basrah province, southern Iraq (Figure 1).

Figure 1: The yellow pin on the map shows the location of the fish farm in Khor al-Zubair, Basrah province.

The farm completely depends on well water 24 m depth, which is pumped to an earthen pond before it is distributed to the fish culturing system. The initial weight of the fish was 100 g and fish were kept in the rearing system 12×3×2 m for 10 months (from March 2018 to January 2019). During the rearing period, fish were fed different types of fish diets including locally made fish diets and imported fish feed depending on availability in the local market and the purchasing power of the farmers.

A total of 10 fresh common carp were purchased from the farm gate and immediately placed in polyethylene bags, put into an icebox, and brought to the laboratory. The fish length was measured from tip to tail and weighed; data for the individual fish were recorded. The samples were prepared according to AOAC (12). Briefly, fish samples were cleaned using a stainless steel knife; the edible part of the fish (tissue from the dorsal side of the body, which is the consumed part of fish) was taken without the skin, cut to small pieces, and washed with distilled water. The samples were dried in a drying oven at 65ºC for approximately 24 hours and homogenized in a porcelain mortar. Approximately 0.5 g of the powdered fish of each sample (10 samples) was weighed and mixed with 3 drops of organic binder and the sample was pressed to form pellets ready for metal analysis

XRF Spectro, Shimadzu, Japan was used to qualitatively and quantitatively determine the metals present in the tissue samples according to Klockenkämper )13). The results were expressed in mg/ kg dry weight and the results were compared, where available, to WHO Guidelines, EC, Environmental Quality Standards, and FDA Consumer Advisory.

Statistical analysis

The statistical analysis (min, max, mean, standard deviation, and range) was conducted using Microsoft excel. All the results are presented as mean ± standard devotion (SD). SPSS program was used to test the normality of distribution and the Pearson correlation.

Results

In the current study, we determined the concentration of heavy metals in common carp reared in earthen ponds that were filled with water from a well (24 m depth) in Khor al-Zubair, Basrah, southern Iraq. The average weight and length of the fish used in the current study were 755.7 ±76.8 g and 29.74 ±1.65 cm respectively.

The results of the present study were compared with the international standard limits to determine if these fish were suitable for human consumption. The minimum, maximum, mean concentration, and SD of heavy metals in the edible tissues of the common carp from Khor Al-Zubair farm compared with the standard limits are given in Table 1. Table 2 and 3 represent the descriptive statistics of heavy metal concentrations (mg/kg) in the muscle of common carp.

Discussion

The maximum concentration of Cu in the muscle tissue was 5.30 mg/ kg, which was lower than the FDA (14) maximum guidelines (30 mg/ kg). Similarly, the mean concentration of Zn recorded in the current study was 42 mg/ kg, which was lower than the FAO/WHO (7) maximum guidelines (50 mg/ kg). It has been suggested that Cu and Zn, as essential metals, tend to accumulate in the liver of fish and not in the edible part owing to their role in metabolism (15).

Table 1: Content of heavy metals (mg/ kg dry weight) in the edible parts of the common carp (Cyprinus carpio) reared in ground water compared with standard limits

Table 2: Descriptive statistics of heavy metal concentrations (mg/kg) in the muscle of common carp

Table 3: Pearson correlation between heavy metals concentrations (mg/kg) in the muscle of common carp

* Correlated at 5% significance level, ** Correlated at 1% significance level.