Introduction

Pollution of heavy metals in aquatic ecosystem is a growing threat in the world. Growing industrialization and population further aggravate the condition. As heavy metals cannot be degraded, they are deposited, assimilated and incorporated in the water bodies. These heavy metals get bioaccumulated and biomagnified via the food chain and finally reach the human consumers and create health risk (Davydova 2005). Fishes are one of the most widely distributed organisms in aquatic environment and considered as one of the main protein sources of food for human (Sen et al., 2000). Sea water fishes play an important role in determining residents’ diet. They are good source of proteins and polyunsaturated fatty acids (PUFA) (Kris Etherton et al.,2002). Fish has been found excellent indicator for heavy metal contamination level in aquatic system because it occupies different food chain levels (Even and Ghaffari, 2011). The bioaccumulation of heavy metal in tissues of marine organism is an indirect measure of the abundance and availability of metals in the marine environment. Marine organisms are characterized by a greater spatial ability to accumulate metals compared to bottom sediments, for this reason fish are widely used as bio-indicator for aquatic or marine pollution by metals and have also been used to evaluate ecological risk. Therefore heavy metal analysis of fish ensures dietary safety of the fish from a particular body of water (Javed and Usmani, 2016). The establishment of an export-processing zone has resulted in increased volume of waste generated by industries and other commercial activities. A number of studies have been carried out on the accumulation of heavy metals in fish and other sea organisms, water and sediments in the past. The main aim of this study is to assess the level of the selected heavy metals like cadmium and lead in the muscles and bones of the most commonly found and consumed fish species Sardine (Sardinella longiceps) sold in and around Thrissur.

Materials and Methods

The fish samples of Sardine were collected from six different vendors in and around Thrissur, Kerala. From each vendor six fish of sardines were collected and forms the six groups. These six groups of fish considered as the materials to be studied. It was carefully conserved with ice in clean polythene bag. Ice was used to minimize the tissue decay and to maintain the moist condition during transportation and stored at -20⁰C. The frozen fish samples were thawed at room temperature. The samples were cleaned and scales were removed. Bones and muscles without bone were collected in twelve different containers for heavy metal estimation. Bone and muscles were cut into small pieces and weighed. Approximately 0.5 grams of bones and 1.5 grams of muscles from each sample were taken for estimation. For microwave digestion, the muscle tissues was dissolved using 12mL of 70% nitric acid and for bone tissue 12 mL of 70% nitric acid and 3 mL of 37% hydrochloric acids were used for microwave digestion using standard temperature and pressure using microwave sample preparation system (TITAN MPS, Perkin Elmer, Serial No: 313X3121101). All organic materials in each sample were completely digested. The digests were allowed to cool and diluted with deionised water to a total volume of 50 mL in a standard flask.

The digested samples were kept in plastic containers and assessed with atomic absorption spectrometry (AAS) using Perkin Elmer, Model no: PinAAcle 900H. Nitric acid and nitric acid: hydrochloric acid (4:1) used in the digestion of muscle and bones were kept as reagent blanks respectively. The values were used to plot a standard curve. The standards and reagent blank were treated in the same way to minimize the matrix interference during the analysis (Khalifa et al.,2010).

Results and Discussion

Six samples collected from different vendors were subjected to examination and the different levels of cadmium and lead found in sardine are presented in Table 1.

Table 1: The concentration of Cadmium (Cd) and Lead (Pb) in bone and muscles of fish samples

*- NDL- non detectable level, ppb**- parts per billion

The lead concentration in bones of Group 2 and 3 was 15.26 and 13.54 ppb respectively. In all other groups the concentrations of lead is minimum or undetectable level. In Group No. 1, the cadmium and lead concentration in muscles were 9.06 and 10.32 ppb respectively. In all other groups the cadmium and lead concentration is minimum or non detectable level. The ranges of heavy metal cadmium in bones and muscle was 0.45-6.26, 0.61-9.06 ppb respectively. The ranges of heavy metal lead in bones and muscle was 0.87-15.26, 1.05-10.32 ppb respectively. These results were compared with WHO limits of cadmium and lead in fishes. Maximum permissible levels of Cd and Pb in fish are 30ppb and 50 ppb respectively.

The result obtained in this experiment revealed values within the maximum permissible level. Cadmium and lead belongs to non-essential and toxic metals. However the accumulation of these metals due to prolonged consumption has to be elucidated. As the heavy metals are not excreted from the body that accumulates in the tissues of fish pursues threat to humans who consume the culminating poison. The accumulation of lead and cadmium in the edible parts of the fish, though not exceeding the maximum permissible level, indicate a slow process of these metals entering into the food chain. Heavy metals like cadmium, lead and mercury can be bioaccumulate and biomagnify in aquatic organisms (Dietz et al., 2000). This bioaccumulation can result in a negative impact on fish metabolism, their behaviour, physiology and ecology (Gbem et al., 2001; Boalt et al., 2012).

Because of their toxicity, long persistence, bioaccumulation and biomagnifications, heavy metals are serious threat to human food chain (Eisler, 1988). The patterns of biomagnifications and bioaccumulation are differing between the tissues and organs (Vieira et al., 2011). So, further studies regarding the hazard involved in prolonged consumption resulting in bioaccumulation and their magnification in different organs and different species has to be investigated.

Conclusion

Estimation of cadmium and lead in Sardinella longiceps fish muscle and bone reveals within the normal permissible level of cadmium and lead in muscle and bone. As heavy metals, bio accumulated and they are not excreted from the body. So, periodical checking of heavy metals in the aquatic environment and further studies needed to find the bioaccumulation of prolonged consumption of fish and their products.

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