Accumulation of heavy metals (Fe, Zn, Cu, Mn, Cd and Pb) in mullets Planiliza subviridis (Valenciennes, 1836) and Ellochelon vaigiensis(Quoy & Gaimard, 1825) from Damb Harbor, Balochistan, Pakistan

Due to the increasing levels of pollution and its effects on human health, heavy metal pollution has become a serious problem worldwide. Various contaminants are highly toxic and can accumulate in seafood. Thus, the fact that people can be easily exposed to these contaminants means that ultimately human health can also be at risk. When toxic elements are absorbed for a long period of time, they can be highly harmful even at low concentrations (Biswas et al. 2012). Industrial effluents, agricultural runoff, transport, burning of fossil fuels, animal and human excretion, geological weathering, and domestic waste cause pollution of water bodies with heavy metals (Olowu et al. 2010).

Various marine pollution monitoring programs have been initiated due to the fact that marine organisms accumulate contaminants such as metals from the environment (Linde et al. 1998; de Mora et al. 2004). Fish accumulate metals contained in water, their food, seabed sediments and some other particulate material. In many countries, industrial waste, geochemical structure and mining of metals are known to cause heavy metal pollution in the aquatic environment owing to their toxicity and accumulation behavior. Under certain environmental conditions, these heavy metals can accumulate up to toxic concentration levels and cause ecological damage (Sivaperumal et al. 2007).

It should be noted that the level and variety of heavy metals among different fish species depend on their feeding, habits, age, size (including length and weight) and habitats (Amudsen et al. 1997). Contamination caused by heavy metals in animal-based food is a severe threat to human health due to their toxicity, persistence, bioaccumulation, and biomagnification (Kannan et al. 2007; Chary et al. 2008).

Fish can accumulate high concentrations of metals in their tissues, especially in muscles, which makes them a major source of these metals in the human diet (Rose et al. 1999). In general, residents of our coastal areas are very poor and do not have a good sense of food security. It is therefore the responsibility of relevant authorities to focus on this aspect and ensure that food security objectives are incorporated into national poverty reduction strategies. This would have an impact at national, sub-national, household and individual levels, with a particular emphasis on reducing hunger and extreme poverty (FAO 1983).

A large number of commercial fisheries are located in the coastal waters of Balochistan. The fishing pressure on pelagic and demersal fisheries resources has been gradually increasing. However, information on fishing pressure and sustainable stock status is limited and little information on the population dynamics and the status of exploitation in the coastal waters of Pakistan is available. The mullet P. subviridis is the cheapest popular food among the population of the coastal area of Balochistan since ancient times. P. subviridis is an inhabitant of coastal waters in estuaries and bays (Das 1992).

The objective of this study was to determine concentrations of heavy metals (Fe, Zn, Cu, Mn, Cd and Pb) in the mullets P. subviridis and E. vaigiensis caught between January and December 2015.

The carnivorous mullets P. subviridis and E. vaigiensis are important in research on heavy metal accumulation as they are abundantly harvested and are one of the most exported species. In addition, they are commercially important and most often consumed. Therefore, the health risk associated with heavy metals in fish has been assessed using the provisional tolerable weekly intake (PTWI) for both fishes.

P. subviridis and E. vaigiensis samples were collected from the Damb Fish Landing Centre in Sonmiani (Miani Hor, Balochistan) between January and December 2015, during the northeast monsoon (December to February), the post-monsoon season (October and November), the pre-monsoon season (March and April) and the southwest monsoon (May to September), using a Thukri net (length 180–190 m, width 1.5–2.0 m). Fishing was banned on the scheduled days in June and July due to rough weather conditions and in the closed season by the Directorate of Fisheries, the Government of Balochistan. Sonmiani (Miani Hor) is a lagoon located about 10 km north-west of Karachi and mostly on the eastern part of the Balochistan coast between 25°35’N and 66°20’E. It is a 50 km long and 7 km wide contorted body of water, which is connected with the sea through a 4 km wide mouth. Two seasonal rivers enter the lagoon: the Porali River empties into the central part from the northern side and the Winder River enters near the mouth of the lagoon from the eastern side. Sonmiani is one of the important fish landing centers and comprises three villages (Sonmiani, Damb, Bhira and Balochi Goth). Fish samples were transported to a freezer in the laboratory immediately after collection, and then thawed and rinsed in distilled water to remove any foreign particles. The length (cm) and weight (g) of the collected fish were measured. Fish were labelled for identification and then frozen until analyzed. Following the biometric measurements, approximately 2 g of the epaxial muscle from the dorsal surface of each sample were dissected, washed with distilled water, dried on filter paper, weighed, packed in polyethylene bags and kept at −20°C until analysis. An AAnalyst 700 Atomic Absorption Spectrophotometer was used to perform the analysis in the Centralized Science Laboratory of the University of Karachi. Absorption wavelengths (λ) used for the determination of the analyzed metals were as follows: Fe – 248.30 nm, Zn – 213.90 nm, Cu – 324.70 nm, Mn – 279.50 nm, Cd – 228.80 nm and Pb – 217.00 nm. Due to the absence of a standard reference material, the accuracy of the analysis and the effect of matrices in the media were controlled with the standard addition method using three randomly selected samples for each analyzed element, cut into the smallest possible pieces. The typical limits of detection were as follows: 0.1, 0.018, 0.035, 0.01, 0.025 and 0.012 μg ml⁻¹ for iron, zinc, copper, manganese, cadmium and lead, respectively, calculated by regression analysis as suggested by the U.S. Environmental Protection Agency (EPA 2004).

A 1–2 g aliquot of each dry sample was placed in a cylindrical Teflon vessel and digested with 3 ml of a 1:2 v/v mixture of H₂O₂ and HNO₃ at 250°C. The organic part was discarded and the remaining part was diluted with demineralized water to 50 ml in a graduated flask (Bernhard 1976).

Concentrations of metals in P. subviridis and E. vaigiensis in the muscle tissues in different seasons were determined by analysis of variance (ANOVA) using Tukey’s HDS post-hoc comparison method. The results were evaluated on the basis of homogenous groups with a significance level of p < 0.05. The common elements in the muscle tissue of P. subviridis and E. vaigiensis were assessed using Pearson’s correlation coefficients. Data collection and statistical calculations were performed using the SPSS software (Ver 22).

The length and weight (min.–max) of P. subviridis and E. vaigiensis determined in our samples were in the range of 11.00–23.00 cm and 24.00–82.00 g, and 13.50–26.50 cm and 31.00–106.00 g, respectively (Table 1). Seasonal and average distributions of metal (Fe, Zn, Cu, Mn, Cd and Pb) concentrations are presented in Table 2.

The accumulation of the metals in the muscles of P. subviridisand E. vaigiensis followed the order: Fe > Zn > Cu > Mn > Pb > Cd. Accordingly, the accumulation of the metals in the muscles of these two species during the northeast monsoon and the post-monsoon season followed the same order of Fe > Zn > Cu > Mn > Pb > Cd (Table 2).

The highest accumulation of Fe and Pb in P. subviridis was recorded in the pre-monsoon season. Fe and Zn were found to accumulate in the smallest amounts in the northeast monsoon season. Cu, Cd and Pb were recorded in the smallest amounts in the southwest monsoon season. The highest accumulation of Zn, Cd and Pb in E. vaigiensis was determined in the pre-monsoon season. The smallest amounts of Fe, Cu, Mn and Cd were detected in the southwest monsoon season, while the smallest accumulation of Zn was found in the northeast monsoon season.

In the northeast, post-monsoon and southwest monsoon seasons, the concentration of all elements was higher in E. vaigiensis than in P. subviridis. Only the accumulation of Fe in P. subviridis in the pre-monsoon season was higher compared to E. vaigiensis, while the accumulated amounts of other elements in the same season were higher in E. vaigiensis than in P. subviridis (Table 2).

The accumulation of Fe found in the muscles is lower than that reported by Al-Najare (2012) for P. subviridis and E. vaigiensis. However, it is higher compared to that reported by Al-Khafajy et al. (1997) for P. subviridisand E. vaigiensis. On the one hand, the Zn values are lower compared to some of the reported data (Mitra & Ghosh 2014; Chakraborty et al. 2016), but on the other, they are higher than those reported by other authors (Al-Khafajy et al. 1997; Ali et al. 2013) for P. subviridisand E. vaigiensis. The Cu values (Table 3) are lower than those reported in the literature (Al-Khafajy et al. 1997; Al-Najare 2012; Mitra & Ghosh 2014; Chakraborty et al. 2016). However, they are higher than those reported by Ali et al. 2013 for P. subviridisand E. vaigiensis. The Mn values are lower compared to the published data (Al-Khafajy et al. 1997; Al-Najare 2012) for P. subviridisand E. vaigiensis. The Cd values are lower than those reported in the literature for P. subviridisand E. vaigiensis (Al-Khafajy et al. 1997; Al-Najare, 2012; Norouzi et al. 2012; Ali et al. 2013). However, they are higher than those reported by Sai Su et al. (2009) for P. subviridisand E. vaigiensis. On the one hand, the Pb values (Table 3) are lower compared to some of the reported data (Al-Khafajy et al. 1997; Mitra & Ghosh 2014; Chakraborty et al. 2016), but higher than those reported by Sai Su et al. (2009), Norouzi et al. (2012) and Ali et al. (2013).

There is no difference (p> 0.05) between seasonal accumulations of Fe, Cu, Cd and Pb in P. subviridis. There is also no difference (p> 0.05) between seasonal accumulations of Zn in E. vaigiensis. Zn and Mn accumulations were found to be different (p > 0.05) for P. subviridis in all seasons. Fe, Cu, Mn, Cd and Pb accumulations were found to be different (p> 0.05) for E. vaigiensis in all seasons.

Table 4 demonstrates that there is no high correlation between the metals for P. subviridis and E. vaigiensis. Cd showed a low correlation with Fe, whereas Pb showed a low correlation with Zn.

Concentrations of Fe, Zn, Cu, Mn, Cd and Pb found in daily fish consumption per capita are calculated to assess a potential health risk to Pakistanis. The average daily fish consumption by Pakistanis is 33 g per capita (Chughtai & Mahmood 2012). The Provisional Permissible Tolerable Weekly Intake (PTWI) of Fe, Zn, Cu, Mn, Cd and Pb (for a 60 kg adult person; g/week/60 kg body weight) was 5600, 7000, 3500, 980, 7 and 25, respectively, expressed in g/week/60 kg body weight (FAO/WHO 2004). The heavy metal accumulation in the muscles of P. subviridisand E. vaigiensiswas found to be below nationally and internationally stipulated values and does not pose a serious health risk (Table 5).

The accumulation of Fe, Zn, Cu, Mn, Cd and Pd in the muscles of P. subviridis and E. vaigiensis (Table 3) is lower than the international limits (FAO 1983; WHO 1989).

The results of this study show that the accumulation of Fe, Zn, Cu, Mn, Cd and Pd in P. subviridis and E. vaigiensis caught at the Balochistan coast was generally below the international limits. The present study shows that the largest Pb accumulation (0.56 μg g⁻¹ and 0.61 μg g⁻¹) detected in P. subviridis and E. vaigiensis is higher than the limit value (0.50 μg g⁻¹) reported by FAO (1983). The accumulation of Pb in these two fishes should be monitored in the future. Otherwise, the pollutants may be harmful to the health of the fish populations and humans who consume them.