Seafood is a major source of diet for a large populous of the world, particularly coastal communities [1
]. Fish is an important source of protein associated with many beneficial health effects [2
]. Owing to its nutritional importance, its safety and quality is of prime importance [3
]. Fish has extensively been studied around the world for heavy metals [4
]. Ingestion of contaminated fish is an important route of human exposure to heavy metals [5
]. Heavy metals are non-degradable; once they enter the ocean, they accumulate in organisms and bio-magnify in the apex predators, and then transfer the toxic pollutant load through the food web [4
]. This in turn lowers the quality of seafood and is a potential human health risk. Fish consumed worldwide are found to be highly contaminated by heavy metals [6
The shipbreaking industry has imported millions of tons of toxic waste to the coastal beaches of South Asia [7
]. These end-of-life ships’ waste is of different nature, like oil, asbestos, organotins, Polycyclic Aromatic Hydrocarbons (PAHs), Polychlorinated Biphenyls (PCBs), and heavy metals. These often get mixed with beach sediment and seawater, which in turn has a negative impact on the coastal environment and biodiversity [8
Shipbreaking activities have impacted the marine environment and various studies have already expressed concerns [9
]. The metals released into the water system are absorbed and deposited by suspended sediments [10
]. This reduces the concentration of heavy metals in the water column and makes surface sediments a reservoir for particle-related pollution [11
]. Due to the bioaccumulation and bioconcentration process, the influence of heavy metals can be detected on land via the food web [11
]. It is important to constantly monitor fish species for concentration of heavy metals as it provides a good indication of pollution status [12
]. Contaminated fish is an important source of heavy metal exposure in humans. Therefore, the assessment of metal content in the organs and tissues of fish is of high significance.
Pakistan has a coastline of 990 km, comprised of two parts: the coast of Balochistan (745 km) and the coast of Sindh (245 km) [13
]. Balochistan coastline is mainly unpopulated, and its beaches are one of the cleanest in the world. This coastal region provides valuable habitat for numerous species and is a key resource for the coastal populous [14
]. The coastal people rely primarily on fisheries and boat making for their livelihood [15
Currently, Pakistan is one of the world’s largest shipbreaking countries and ranks third in the world in terms of scrapped tonnage and in number of ships, followed by Bangladesh and India [15
]. This sector has been ignored so far, by both the provincial and federal government [16
End-of-life tankers, bunkers, and container ships are beached on designated slots for almost two to three months to be dismantled. Hundreds of accidents have led to severe environmental as well as human health issues in the region. According to the World Bank usual scenario-based study, the projected accumulation of heavy metals that will remain at the yards or in beach sediments for 2010–2030 will be 22 tons [16
]. The area contains 314 plots of different sizes—135 of them are active shipbreaking yards. The beach of Gadani is sandy and the water level is deep with a tidal range between 1 and 3 m.
The Balochistan coastal zone, up to about 20 km inland from the coast, is delineated by the Makran Coastal Mountain Range (MCMR). MCMR runs parallel to the coast and separates it physically, socially, and economically from the rest of the province. Balochistan’s coastal water is defined by a narrow continental shelf, mostly 15–50 kilometers wide at the 200 meter isobaths. From here, the continental slope dips sharply, delimiting an extensive, deep offshore zone.
The sole large island along the Balochistan coast is Astola (Haft Talar), 7 square km, 39 km from Pasni. Astola is uninhabited. It is endowed with 29 different species of corals, endangered green and hawksbill turtles, and is a breeding ground for many water-birds. It is one of the four Ramsar Convention sites in Balochistan. Corals are also found at 9 locations along the Balochistan coastline, e.g., they have recently been discovered at Daraan and Gunz, which add to the ecological importance of the coastal areas of the province.
As a result of monsoon dynamics and strong seasonal upwelling of nutrient-rich water from the depths along the narrow continental shelf, there is high surface productivity in the Arabian Sea and the area is known to be rich in marine biodiversity. The Balochistan coastal zone is rich in marine fisheries, which include about 350 different species. Some 240 are demersal fish, 50 are small pelagic, 10 are medium-sized pelagic, and 18 are large pelagic fish. In addition, there are 15 species of shrimps, 12 of squid/cuttlefish/octopus, and 5 species of lobsters.
The shipbreaking industry has introduced hazardous waste to the coastline of Pakistan. No comprehensive study has yet been carried out to determine the contamination level and its impact on water and marine fish and associated human health from the consumption of fish in the Gadani coastal area. Therefore, the aim of this study is to investigate the accumulation of heavy metals in water and in fish along the shipbreaking area, and human health risk from the toxicity of selected heavy metals owing to the ingestion of fish.
The physiochemical properties determine the overall health of any aquatic system [37
]. Shipbreaking severely affects the physiochemical properties of seawater due to the continuous discharge of liquid waste, such as salt, caustic agent, paints, and various other contaminants [7
]. Several studies reported high pH, turbidity, TSS (Total Suspended Solids), TDS (Total Dissolved Solids), ammonia, and high EC in adjacent seawater [7
]. Metals released into the aquatic system are adsorbed by suspended sediments and settle down. This results in the lower concentration of heavy metals in the water column [40
]. The accumulated heavy metals in sediments are released into the water column by certain variables, like redox potential, altering pH, mobilization of benthic biota, and sediment re-suspension [41
]. The results of this study were in accordance with other regional studies conducted on shipbreaking areas reported in Table 6
. Fish uptake heavy metals by two pathways: through the digestive tract by diet exposure and through the gills’ surface by water exposure [42
]. The correlation study showed that there is a positive correlation between metal concentration in seawater and muscles (p
< 0.05) and a negative correlation between seawater and gills (p
> 0.05). It is important to constantly monitor seawater for concentration of heavy metals as it provides a good indication of pollution status [43
The average Pb concentration for all fish species was higher than the study conducted in the Bay of Bengal, Palk bay, India, Turkish Sea, Mian kaley, Lake Iran, and lower than the study conducted in the Mediterranean Sea (7.33–9.11 μg/g), Jedda Coast, and Saint Martin Island. Whereas, the Pb concentration in examined fish species agreed with the reported studies for the Gulf of Aqaba and Bangshi river (Table 6
). The Pb concentration was also higher than the permissible limits (0.5 μg/g—FAO, 2 μg/g—USEPA (United States Environmental Protection Agency), 2 μg/g—WHO (World Health Organization) [34
]. This high level of Pb may be attributed to the highly contaminated sediments of the shipbreaking area. In Chittagang, it was reported that the Pb concentration in sediments ranged from 4232 to 5733 mg/kg [44
]. Pb is mainly released from paints, batteries, and electrical equipment, during the ship dismantling process into the beach sediments that are carried away by the waves and tides. This then makes its way into the food chain. Thus, this high level of Pb in selected fish species may be attributed to a high level of Pb in seawater (Table 2
). Thus, the continuous mobilization of contaminated sediments may be a source for metal uptake by the adjacent biota. Through ingestion of contaminated fish, Pb toxicity can inhibit or mimic the actions of calcium, and in the same way, Pb also has an affinity for sulfhydryl group that in turn disturbs multiple enzyme systems.
Ni levels in the muscles were higher than those from the Turkish seas [56
]. And the South East Coast of India [57
], and lower than the studies reported in the South West coast of India and the Red Sea [45
The Cd level was lower than those observed in the fish species of the Santa Maria Bay (1.52–14.09) [58
], while higher than the values displayed for other fish in the Red Sea [59
], the SE coast of India [57
], and the Bay of Bengal [60
]. However, the results of Cd in analyzed samples were in line with the study conducted in the oil-spilled area and the Red Sea [61
]. The mean Cd concentrations in the muscles of individual fish species were in the range of permissible limits [62
The Mn concentration in this study was lower than the values observed for Mn in the fish species reported from the Red Sea and the Bay of Bengal. However, the Mn concentration exceeded the values observed for other fish species in the Gulf of Aqaba [47
] and the SE coast of India [57
Variation between fish species regarding metal accumulation was shown in Figure 3
. There was a slight difference between metal accumulation in gills and muscles of different fish species. Among the observed fish species, Pb was highly accumulated, followed by Ni, Mn, and Cd. As mentioned earlier, fish uptake heavy metals by two pathways: through the digestive tract by diet exposure and through the gills’ surface by water exposure. Gills and muscles are important to monitor as they reflect the surrounding environment. This study correlated metal concentration in aqueous medium with that of metal concentration in gills and muscles. The correlation study showed that there is a positive correlation between metal concentration in seawater with muscles and a negative correlation of metal concentration in seawater with gills. Muscles are important for dietary exposure.
The metals released into the water system are absorbed by suspended sediment, and then settle down [10
]. This causes a lower concentration of heavy metals in the water column and makes surface sediments a reserve for pollution [63
]. Bottom fauna depends on bottom sediments in terms of habitat and food source [4
]. Thus, the metals associated with the particles are a source of pollution [11
]. Fish that have been affected are an important way of exposing heavy metals to humans. Therefore, this poses a risk to human health.
Bioaccumulation refers to an organism’s intake ability of chemicals from the aqueous medium. Bioconcentration is different from bioaccumulation in a way that bioconcentration depends only on water exposure, while bioaccumulation considers both dietary and water exposure [64
]. Bioconcentration is mainly done for lab data and bioaccumulation for field data, as one cannot be sure that the heavy metals’ concentration in fish is only because of water in the field. The value of BAF > 1 generally declares that the biota can potentially accumulate heavy metals in their body, but this becomes significant when the value exceeds 100 or more. BAF values greater than 1000 indicate a hazard, as declared by many regulatory agencies. This value has its origin from non-polar compounds. The BAF value > 1000 signifies slow and significant accumulation, which means that there is a potential for chronic effects and a chance for tropic transfer. BAF of Mn was greater than 100 and less than 300, while the average Ni, Pb, and Cd BAF values were less than 100, showing potential accumulation.
The mean of all EDI was below the tolerable daily intake (TDI) (0.0015) [30
]. The mean EDI values compared with the TDI values showed that there is no detrimental health risk from the consumption of selected fish in the study area.
THQ is actually a model for relating the average chronic daily intake of contaminants in diet with the reference dose, calculated by International and National Agencies (ASTDR, IRIS, EPA (Environment Protection Authority)). THQ > 1 signifies a potential risk, whereas THQ < 1 means there is no risk, or a minimal potential risk that is negligible. THQ is actually suggested for non-carcinogenic metals. The human population is exposed to the combined impact of contaminants. An accumulative risk index including selected heavy metals is required for all individual fish species. This is called the hazard index (HI) [30
], and it is the sum of all THQs for each individual fish. The HI values for all individual fish species were greater than unity and are presented in Table 5
. Based on HI values, Small-scale terapon, Torpedo scade, Sickle fish, Saddle grunt, Gold silk seabream, Indian mackerel, and Spotted sickle fish may cause potential health risks. The continuous exposure from these metals through the consumption of selected fish species may cause chronic health effects.
This study considered only Ni, owing to its availability of carcinogenic slop factor, for the TR Index [30
]. The TR value is calculated for those metals that have a known carcinogenic effect upon lifetime exposure. TR values for Ni are listed in Table 5
TR values > 1 in a million (10−6
) are considered a risk, and this is unacceptable by the USEPA standards (2000). However, the acceptable level may vary somewhere else in accordance with national standards and environmental policies and may be as high as 10−4
. Risks that lie between 10−4
are considered acceptable [29
]. The TR values indicate that there is a low or minimal carcinogenic risk from the consumption of the examined fish species.