Environmental pollution can be described as the consistent or intermittent release of compounds that are hazardous to living organisms into water, soil, or the atmosphere [1]. Given their toxicity to humans and environmental degradation, heavy metals are typical contaminants found in urban areas and should be taken very seriously [2]. Both naturally occurring and man-made sources can introduce heavy metals into the aquatic environment, which can have detrimental effects on aquatic ecosystems, the food chain, and human health [3]. The integrity of the environment has been threatened by anthropogenic heavy metal input, particularly when it comes to aquatic life such as fish species [4]. It has been documented that fish and other aquatic species' genetic, physiological, biochemical, and behavioral characteristics can be altered by heavy metals [5]. Due to their importance as a source of protein for humans and their presumed superior understanding of the aquatic environment, fish have been the most frequently chosen organisms to test for heavy metals [6]. Because heavy metals are extremely persistent, unbreakable, and have the capacity to bio accumulate, their pollution of the aquatic environment is an issue that is becoming more and more widespread worldwide [7]. Because they vary in size and trophic level, fish are useful markers of heavy metal pollution and are frequently used to assess the health of aquatic ecosystems [8]. Fish are indicators of pollution and water quality since they are at the top of the aquatic food chain. After consuming such aquatic organisms, humans risk experiencing health problems due to the accumulation of metals in the food chain and ecological harm [9]. Therefore, keeping an eye on fish tissue pollution is crucial for serving as an early warning system for contaminated water and for assisting in the appropriate implementation of corrective measures to protect the environment and public health [10].

Since trace metals do not leave aquatic environments by self-purification [11] and instead concentrate in suspended particulates and sediments [12], there has long been concern about their toxicity and possible hazard to human health as well as ecosystems through the food web. As a result, assessing the ecological risk associated with trace elements has gained popularity recently [13]. Both naturally occurring channels and a range of human-caused sources allow heavy metals to reach the aquatic environment [14], where they can negatively affect aquatic ecosystems, the food chain, and human health [15]. A complex interplay of biological and ecological factors determines the concentration of heavy metals in biological compartments, like fish muscle [16]. In fish, these components can induce difficulties in growth and reproduction, as well as histological abnormalities in the skin, gills, liver, spleen, and kidneys [17]. Given the popularity of smoked fish products among the local population, this study was necessary due to the lack of information on the amounts of heavy metals in smoked fish offered in many markets in Nigeria.

Thermal treatments and processing of food at high temperatures that have direct contact with combustion gases, such as smoking, lead to direct microbial annihilation [18]. It also causes a decrease in the moisture content of such food, which concentrates other nutrients, some of which may be heavy metals [19,20]. Even though fish may consume heavy metals from their contaminated surroundings, processing and preservation techniques like smoking and the use of chemicals to lengthen and preserve food may affect the amount of heavy metal contamination. For this reason, it is important to evaluate the effects of these techniques in order to effectively manage health risks. One of the most popular fish products in the markets of Awka, the capital of Anambra State, Nigeria, is smoked fish, which is consumed by a large number of local families.  Awka, the capital of Anambra State, Nigeria, has three big marketplaces where dried fish is sold on a daily basis. The study therefore evaluated certain heavy metals in these fishes.

Study Location

The study was carried out in three local markets, namely: Eke-Awka main market, Nkwo-Amenyi market, and Kwata market. These markets were all located in Awka metropolis.

Sample collection

The dried consumer fish samples were procured from three selected markets on three occasions. The species were: stockfish (cod): Gadus morhua; horse mackerel (shina): Trachurus trachurus; and African catfish: Clarias gariepinus. Three samples of each fish species were purchased directly from each of the selected markets, making a total of twenty-seven samples. The fish are commonly consumed by the local populace in Awka metropolis.

Heavy Metal Analysis

The muscles from each of the sampled fish were rinsed with distilled water to remove dust and other external adherents. They were then dried in an oven at 1050 °C. They were later homogenized using a mortar and pestle. 10g of the homogenate was digested as described by APHA (1998). The sample was digested using a 1:5:1 mixture of 70% perchloric acid, concentrated nitric acid, and sulfuric acid at 800 °C in a fume chamber until a colorless liquid was obtained. The metal concentrations, which include Fe, Cr, Pb, Cd, and as, were determined by atomic absorption spectrophotometry using the Buck Scientific 200A model. Levels of heavy metals were expressed in mg/kg dry weight.

Statistical Analysis

The data obtained from the study were collated and analyzed using the Statistical Package for Social Sciences (SPSS 22.0). A one-way analysis of variance (ANOVA) was employed to reveal significant differences in measured variables. When a difference was detected (P < 0.05), Turkey’s multiple comparison test was applied to identify differences between the means.

Table 1 represents the concentration of heavy metals in some dried consumer fish sold in Nkwo Amaenyi Market, Awka, compared with the World Health Organization (WHO) and Federal Ministry of Environment (FME) acceptable limits. The result of the study revealed that G. morhua has the highest concentration of cadmium (0.18±0.02), followed by T. trachurus (0.06±0.01). G. morhua has the highest concentration (0.32±0.06), while C. gariepinus has the lowest (0.10±0.01). Mercury was highest in G. morhua (0.07±0.02), followed by T. trachurus (0.02±0.00), and C. gariepinus was the least. The highest concentration of Arsenic was observed in G. morhua (0.03±0.00), while Zinc has the highest concentration (4.76±0.12) in T. trachurus.

Table 2 represents the concentration of heavy metals in some dried consumer fish sold in Eke-Awka Market, Awka, compared with the World Health Organization (WHO) and Federal Ministry of Environment (FME) acceptable limits. The result of the study revealed that G. morhua has the highest concentration of cadmium (0.20±0.03a), G. morhua has a concentration (0.29±0.04), and Mercury has the highest concentration on G. morhua, same as Arsenic. T. trachurus has the highest concentration of Zinc (4.99±0.23).

Table 3 represents the concentration of heavy metals in some dried consumer fish sold in Kwata Market, Awka, compared with the World Health Organization (WHO) and Federal Ministry of Environment (FME) acceptable limits. The result of the study revealed that G. morhua has the highest concentration of Cadmium (0.17±0.02), Lead (0.28±0.03), Mercury (0.06±0.0), and Arsenic (0.03±0.00), while C. gariepinus has the lowest concentration. T. trachurus has the highest concentration of Zinc (4.74±0.23), followed by G. morhua (4.07±0.69), while C. gariepinus has the lowest concentration (1.02±0.02).

The comparative values of Cd in some dried consumer fish sold in three markets in Awka Metropolis are shown in Figure 1. The results indicated that the highest value of 0.20 was observed in the species G. morhua in Eke Awka, while the lowest value of 0.04 was observed in C. gariepinus in all the markets. The comparative values of Lead in some dried consumer fish sold in three markets in Awka Metropolis are shown in figure 4.2. The results indicated that the highest value of 0.32 was observed in the species G. mrhua in the Nkwo market, while the lowest value of 0.10 was observed in C. gariepinus in the same market.

The comparative values of Mercury in some dried consumer fish sold in three markets in Awka Metropolis are shown in figure 4.3. The results indicated that the highest value of 0.09 was observed in the species G. morhua in the Nkwo market, while the lowest value of 0.01 was observed in C. gariepinus in all the markets. The comparative values of Arsenic in some dried consumer fish sold in three markets in Awka Metropolis are shown in figure 4.4. The results indicated that the highest value of 0.03 was observed in the species G. morhua in all the markets, while the lowest value of 0.01 was observed in C. gariepinus in all the markets. The comparative values of Arsenic in some dried consumer fish sold in three markets in Awka Metropolis are shown in figure 4.5. The results indicated that the highest value of 0.4.99 was observed in the species T. trachuru in Eke Awka market, while the lowest value of 1.01 was observed in C. gariepinus in Kwata market.

Table 1: Heavy Metals in Some Dried Consumer Fish Sold in Nkwo Amaenyi Market, Awka (Mean± SD).

Means with the same row with different superscripts are significantly different (P<0.05)

Table 2: Heavy Metals in Some Dried Consumer Fish Sold in Eke-Awka Market, Awka (Mean± SD).

Means with the same row with different superscripts are significantly different (P<0.05)

Table 3: Heavy Metals in Some Dried Consumer Fish Sold in Kwata Market, Awka (Mean± SD).

Means with the same row with different superscripts are significantly different (P<0.05)

Figure 1: Concentrations of Cadmium (Cd) in Some Dried Consumer Fish Sold in Three Markets in Awka Metropolis.

Figure 2: Concentrations of Lead (Pb) in Some Dried Consumer Fish Sold in Three Markets in Awka Metropolis.

Figure 3: Concentrations of Mercury (Hg) in Some Dried Consumer Fish Sold in Three Markets in Awka Metropolis.

Figure 4: Concentrations of Arsenic (As) in Some Dried Consumer Fish Sold in Three Markets in Awka Metropolis.

Figure 5: Concentrations of Zinc (Zn) in Some Dried Consumer Fish Sold in Three Markets in Awka Metropolis.

Heavy metals have been a concern in recent times because they are persistent in the natural environment and biodegradable. They have become a cause for concern [21]. Fish intake of heavy metals in contaminated aquatic environments is influenced by a variety of parameters, including salinity, diet, sediment, water pollution level, ecological needs, and metabolism [22]. With the exception of Zn, which is greater in T. trachurus, the concentrations of Pb, Cd, and Mg in smoked fish species were highest in G. morhua. This suggests that the fish species may have bioaccumulated more Zn than the other metals in its host aquatic medium. For CD, the circumstances might have been the opposite in the same domain. Bioaccumulation measurements relate to studies or procedures tracking the uptake and retention of contaminants like metals or biocides in the tissues of organisms such as fish [23]. The processes and channels of pollutants from one trophic level to another are described by the bioaccumulation of heavy metals in living organisms and biomagnifications. Toxic metals can enter the tissues of a living thing during the bioaccumulation phase if they are highly available in the environment or food [24].

In addition to elements absorbed through the gills from the surrounding water or by the ingestion of contaminated food, it has been observed that elements can be introduced by smoke, and concentrations can approach hazardous levels [25]. The amounts of heavy metals in fish were found to change significantly (P > 0.05) amongst markets, most likely as a result of the smoked fish product coming from various wholesalers or processors [26]. Additionally, there were notable variations (P<0.05) in the heavy metal concentrations among different fish species, most likely due to seasonal fluctuations in the metal levels in fish aquatic media. When compared to the results of this study, Wangboje and Oghenesode [27] measured the levels of several heavy metals in smoke-dried Bonga fish (Ethmalosa fimbriata) sold in fish markets in Warri, Niger Delta, Nigeria, and found significantly higher values for Pb (2.62 mg/kg), Cd (0.19 mg/kg), Zn (87.90 mg/kg), and Cu (18.89 mg/kg). The anomalies of the host environment could be the cause of the values that seemed to be greater in the earlier investigation than those in this study. For instance, comparatively, there is more crude oil exploration activity in Warri than in Anambra State.

Even though it was not above the FAO limit, cadmium had the greatest concentration and quota of all the metals among the fish species. This indicates that potential consumers of the smoked fish are not in any way endangered by ingesting this level of heavy metals. As per the findings of Hassan et al. [28], in the event that the concentration surpasses the FAO limit, the period of exposure determines whether it is acute or chronic. When heavy metals build up in soft tissues without being absorbed by the organism, they become poisonous [29]. According to the overall heavy metal load in fish by market, the Eke-Awka market had the highest values. This research implies that, should the restriction surpass the WHO limit, future consumers of smoked fish may theoretically be exposed to a higher metal load.

In this study, the FAO/WHO limits for heavy metals were 0.05 mg/kg for Cd, Mg, As, and Pb, and 99.40 mg/kg for Zn. The fact that the mean concentrations of each heavy metal—Pb, Mg, As, Cd, and Zn—in fish did not exceed the previously specified thresholds for heavy metals in fish means that this claim is not problematic. In addition to natural or lithogenic sources, Pb can be obtained from batteries, paints, alloys, solders, batteries, plastics, and fossil fuels. On the other hand, Cd can be obtained from paints, batteries, fertilizers, batteries, and alloys. Batteries, metallurgical processes, insecticides, alloys, colors, fossil fuels, and landfills are among the sources of zinc [30].

The study effectively determined and measured the levels of Pb, Cd, Mg, Zn, and As. Purchased from Nkwo Amaenyi, Eke-Awka, and Kwata markets in Awka metropolis, Anambra State. Since the levels of heavy metals in the fish species utilized in this study were within the limits stipulated by the Federal Ministry of Environment (FME) and the World Health Organization (WHO), the fish were deemed safe for human consumption. In the study, the species G. morhua had the highest concentration of heavy metals, followed by T. trachurus and C. gariepinus, which had the lowest concentrations. Given the low concentrations of heavy metals found in the smoked fish samples examined in this study, ongoing monitoring is necessary to determine when the levels of heavy metals exceed the permissible limit for safe consumption. This is because exposure to these metals can pose a risk to human health. To keep these metals and other pollutants out of the environment, safe disposal practices for household trash and industrial effluents should be followed.

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