Background Information                                                                     

Water is the most important natural resources and there are many conflicting demand for them. Skillful management of water bodies is required if they are to be used for such diverse purpose as  domestic and industrial supply, crop irrigation, transport, recreation, sports, commercial fisheries, power generation and waste disposal.         

Fishes are dependent on the water as a medium in which to live. All vital metabolic functions of fish such as respiration, feeding, movement, growth and reproduction are all dependent on water. Water is therefore the high way, byway, communication medium, nursery, playground, school, room, bed, drink, toilet and grave for a fish. 

Water bodies are vulnerable to contamination accident and bioterrorism attacks because they are relatively unprotected, easily accessible, often isolated and their various use by human pre-disposes them to contamination. [4].

Environmental exposure to toxic metals is a critical issue in environmental and public health.

Heavy metals are well known pollutants in aquatic systems where industrial wastes are discharge, petroleum production and refining, gas flaring, gas processing plants and conveyance pipelines. Gas flares are operated in relatively uncontrolled manner in the study area of the Niger Delta region. These effluents discharge and atmospheric emissions from flow stations and refineries often settle in the aquatic environment. When contamination reaches levels in excess of the assimilative capacity of the receiving waters, they may affect the survival, reproduction capacity, growth and behavioral condition of organisms. [5]. 

Contaminants can persist for many years in sediments in both freshwater and marine system where they hold the potential to affect human and the environment. All metals are virtually toxic if the exposure level is sufficiently high to exceed tolerance limit. Pollution of aquatic system by heavy metals inhibits primary production, nitrogen fixation, mineralization of carbon, nitrogen, phosphorus, litter decomposition and enzyme synthesis. The accumulation of these toxic metals by primary producers constitutes a potential danger to the organisms at higher trophic level [6]. The potential adverse impacts of heavy metals on aquatic life as well as humans are diverse.

The fish of interest include the African Mud Catfish (Clarias geriepinus) and Tilapia (Tilapia niloticus) which are tropical freshwater fish inhabiting Orashi River. These two species were chosen for the study because they are sedentary or resident in the area of interest, easy to identify, abundant and available for sampling all year round, long lived and has economic value and mostly consumed by the inhabitant of the area. 

The heavy metals of interest include the non-essential trace element - Cd, Cr and Pb) and the essential metals such as Cu, Zn, and Fe which have important biochemical functions to the organism at very low concentration. These heavy metals are blacklisted in European Economic Commission (EEC) Directive as dangerous substances in the aquatic environment. These heavy metals can be hazardous to human, even in very small amount. They are taken up by aquatic organisms and passed up the food chain through the process known as bio-magnification.

The increase level of heavy metals in human has often been traced to heavy metal contamination in the aquatic system [7].

A general concern about the safety of foods for human consumption has been on the increase in recent years [8]. The concentration of natural and synthetic chemical compounds in food contribute to its safety, it is therefore necessary to quantify the traditional nutrients, heavy metals, pesticides and various other constituents for the safety of consumers [9]. Improving the nutritional quality of food is imperative for farmers, fishermen and food industries.  Fish are proteinous, eaten as support or main dish and used as African’s cultural heritage. They play important roles in the customs, traditions and food culture of the African household. Nigeria is endowed with variety of fish and different types are consumed by the various ethnic groups for different reasons.

Heavy metals are generally used to describe chemical elements with a specific gravity that is at least five times the specific gravity of water [10]. 

Considering the potential toxicity, recalcitrant nature and cumulative behavior of heavy metals, the frequency of fish consumption, its safety and health concerns, more research work is still needed to be done on all species of fish consumed in Nigeria. Thus this study was designed to assess the level of some heavy metals in two most consumed fish species in ONELGA and also to assess the potential health risk associated with their daily intake.

Aim 

To determine the concentrations of heavy metals in fish and its potential human health risk associated with its consumption from Orashi River in Ogba/Egbema/Ndoni Local Government Area of Rivers State, Nigeria. This is to enable the assessment of potential hazardous levels from human nutritional stand point.

The Study Area

The study area is Ogba/Egbema/Ndoni LGA in Rivers State of Nigeria. (Figure: 1a, b and c). The area has a number of oil wells and major flow stations within the Niger Delta region of Nigeria. Nigerian Agip Oil Company (NAOC) and Total E and P Nigeria Limited explores, exploit crude oil and flare gases indiscriminately in the area at Ebocha, Obrikom and Obitte. 

The inhabitants of the area are predominantly farmers and fishermen which is their basic source of livelihood. The area has a growing population of 283, 294 in 2006 and a projection of 398,000 in 2016 (National population commission of Nigeria (web), National Bureau of statistics (web).

The site is Orashi River, a non-tidal freshwater of the lower Niger basin that runs through some communities in Imo State, Egbema, Ndoni and Ogba communities in Rivers State. The river is a freshwater swamp forest river with several tributaries and originate from River Niger and empties into Sombrieiro river in Ahoada. The area is tropical with two seasons- the rainy (April – October) and dry season (November – March) which are usually flooded in the rainy seasons.

Sampling Stations

A reconnaissance survey was carried out in the study area on November 2018 and then sampling stations were established  at  five  locations  along  the  Orashi River, 5km distance from each other using Global positioning system navigator (GPS) as shown in Table 5 and represented by station 1 – 5.

Figure 1: Maps Showing the Study Area

Figure 2: T.Nnoliticus

Field Collection of Fish Samples

Fish samples were collected from the 5 sampling stations monthly beginning from September 2019 to August 2020.

The fish samples - Catfish (Clarias gariepinus) and Tilapia (Tilapia niloticus) were purchased from artisanal fishermen from the five (5) stations for each sampling period. The samples were washed in distilled water, kept in labeled air tight plastic containers and packed in a cooler and subsequently transferred to the Institut Pollution Studies (IPS) laboratory of Rivers state University Port-Harcourt for tissue analysis. The samples were frozen until analysis in order to prevent post Mortem changes which may be either putrefactive or auto-lytic in nature.

Determination of Heavy Metals in Fishes 

In the laboratory, the fish samples (C. gariepinus and T. niloticus) were each thawed, washed, scaled and eviscerated. The organs were removed with a stainless steel knife and the whole body tissue were taken and immediately transferred according to the stations and months for the heavy metal analysis. The samples were oven dried at 100℃, cooled and then transferred to a plastic high speed blender. Blending continued for several minutes until samples were crushed into fine powders.

The dried and ground samples were taken and digested by micro- wave digestion method.

In this method of digestion, nitric acid (Analar grade) and hydrogen peroxide (Analar Grader) in the ratio of 3:1 were added to the samples. The mixture was digested at 150℃ for 30 minutes in microwave oven.

Figure 3: C. Gariepinus

The hydrogen peroxide added to the sample with nitric acid reduces nitrous vapor and speeds up digestion of organic substances by increasing the temperature of reaction in the digestion process. The digested samples were filtered with 20ml of de-ionized water. The filtrate was collected with clean acid-washed and appropriately labeled 50ml polyethylene sampling containers for analysis by Atomic Absorption Spectrometric method using standard method [11].

Data Analysis   

All statistical analysis and presentation of results was done using Microsoft excel and Minitab 16 software.

Raw data was subjected to a two way analysis of variance (ANOVA) with replication using MINITAB. 

The potential health risks of heavy metal consumption through fish were assessed based on the daily intake of metal (DIM), health risk index (HRI) and the target hazard quotient (THQ) whose bench mark is FAO/WHO 1993, [12]. The DIM was calculated to averagely estimate the daily metal loading into the body system of a specified body weight of a consumer. This will inform the relative bioavailability of metals. This does not take into cognizance the possible ingestion rate of a particular metal.

The estimated daily intake of metal in this study was calculated based on the formula below:

DIM =

Where C_metal is the heavy metal concentration in fish (mg/kg)

C_factor is the conversion factor = 0.208

C_food intake is the daily intake of fish = 65g/day. Average weight used was 65kg for the study. The health risk index (HRI) was calculated using the formula below:

HRI =

The THQ was calculated using the formula below:

THQ = x 10^-3

Where Ef = Exposure frequency (365 day/year)

E_D = Exposure duration (54years, equivalent the average life time of the Nigerian population.

F_IR = Food ingestion rate (fish consume value for South adult Nigeria is 104g/person/day.

Cf =Conversion factor = 0.208

Cm = the metal concentration in the edible part of fish (mg/kg) obtained from laboratory analysis.                                                                                       

RFD = Oral reference dose – Pb = 0.0035; Cd = 0.001; Cu = 0.040; Cr = 0.003; Zn = 0.3; Fe = 0.007µg/kg/day.

USEPA (2006)¹⁹ =maximum accepted dose of toxic substance.                 

WAB = Average body weight = 65kg for adult consumer in South Nigeria.                                                                                                                                                                           

TA = Average exposure time to cause obvious adverse effects (It is equal to E_f x E_D as used by many previous studies.

Heavy Metals Concentration in Fishes

Six heavy metals were analyzed in Catfish and Tilapia samples from Orashi River in 5 stations during the monitoring period. These include: Chromium, Cadmium, Copper, Lead, Zinc and Iron. Of these metals monitored in fish, Chromium and Lead was not detected throughout in the stations during the monitoring period. The level of heavy metals in fish samples were presented in Table 1 and 2 and represented in the graphs (Figure 2 to 9) below.

There were both spatial and seasonal variations of the heavy metals in the flesh of the two fish in all stations sampled.

Cadmium (Cd) Level in Catfish and Tilapia Muscle/Tissues

Cadmium was obtained in the muscle of both Catfish and Tilapia analyzed in the present study.

The results indicate range of Catfish muscle from 1.0 – 3.9mg/kg and mean levels for the various stations are station 1(3.07), station 2(3.47), station 3(2.93), station 4(3.06) and station 5(1.7) mg/kg  and Tilapia range from  0.1 – 4.2mg/kg with mean values of station 1(2.56), station 2(3.39), station 3(1.07), station 4(2.23) and station 5(2.17) mg/kg.

The concentration of Cadmium recorded in Catfish and Tilapia is presented in appendix 1a and b and shown in figure. 2 and 3 below.

There are both spatial and seasonal variations of Cadmium in both Catfish and Tilapia.

The analysis of variance (ANOVA) result showed that there were significant difference (p<0.05) between stations and months in Cd level observed in Catfish muscle during the study period (P = 0.015). There were no significant difference (p<0.05) in Cd levels observed in Tilapia muscle during the study period (P=0.979).

Figure 4: Cadmium (Cd) in Catfish

Figure 5: Cadmium (Cd) in Tilapia

Table 1: Geographical Positioning System (Gps)

Copper (Cu) Level in Catfish and Tilapia Muscle/Tissues

The concentration of Copper recorded in Catfish and Tilapia muscle is shown in figure 4 and 5. Copper concentration in the muscle ranged from 10.9 – 33mg/kg and 17.3 – 40.6mg/kg for Catfish and Tilapia respectively analyzed in the present study. 

The mean levels for stations 1, 2, 3, 4 and 5 were 28.94, 28.23, 18.18, 26.58 and 19.08mg/kg for Catfish and 19.24, 33.68, 28.12, 27.33 and 25.26mg/kg for Tilapia respectively.

There are both spatial and seasonal variations of Copper in both Catfish and Tilapia.

Seasonal variations show that Copper level was higher for Catfish during dry season than in the rainy season while rainy season recorded higher values than dry season in the case of Tilapia (Table 3). 

The analysis of variance (ANOVA) result showed that there were no significant difference (P<0.05)  between stations and months in Cu level observed in Catfish (P=0.837) while there were significant difference (P<0.05) in Cu levels observed in Tilapia muscle during the study period (P=0.019).

Zinc (Zn) In Catfish and Tilapia Muscle/Tissues

The concentration of Zinc recorded in Catfish and Tilapia muscle/tissue is shown in figure 6 and 7 respectively. The results indicate that Zinc level in this study ranged from 22 – 213.2mg/kg and 30.1 – 196mg/kg for Catfish and Tilapia respectively.

The mean levels for stations 1(59.6), station 2(37.03), station 3(51.87), station 4(139.34) and station 5(80.77mg/kg) for Catfish and station 1(40.19), station 2(82.29), station 3(79.68), station 4(62.28) and station 5(158.73mg/kg) for Tilapia.

There are both spatial and seasonal variations of Zinc in both Catfish and Tilapia.  

Figure 6: Copper (Cu) in Catfish

Figure 7: Copper (Cu) in Tilapia

Figure 8: Zinc (Zn) in Catfish

High values are observed in dry season than in the rainy season for the two fish species (Table 2). 

The analysis of variance (ANOVA) result showed that there were no significant difference (P<0.05) between stations   and   months   in   Zn   level   observed In Catfish (P=0.625) and Tilapia muscle (P=0.696) during the study period.

Iron (Fe) In Catfish and Tilapia Muscle/Tissues

The concentration of Iron recorded in Catfish and Tilapia muscle/tissue is shown in fig 8 and 9. The results indicate that Iron level in this study ranged from 0.02 – 2.5mg/kg and 0.1 –5.6mg/kg for Catfish and Tilapia respectively.

The mean values recorded for two edible fish for stations 1(0.72), Station 2 (1.09), Station 3 (1.21), Station 

Figure 9: Iron (Fe) in Catfish

Figure 10: Iron (Fe) in Tilapia

4 (1.46), and Station 5 (1.15mg/kg) for Catfish and 2.9, 1.76, 1.15, 1.17 and 0.91mg/kg for Tilapia.

There are both spatial and seasonal variations of Iron for both Catfish and Tilapia.

High values are observed in dry season than in the rainy season for Catfish while reverse is the case with Tilapia (Table 3).                                                                                                                                                                     

The analysis of variance (ANOVA) result showed that there was significant difference (P<0.05) between stations and months in Fe level observed in Catfish (P=0.026) and Tilapia muscle (P=0.007) during the study period.

The order of heavy metal concentration in fish is found to be in the order: Zn>Cu>Fe> Cd.

Heavy Metal Analysis of Fish Muscles/Tissue

Aquatic organisms tend to bio-accumulate heavy metals and therefore become susceptible to its effect [13], [14].

Fishes have been used for many years to determine the pollution status of water and thus regarded as excellent biological markers of metals in aquatic ecosystem [15]. The above statements are confirmed in this study. 

The bioaccumulation of toxic levels of heavy metals are detrimental to organisms generally as they biomagnified them through the food chain and later transferred to human. Increased level of heavy metals in humans has been traced to heavy metal contamination in aquatic systems. Study of fish muscle is one of the means to investigate the amount of heavy metals entering the human body in fish [16].

Generally heavy metals detected in this study were high in the fish and proves an evidence of bioaccumulation. Both essential and non-essential heavy metals were detected in flesh of Catfish and Tilapia except Chromium and Lead that were below detection limit.

There were both spatial and seasonal variations of the heavy metals in the flesh of the two fish in all stations sampled.

The results found in Catfish and Tilapia was compared with [17], [1], and [18] permissible limits to ascertain its level of toxicity.

Cadmium (Cd) Level in Catfish and Tilapia Muscle/Tissues

The results showed that Cadmium level in Catfish and Tilapia found in Orashi River exceeded [17], and [18] permissible limit of 0.01mg/kg. Cadmium is a heavy metal with high toxicity, a non-essential element in foods and natural waters and it accumulates principally in the kidney and liver.

Generally Cadmium detected was higher in Catfish than in Tilapia. This can be attributed to their habitat, age and feeding methods. Catfish are nocturnal and bottom dweller in contact with sediment while Tilapia is diurnal and surface dweller deriving its Cd contamination only from water alone. 

Copper (Cu) Level in Catfish and Tilapia Muscle/Tissues

The concentration of Copper recorded in Catfish and Tilapia muscle is shown in figure 4 and 5.

Copper concentration in the muscle ranged from 10.9 – 33mg/kg and 17.3 – 40.6mg/kg for Catfish and Tilapia respectively analyzed in the present study. 

These results are within the FAO [17] permissible limit of 30mg/kg. Copper is one of the most toxic metal to marine organisms although is not often considered a threat to human health except when present in abnormally high value. It is an essential element and most animals apparently possess a well-developed regulatory mechanism for this metal. It is advisable to guard against the indiscriminate discharge of industrial and domestic waste into Orashi aquatic environment to avoid any possible transfer of this contaminant to aquatic biological resources that are the primary animals’ protein source to the local communities.

From the result of Cu concentration in Orashi River, one can infer that the consumption of these fishes do not pose a health risk for its consumers. 

Zinc (Zn) In Catfish and Tilapia Muscle/Tissues

The concentration of Zinc recorded in Catfish and Tilapia muscle/tissue is shown in figure 6 and 7 respectively. The results indicate that Zinc level in this study ranged from 22 – 213.2mg/kg and 30.1 – 196mg/kg for Catfish and Tilapia respectively.

These results were above the FAO [17] permissible limit of 30mg/kg for Zinc in seafood. This is an indication of anthropogenic input of abattoir from runoff, tyre ash and animal blood which is rich in Zn and Fe [20]. Zinc is an essential element in animals and human diet and it is required to maintain the functioning of the immune system. Its deficiency results in stunted growth, loss of taste and hypo-gonadism leading to decreased fertility [19]. Zinc toxicity is rare but at concentration up to 40mg/kg, it may induce toxicity such as irritability, muscular stiffness and pain, loss of appetite and nausea. Others include severe vomiting, diarrhea, bloody urine, liver and kidney failure and anemia.

It is advisable to guard against the indiscriminate discharge of industrial, domestic and abattoir waste into Orashi aquatic environment to avoid any possible transfer of this contaminant to aquatic biological resources that are the primary animals’ protein source to the local communities.

Iron (Fe) In Catfish and Tilapia Muscle/Tissues

The concentration of Iron recorded in Catfish and Tilapia muscle/tissue is shown in figure 8 and 9. The results indicate that Iron level in this study ranged from 0.02 – 2.5mg/kg and 0.1 –5.6mg/kg for Catfish and Tilapia respectively.

These results are within the FEMNV permissible limit of 3mg/kg for Iron in seafood. This is an indication of the presence of anthropogenic input of abattoir from runoff, tyre ash and animal blood which is rich in Zn and Fe [20]. Fe is an essential element in animals and human and its deficiency results in stunted growth, loss of taste and hypogonadism leading to decreased fertility [19].

Health Risk Assessment

Risk assessment is one of the fastest methods currently used to evaluate the impact of heavy metal hazard on human health and also used to determine the level of treatment which attend to solve the environmental problem that occur in daily life [21]. Health risk assessment is an important tool that evaluates the consequences of human activities and weighs the adverse effects to public health against the contribution to economic development.

The potential health risks of heavy metal consumption through fish were assessed based on daily intake of metal (DIM) [22]. Health risk index (HRI) and the target hazard quotient (THQ) whose bench mark is FAO/WHO 1993, [12] using equation 1 – 3 above and the results of this study are presented in table 2, 3 and 4. This method of risk assessment has recently been used by many researchers and has been shown to be valid and useful.

The DIM results are shown in table 2 and were compared with the recommended daily intake of metals and upper tolerable daily intake level (UL).

The HRI for the two fish species in the 5 stations ranged from ND to 2.8. The HRI for Cd and Cu from this study were far greater than 1. Generally HRI < 1 means that the exposed population is safe of metal health risk while HRI > 1 means the reverse (Khan et al, 2008)^14b. The population is therefore at greater risk of Cadmium (Cd). Copper (Cu).  Zinc (Zn) and Iron (Fe) are essential element and the concentration is below the FAO [17], FAO/WHO [1] and EEC [18] permissible limit and the population is safe and not at risk as HRI is less than 1.                           

The THQ is a ratio between the measured concentration and the RFD, weighed by the length and frequency of exposure, amount and body weight. The parameter defines the exposure duration and the risk within that period. The THQ was calculated using the formula below:

THQ = x 10^-3

Where Ef = 365, Ed = 65, FIR = 104, Cf = 0.208, Cm =heavy metal level in fish. RFD = Cd = 0.001, Cu = 0.0371, Zn = 0.3, Fe = 0.007. WAB = 65, TA (Ef x Ed) = 365 x 65. The THQ values for Cd, Cu, Zn, Pb and Cr due to fish consumption for the populace (adults) of the study area are shown in Table 4. 

The result reflected the risk associated with Cd, Cu, Zn, Fe, Pb and Cr exposure for the period of life expectancy considered. The inhabitant are highly exposed to health risks associated to these metals in the order Zn>Cu>Cd>Fe. Generally Zn, Cu, and Fe are important nutrients for humans than Cd, Pb and Cr. In this study, THQ in all metals is far more than 1 in the two fish species except Pb and Cr that were not detected, therefore it pose health risk concern. The potential health risks of heavy metal in ONELGA, Rivers State were likely to be higher as fish consumption was usually high and just one part of food consumed, other foods like meat, vegetable, tobacco, rice and cassava [8,3]. 

For ONELGA populace, food consumption, air pollution, drinking water are the important pathways for human exposure to toxic material [14]. Consequently, the potential health risks for the residents were actually higher than the results from this study.

The health risk index was calculated using the formula: 

HRI =

Where RFD for the metals of interest are: Cd = 0.001, Cu = 0.0371, Zn = 0.3, Fe = 0.007, Pb = 0.0035, Cr = 0.003. 

Table 2: Daily Intake Rate (Mg/Kg/Person/Day) Of Heavy Metals in Fish Samples in All Stations

Recommended daily intake (DI) and upper tolerable daily intake (UL) levels of heavy metals in foodstuffs [23]

Table 3: Result of Hri in the Fish Species Sampled In All the 5 Stations

Table 4: Heavy Metal Concentration in Fish Samples

Table 5: Seasonal Variation in Heavy Metal Concentration in Fish Samples

The research was intended to study heavy metal concentration as a result of the influence of human and industrial activities on the two bony fish (Clarias gariepinus and Tilapia nilotica) on the Orashi River and its potential risk in consuming contaminated fish species.                                     

The results from this study have provided information on the heavy metals profile of the fish (Catfish and Tilapia) of the river as well as its attendant health risk which were close to or exceeded World Health Organization (WHO) and Federal Ministry of Environment (FMEnv) recommended limits for drinking water and seafood. 

Essential heavy metals detected in water were Copper (Cu), Zinc (Zn) and Iron (Fe) and were low when compared with international standard [24], [25] recommended limits of 30mg/kg.   The non-essential metals detected were Cadmium (Cd) which exceed the international standard of 0.01mg/kg and therefore constitute health risk. This was confirmed by the determination of DIM, HRI and THQ in the study.                                                                                                                                                                      

The high level of Heavy metals in the fish calls for concern as it can have some health – risk implication in humans who are the final consumers.

Recommendation

This study has shown that Heavy metals accumulate over time within the Orashi aquatic resources which are the major source of protein to the indigenous community who are exposed to the hazardous effects of these pollutants.  It is therefore recommended that:

• The companies operating in the area should adopt improved waste management plans to reduce the levels of the pollutants discharge in the environment

• Comparative studies in other biota and other food sources of the environment would be useful in monitoring the rate and mechanism of uptake of the metals in the fish of Orashi River and other food source. This will provide data for informed decision on uptake of what is available in a polluted environment

• The local communities should be enlightened about the adverse effects of anthropogenic activities, oil pipeline vandalization/sabotage and bunkering activities to make money without consideration of the environmental effects

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