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This study investigated hazard indices and human health risks associated with consumption of bivalve mollusc from the Niger delta. The research was conducted in two phases; The first was the analysis of consumption pattern and perception of  bivalve molluscs in the studied locations through the use of structured questionnaire while in the second phase, four species of  bivalve mollusc; Bloody cockle (Anadara senilis), Donax clam (Donax rugosus), Knife clam (Tagelus adansonaii) and Mangrove oyster (Crassosstra gasar) collected from four different  locations in the Niger delta were assessed for microbiological hazards, toxic element contents, concentrations and compositional pattern of polycyclic aromatic hydrocarbons (PAHs), levels of polychlorinated biphenyls (PCB) congeners in bivalves as well as the estimated  human health risk associated with the consumption of bivalve with these chemical contaminants. Standard methods of analyses were employed in the determination of microbiological hazards and levels of toxic elements, PAHs and PCBs concentrations in bivalves were determined through the use of gas chromatography while the United State Environmental Protection Agency (US EPA) method was used to estimate the human health risk of chemical contaminants in bivalves consumed in the Niger delta. Results of consumption pattern and perception indicated that bivalve consumption were driven by attributes such as easy availability, low-price and pleasant flavor. A chi-square test indicated a positive significant relationship (p<0.05) between demographic variables of the respondents and the level of perception toward bivalve consumption. Microbiological hazards isolated from bivalve samples included; Vibrio choleraeVibrio parahaemolyticusSalmonella spp, Shigella spp, Listeria monocytogenes and Escherichia coli in levels above the stipulated FDA standard for shellfish which rendered the product unsafe for human consumption.  Lead concentration was within the 1.5mg/kg acceptable limits while cadmium, arsenic and mercury were higher than FAO limits of 0.5, 0, 0.5 mg/kg respectively. The individual tissue burden of PAHs indicated bloody cockle (53.75μg/kg), knife clam (50.00μg/kg), mangrove oyster (40.34μg/kg) and Donax clam (36.94μg/kg) while the compositional pattern of PAHs indicated the dominant of  low molecular weight PAHs in Andoni location while other study locations were dominated with high molecular weight PAHs. The diagnostic ratio showed that the PAH burden originated from petrogenic source at Andoni while that of other locations were from pyrogenic or combustion processes. The result of analysis for PCBs indicated the presence of lower chlorinated PCB congeners in all the study location. The total concentrations of PCB congener were highest at Bonny (1.75ng/g) while Andoni had the least (0.36ng/g). The PCB burden in bivalve tissue indicated that mangrove oyster accumulated the highest concentration (1.38ng/g) followed by bloody cockle (1.11ng/g) while Donax clam (0.60ng/g) had the least. The estimated human health risk indicated a non-carcinogenic values and hazard indices higher than threshold value of one for cadmium, total arsenic and methylmercury while risk values for carcinogens were higher than the US EPA stipulated one in one million chances for developing cancer. This implies that toxic elements apart from lead, carcinogenic PAHs and PCBs in bivalves from these locations can induce potential deleterious health effects to the consumers.


CHAPTER      TITLE                                                                                       PAGE

Title Page        ..          ..          ..          ..          ..          ..          ..          ..          ..          i

Declaration      ..          ..          ..          ..          ..          ..          ..          ..          ..          ii

Certification    ..          ..          ..          ..          ..          ..          ..          ..          ..          iii

Dedication      ..          ..          ..          ..          ..          ..          ..          ..          ..          iv

Acknowledgement      ..          ..          ..          ..          ..          ..          ..          ..          v

Table of Contents       ..          ..          ..          ..          ..          ..          ..          ..          vi

List of Tables  ..          ..          ..          ..          ..          ..          ..          ..          ..          ix

List of Figures ..          ..          ..          ..          ..          ..          ..          ..          ..          xi

List of Plates   ..          ..          ..          ..          ..          ..          ..          ..          ..          xii

Abstract          ..          ..          ..          ..          ..          ..          ..          ..          ..          xiii


1.1                   Background of the Study      ..          ..          ..          ..          ..          1

1.2                   Statement of Problem                        ..          ..          ..          ..          ..          5

1.3                   Justification                ..          ..          ..          ..          ..          ..          7

1.4                   Objective of the study           ..          ..          ..          ..          ..          8


2.1                   The Bivalve Mollusc   ..          ..          ..          ..          ..          ..          9

2.2                   Global Bivalve Production, Distribution and Consumption               10

2.3                   Bivalve Production and Consumption in Nigeria       ..          ..          12

2.4                   Food Safety Issues Related to Bivalve Molluscs Consumption         14

2.5                   Benefits Associated with Bivalve Molluscs Consumption                 15

2.6                   Risk Factors Associated with Bivalve Molluscs Consumption          17

2.7                   Toxic elements            ..          ..          ..          ..          ..          ..          24

2.8                   Organic Contaminants            ..          ..          ..          ..          ..          29

2.9                   Food Safety Risk Assessment            ..          ..          ..          ..          35

2.10                 Components of Risk Assessment       ..          ..          ..          ..          37

2.11                 Method of Food Safety Risk Assessment                  ..          ..          42

2.12                 Determination of Human Health Risk Assessment of

Chemical Contaminants          ..          ..          ..          ..          ..          44



3.1                   Study Area      ..          ..          ..          ..          ..          ..          ..          47

3.2                   Sample Collection       ..          ..          ..          ..          ..          ..          49

3.3                   Sample Preparation and Treatments   ..          ..          ..          ..          52

3.4                   Consumption Pattern and Perceptions of Bivalve Molluscs               52

3.5                   Proximate Analyses    ..          ..          ..          ..          ..          ..          53

3.6                   Assessment of Microbiological Composition of Bivalves      ..          56

3.7                   Determination of Mineral Composition in Bivalve Mollusc   ..          59

3.8                   Determination of Concentration of Organic Contaminants in

Bivalve Samples          ..          ..          ..          ..          ..          ..          61

3.9                   Human Health Risk Assessment Procedure               ..          ..          64

3.10                 Experimental Design   ..          ..          ..          ..          ..          ..          67

3.11                 Analysis of Data         ..          ..          ..          ..          ..          ..          68



4.1                   Consumption Pattern and Perception of Bivalve Molluscs                69

4.2                   Bivalve Consumption Pattern ..          ..          ..          ..          ..          72

4.3                   Perception toward Bivalve Consumption       ..          ..          ..          81

4.4                   Food Safety and Environmental Concerns     ..          ..          ..          84

4.5                   Proximate Composition          ..          ..          ..          ..          ..          88

4.6                   Macromineral Content of Bivalve Molluscs               ..          ..          97

4.7                   Trace Elements            ..          ..          ..          ..          ..          ..          103

4.8                   Toxic Elements           ..          ..          ..          ..          ..          ..          110

4.9                   Microbiological Composition              ..          ..          ..          ..          119

4.10                 Polycyclic Aromatic Hydrocarbons (PAHs)               ..          ..          126

4.11                 Polychlorinated Biphenyls (PCBs)     ..          ..          ..          ..          137

4.12                 Human Health Risk Assessment         ..          ..          ..          ..          146

4.12.1              Risk assessment of toxic elements from consumption

of bivalve samples       ..          ..          ..          ..          ..          ..          146

4.12.2              Risk assessment of PAHs from consumption of bivalve samples  ..  151

4.12.3              Risk assessment of PCBs from consumption of bivalve samples       153



5.1                   Conclusion     ..          ..          ..          ..          ..          ..          ..          164

5.2                   Recommendations     ..          ..          ..          ..          ..          ..          166

REFERENCES          ..          ..          ..          ..          ..          ..          ..          ..          168

APPENDIXES           ..          ..          ..          ..          ..          ..          ..          ..          189







TABLE                                  TITLE                                                                   PAGE


4.1                   Demographic characteristic of the respondents          ..          ..          70


4.2                   Showing the Rate of Consumption of Bivalve Species                      74


4.3                   The preferred form of consumption    ..          ..          ..          ..          78


4.4                   The preferred place of consumption   ..          ..          ..          ..          80


4.5                   Reasons for consuming bivalve molluscs                    ..          ..          82


4.6                   Health related issues encountered while consuming bivalves             85


4.7                   Effect of location on Proximate composition(%) of bivalve samples  90


4.8                   Effect of species on Proximate composition of bivalve species(%)    91


4.9                   Effect of location and species on the proximate composition (%)

                        of bivalve samples       ..          ..          ..          ..          ..          ..          95


4.10                 Effect of location on Macro mineral content of

                        bivalve samples (mg/100g)      ..          ..          ..          ..          ..          98


4.11                 Effect of species on Macro mineral content of bivalve

                        Samples (mg/100g)      ..          ..          ..          ..          ..          ..          99


4.12                 Effect of location and specie on: Macro minerals content

                        of bivalve samples (mg/100g) ..          ..          ..          ..          ..          101


4.13                 Effect of location on Trace elements content of bivalve

                        samples (mg/100g)      ..          ..          ..          ..          ..          ..          104


4.14                 Effect of species on Trace element content of bivalve

                        samples (mg/100g)      ..          ..          ..          ..          ..          ..          105


4.15                 Effect of location and species on Trace element content

                        of bivalve samples (mg/100g) ..          ..          ..          ..          ..          107


4.16                 Effect of Location on Toxic element content of

                        bivalve samples (mg/kg)  ..      ..          ..          ..          ..          ..          112


4.17                 Effect of species on Toxic element content of

                        bivalve samples(mg/kg)           ..          ..          ..          ..          ..          113


4.18                 Effect of location and species on Toxic metal content

                        of bivalve samples (mg/kg)     ..          ..          ..          ..          ..          114


4. 19                Effect of Location on Microbiological

                        composition (log CFU/g) of  bivalve tissue                ..          ..          120


4.20                 Effect of species on Microbiological composition

                        (log CFU/g) of  bivalve tissue ..          ..          ..          ..          ..          121


4.21                 Effect of Location and species on Microbiological

                        composition (log CFU/g) of bivalve tissue                 ..          ..          122


4.22                 Effect of location on the concentrations and compositional

                        patterns of PAHs (μg/kg) in bivalve samples  ..          ..          ..          127


4.23                 Effect of species on the concentrations and compositional   

                        patterns of PAHs (μg/kg) in bivalve samples  ..          ..          ..          128


4.24                 Effect of location and species on the concentrations and

                        compositional patterns of PAHs (μg/kg) in bavalve samples  ..          131


4.25                 Effect of  location on the concentrations of

                        PCBs (ng/g) in bivalve samples           ..          ..          ..          ..          138


4.26                 Effect of species on the concentrations of

                        PCBs (ng/g) in bivalve samples           ..          ..          ..          ..          139


4.27                 Effect of location and species on the concentrations of

                        PCBs (ng/g) in bivalve samples           ..          ..          ..          ..          141


4.28                 Non-carcinogenic risk value of toxic elements in bivalves samples    147


4.29                 Carcinogenic risk value for toxic elements during bivalve consumption          152


4.30                 Non carcinogenic risk value for PAHs during consumption

                        of  bivalve samples      ..         ..          ..          ..          ..          ..          154


4.31                 Carcinogenic risk value for PAH during consumption          

                        of bivalve samples       ..          ..          ..          ..          ..          ..          157


4.32                 Non carcinogenic risk value for PCBs during bivalve consumption   160


4.33                 Carcinogenic risk value for PCBs during bivalve consumption          162





FIGURE                                TITLE                                                                   PAGE



3.1                   Section of  Niger Delta coastal area showing the

sampling locations       ..          ..          ..          ..          ..          ..          48


4.1                   The source of bivalve species by the respondents in Niger Delta       76       

4.2                   Possible source of Risk Factors in Bivalve Species    ..          ..          87


4.3                   Accumulated toxic element concentrations in bivalve species

from study locations   ..          ..          ..          ..          ..          ..          115


4.4                   Accumulated PAH concentration in bivalves species from

study locations ..         ..          ..          ..          ..          ..          ..          133


4.5                          Accumulated PCB concentration in bivalve species from

study locations ..         ..          ..          ..          ..          ..          ..          142


4.6                   Hazard index of toxic elements of bivalve species in the

study locations            ..          ..          ..          ..          ..          ..          150





PLATE                                  TITLE                                                                    PAGE

1                      Bloody cockle (Anadara senilis)         ..          ..          ..          ..          50

2                      Donax clam (Donax rugosus) ..         ..          ..          ..          ..          50

3                      Knife or Razor clam (Tagelus adansonaii)     ..          ..          ..          51

4                      Mangrove oyster (Crassosstrea gasar)           ..          ..          ..          51








            Feeding an expected global population of 9 billion by 2050 is a daunting task that is engaging researchers, technical experts and leaders of the world over. A relative unappreciated, yet the promising fact is that seafood plays an important function in satisfying the plates of the world’s growing middle-income group while also meetings the food security need of the poor (World Bank, 2013). Already seafood including bivalve mollusc represent 20% of all animal protein consumed globally, and this proportion of the world’s food basket is likely to increase due to higher demand by middle income earners who seek higher-value seafood and as aquaculture step-up to make up with rising demand for bivalve shellfish (FAO, 2016).

            Molluscs are soft-bodied invertebrate which are enclosed in a hard shell. There are six major classes of molluscs of which bivalves are the most organized and specialized (Gosling, 2003). Bivalve mollusc inhabits fresh and marine waters from the abyssal depths of high intertidal areas in tropical to warm temperate waters. They are considered as delicious and healthy food items in several dietary regimes in a different part of the world. The most important species are the clams, mussels, oysters, and scallops (Gosling 2003, Gopalsamy et al., 2014). The pattern of their distribution is characteristically different from one location to another depending on the sediment types, variations in water salinities and tidal movements (Sarker et al., 2008).

            Globally, bivalve production has consistently increased in the last sixty-six years growing from nearly 1 million tonnes in 1950 to about 17.1 million tonnes in 2016, contributing to over 10% of the total amount of income in 2016 (FAO, 2018). China was by far the leading producers of bivalve molluscs with 13.4 million tonnes in 2014 representing 83.3% of the global production and 81.9% of the global aquaculture production in that year. Japan and the Korea Republic were the second and third largest producers far behind China with a production of 376 and 359 thousand tonnes respectively (FAO, 2016). In Nigeria, finfish farming dominates the scanty literature on aquaculture, with much less on bivalve molluscs. Although there is no statistic on the production of bivalve mollusc by species, the Nigeria Bureau of Statistic (NBS) reported that Nigeria produced 5.8 million tonnes of fish including bivalve molluscs between 2010 and 2015 (NBS, 2017).

            As the global human population is increasing, seafood consumption including bivalve is also rising steadily due to the health consciousness of modern-day consumers who are interested in seafood because of their nutritional superiority and the health benefits (Gopalsamy et al., 2014). The consumption of bivalves and other seafood is believed to provide an inexpensive source of protein with high biological value, essential minerals such as selenium, calcium, iron, phosphorus as well as vitamins (Astorga-Espana et al., 2007). The nutritional characteristics of bivalves vary among species, and between individuals of same species. Other factors that affect their nutritional qualities include age, sex, maturation stage, origin, season, seawater, physical/chemical properties and feed composition (Orban et al., 2002).

            Despite the numerous advantages of seafood-based diet, adverse health effects can also exist and seafood harvested from polluted aquatic environments can contain biological and chemical contaminants (FAO/WHO, 2011a). Bivalves molluscs being sedimentary filter feeders, feed by opening their shells for absorption of food particles. Due to their feeding pattern, they filter tiny particles of aquatic plants, animals and inorganic matter. It also accumulates the diversities of other contaminants from the surrounding seawater (Huss et al., 2003, Lees, 2000). These feeding pattern render the bivalve shellfish easily prone to bacteria, heavy metals, biotoxins and other environmental contaminations (Sarkar et al., 2008).

            Most species of bivalve molluscs consumed in Nigeria are harvested from the brackish water that is exposed to varying amounts of chemical and environmental contaminants such as industrial chemicals, toxic residues from various anthropogenic activities. Pollution of the coastal waters in the Niger delta has continue to attract greater attention. This is due to the high level of environmental degradation posed by petroleum production and exploitation in along the coastline (Wala et al., 2016; Zabbey and Babantunde, 2015). Petroleum hydrocarbon from oils spills and human-mediated activities are usually incorporated into sediments where they can persist for years gradually releasing toxic substances into the immediate and remote environments (Zabby and Babatunde, 2015). Some of the deleterious effects associated with dietary intake of these contaminants include diarrhoea and gastrointestinal disorders, immune suppression, neurological disorder, reproductive impairment, developmental retardation, cardiovascular disorder, liver disease, infertility and miscarriage (ASTDR, 2002; De Jager et al., 2012). The groups most vulnerable to dietary exposure of the contaminants are child-bearing women, children below twelve years, and subsistence fish farmers (FAO/WHO, 2011a). For better understanding and characterization of the risk presented by chemical toxins in the environment to human and ecological receptors, most researchers used benthic organisms such as bivalves as biomonitors of the levels and long-term influences of chemical toxins within the ecosystem (Sarker et al., 2008).

            The risk factors that are associated with bivalve mollusc consumption are mostly from the contaminated water body where they live, particularly when they are to be consumed fresh or slightly cooked (Lees et al., 2010). These circumstances make them important sources of foodborne diseases which represent a significant health risk to consumers. The hazards usually encountered include those due to pathogenic bacteria, viruses, parasites as well as intoxication due to chemicals from metallic elements such as methyl mercury, cadmium, arsenic, lead and others. There are also organic pollutants such as dioxins, furans, polychlorinated biphenyls (PCBs), pesticide residues among others (Lees et al., 2008). Marine bivalves are also implicated to be a major source of biotoxins or shellfish toxins which are contacted from species of micro-algal cells (phytoplankton) when consumed (Huss et al., 2003). These toxins, usually linked to microalgae bloom are not harmful to the bivalves but might pose a safety problem to consumers particularly when the bivalves are cooked since the toxins are heat resistant.

            According to Conte et al (2014), seafood generally is perceived as healthy food and as an alternative source of proteins. However, consumers also have the consciousness of some safety risks, e.g. potential adverse effects of shellfish contaminants on health. Also, the ability of bivalve to bioaccumulate and bioconcentrate contaminants leaves those at the highest trophic level at the greatest concentration and risk and depending on the contaminants in contact with, a wide variety of harmful effects have been reported (US EPA, 2009). Fishery products are considered the major sources of human contact to pollutants such as polychlorinated biphenyls, dioxins, organochlorines polycyclic aromatic hydrocarbons, some heavy metals and other environmental toxic substances and according to Conte et al (2014), some differences exist in the type and levels of contaminants among regions and as such risk assessment must be performed locally.


            The Niger Delta coastal waters are an exceptional breeding ground for a vast variety of fish and shellfish. They provide excellent habitat for a diversity of shellfish such as bloody cockle (Anadara senilis) mangrove oyster (Crassostrea gasar), razor or knife clam (Tagelus andansonii) and others. But the prevailing pollution occasioned by exploration and production of petroleum have impacted negatively to the quality and quantity of shellfish in the area (Yakubu, 2017).

            According to Amnesty International (2018) reports, Niger Delta is the most recognized oil-producing region in Africa. It is also known to be the most polluted area on earth. The prevailing widespread pollution has severally impacted negatively on the food product especially seafood obtained from the coastal waters of this area. Amnesty International (2009), reported a study carried out by Friends of the Earth in Akwa Ibom State in 2008, which discovered elevated heavy metals in fish tissue. Natives also complained that fish and fishery products obtained from the coastal waters caused stomach upsets when consumed.

            Research has determined that there is bioaccumulation of Benzo (a) pyrene (BaP), other hydrocarbons and heavy metals has occurred in a toxic amount in major high protein contents seafood such as periwinkle (Tympanotonus fuscatus), mudskipper (Periophtalalius papillio) and other seafood (Ordinioha and Brisbe, 2013). Yakubu (2017), reported a benzene concentration of 0.155-48.2μg/m3 in this area and this concentration to represent 1:10,000 cancer risk as benzene and its associated  compounds such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) are known carcinogens. Also, Gobo et al (2010) argued that the prevalence of diarrhoea in coastal communities of Nigeria is on the rise because of consumption of sea food and other animals products which have been in contact with pathogenic microorganisms.

            Furthermore, the Nigerian Government recently seek European Union to help to counter cancer outbreak in the polluted Niger Delta. This is as a consequent of the reported cases of health issues such as breathing problems, skin lesions and many cases of cancer resulting from oil pollution from water, air and agricultural products. The government explained the “enormity of the menace” and called for immediate action as the capacity of the Nigerian government was inadequate (Euroactiv, 2017). Also, the former Bayelsa State Governor, Hon. Henry Seriake Dickson in 2018 during his mother's death expressed worry over the increasing numbers of illnesses paticularly respiratory problems and cancer in Niger Delta. He stressed that cases of illnesses in the Niger delta were traceable to environmental degradation and called for a serious action to reverse this condition (The Guardian, 2018). Bivalve including mangrove oysters, bloody cockle, clams among others are suspension or filter feeders takes in chemical contaminants and microbes from the polluted Niger delta waters can accumulate in their tissue posing serious concern in their quality and safety. Therefore, the continuous consumption of bivalve molluscs from the Niger Delta waters posits or exemplify the conflict between food benefits and food risks. The State (Government) are encouraged by international law and other Charters to improve upon, protect and provide its citizens with different sources of food. The major food sources should therefore  be seriously protected, not abused or comtaminated by private individuals or organisations thereby preventing peoples’ ability at feeding themselves. The multidisciplinary and multisectoral approach to the sustainable mitigation of health risks resulting from pollution in Niger Delta region is paramount and  the assessment of food safety risks that are associated with the consumption marine bivalve mollusc shellfish offer a valuable strategy.



            The foodborne disease presents a serious public health problem to every country in the world of which a little over 10% is attributable to fish and shellfish product. Reports show that when muscle foods are considered separately over 56% of illnesses are connected to seafood (CDC, 2008). Seafood is not only a high internationally traded commodity but an important component of diets worldwide. According to FAO/WHO (2011a), it is estimated that over one billion people around the globe rely on seafood products as their main source of animal protein. Fishing is also a major occupation of the people of the riverine communities of Nigeria, and various fisheries resources are also important delicacies including bivalve mollusc which are common among small-scale fisheries. Contamination of aquatic ecosystem in the Niger delta region cannot be overemphasized as most of the contaminants can bioaccumulate and become significant along the food chain giving concern on seafood safety to consumers (Davidson et al., 2006). Risk assessment and hazard identification are established protocol or framework that defines an appropriate level of public health hazards in food products and ensures the presentation of  foods that are safe. Since bivalves are economically and nutritionally very important for human consumption, playing a central role in the gastronomy of this region, assessing risk factors relating to its consumption and their health implications are indeed very important. This research work is aimed at correctly predicting food safety risk associated with chemical contaminants in  bivalve mollusc.



            The broad objective of the study is to assess the food risks associated with  bivalve molluscs harvested from the coastal waters of Niger Delta,in Nigeria. The specific objectives of the study are as follows:

1.         To analyse the consumption pattern and perceptions of bivalve molluscs in Niger    Delta through the use of a structured questionnaire.

2.         To assess the microbiological composition of bivalve species harvested from this    area and identify inherent hazards in fresh bivalve.

3.         To access  proximate composition of bivalve species consumed in the Niger Delta.

4.         To assess the macrominerals and trace elements content of bivalve species from the region.

5.         To determine the level of toxic elements accumulated by bivalve molluscs.

6.         To investigate the concentrations and  patterns of composition of the 16 priority    PAHs accumulated by bivalve molluscs

7.         To assess the distribution pattern and bivalve tissue burden of PCB congeners in    the study locations.

8.         To use the US EPA method to estimate the human health risk of toxic elements,    PAHs and PCBs contaminants in bivalve samples consumed in Niger delta.



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