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Commercially available (farmed and wild) fish in the Yola-Jimeta region, Northeastern Nigeria were investigated to find out if they present a public health threat. The fish species used for the study was catfish from the farm and the wild (Benue River) fish. The concentrations of six heavy metals were analyzed. These metals include: cadmium (Cd), iron (Fe), zinc (Zn), manganese (Mn), lead (Pb), and nickel (Ni). The metal concentrations were obtained using atomic absorption spectrophotometer (AAS). The results from AAS were expressed as part per million (ppm) and converted to µg/g dry weight in the result section. The result shows that the gills of farmed fish with exception of manganese accumulate more metals than in the flesh. However, the flesh of wild fish has significantly higher metals concentration than in the gills. Nevertheless, concentrations of heavy metals were higher in the gills of farmed fish but the results were not statistically significant except for lead and zinc. In addition to this, the concentrations of lead, nickel, and cadmium in both the tissues of farmed and wild fish except for zinc in the gills of farmed fish were above the recommended levels provided by WHO.






Freshwater fish. 6

Bioaccumulation in fish. 6

Health impact of heavy metals from fish consumption. 8

Water pollution in developing countries. 8

River Benue, a source of fish in northeast Nigeria. 9




Study site. 13

Sampling. 14

Sample preparation. 14

Heavy metals analysis. 15

Statistical analysis. 15













Water is regarded as contaminated or polluted if there are substances in it to such an extent that it cannot be utilized for a particular reason. Polluted water is not safe for cooking, drinking, and other human uses. Water pollution occurs as a result of anthropogenic activities such as littering, mining, deforestation, and use of pesticides. It also occurs as a result of industrial activities such as oil spillage, sewage leakage, and industrial waste dumping. Other parameters that can contribute to water pollution include salinity pollution (the content of dissolved solids in water) and pathogenic pollution (coliform bacteria) from fecal contamination (UNEP, 2016).


More than 50% of river stretches in Latin America, Asia, and Africa are adversely affected by organic and pathogen pollution, whereas 33% of them are affected by salinity pollution. This deterioration is more pronounced in subdivision of these river stretches. In 1990, water pollution has risen to a serious level and by 2010, it worsened (UNEP, 2016).


In developed countries like China, increasing water pollution has been associated with the increase in industrial and economic activities. An example is Yangtze River in China. Waste from industries, mining process, and sewage discharged into the river weighs about 25 billion tons yearly. About 80% of this waste is not properly treated and 60% of the river’s length is affected by pollution (Yi, Yang, & Zhang, 2011). This pollution eventually spreads out to the other parts of the river, which have high densities of population and industry. The sewage discharge around the river also releases heavy metals into the river, which eventually accumulate in the water sediments, leading to ecological damage.


Heavy metals are those metals with relatively high density. About forty metals are classified as heavy metals based on their chemical properties (Zahra, Kalim, Mahmood, & Naeem, 2017). These metals are mostly found in rock formations, but are also frequently found in soil, aquatic habitats, and in the atmosphere in form of vapors. Some of these metals are not easily degradable and can therefore accumulate in the environment. Heavy metals are also classified as essential such as iron, zinc, manganese, copper, and non-essential metals such as mercury, lead, nickel, and cadmium. Essential metals are needed as micronutrients in the body. For example, zinc and copper act in two ways: as activators for enzyme-catalyzed reactions or as prosthetic group in metalloproteins. In addition, they are needed in redox reactions, electron transfer, and are essential in metabolism of nucleic acids (Zahra et al., 2017).  Essential metals only become toxic in high quantities while non-essential ones are toxic even in low quantities.


Heavy metals are released into aquatic habitats as a result of anthropogenic sources, such as mining of metals, metropolitan and sewage wastes, industrial emissions, and agricultural runoff (Fig. 1). They can also be released into an aquatic ecosystem due to natural causes such as biological weathering of rocks and atmospheric deposition (Iqbal, Tabinda, Yasar, & Mahfooz, 2017).



The high concentration of heavy metals in the aquatic organisms can result in serious ecological changes. One of the most serious consequences of their persistence is the biomagnification of metals in the food chain (Fig. 2). Long-time accumulation of these metals affects the plant, surface water fishes, and benthic fish species. Benthic fish species are those species of fish that live at the bottom of the water body (Yi et al., 2011). In addition, metals transfer through aquatic food web affects human health. However, some organisms act as vectors and are responsible for transferring toxicants from one organism to another to the top level of the food web.

The presence of heavy metals in the water may pose a threat to human health because of the inability of heavy metals to be broken down by natural processes (Milam, Maina, Onyia, & Ozoemena, 2012). According to WHO (2017), the accumulation of heavy metals such as lead, arsenic, and mercury in water affects human and aquatic organisms such as fish. Some heavy metals, such as mercury, cadmium, and lead, are toxic to fishes even in low quantities (Uysal, 2011).



Lead gets into water bodies through anthropogenic activities such as mining activities and burning of fossil fuels. Also, lead can occur naturally in the environment. It has been reported that the United States is at high risk because it still uses high amounts of lead contaminated materials such as adjacent bare soil and  lead deteriorated paints (Tchounwou, Yedjou, Patlolla, & Sutton, 2012).



Exposure to arsenic from water may result in arsenic poisoning. It is responsible for the death of many people worldwide. An example is the incidence of arsenic poisoning in Bangladesh. According to World Health Organization, Bangladesh holds the record for the most people affected by arsenic poisoning: it affected about 35 to 77 million of their total population of 125 million. It led to cancer (including internal cancers) outbreak and some other deadly diseases, and claimed the lives of several people (WHO, 2000).  In the same article, high levels of arsenic were reported in the local water systems of Andes and Amazon regions of South America. People from these regions were exposed to high level of arsenic, which resulted in many health issues.



Mercury is known as the most toxic metal in the aquatic environment. It gets transformed to methylmercury after accumulating into the top trophic level (Benson, Essien, Williams, & Bassey, 2007). It is very dangerous to health; mercury poisoning may affect the fetus if a pregnant woman is exposed to it (Tchounwou et al., 2012).

Water pollution also causes several health problems, such as gastrointestinal diseases, reproductive problems, neurological disorder, diarrhea, and cancer. Pregnant women and children are more vulnerable to these diseases because their immune system is weak (WHO, 2017).


Freshwater fish

Fish consumption provides nutritional benefits to humans. This is because of the high level of protein content in the fish. Fish supplies 17% of the total animal protein for people worldwide and comprises 6% of the total human diet globally. Fish consumption has been growing at the rate of 3.6% yearly, whereas the world’s population has been increasing at the rate of 1.8% yearly. Available data show an increase in the per-capita annual consumption. The consumption of fish has almost doubled in 37 years: from 9kg in 1960s to 16kg in 1997. Fish accounts to about 180kcal per-capita every day for countries that lack any other source of protein apart from fish and those that prefer fish as their main source of protein. Examples of such countries are Iceland, Japan, and other island nations. Generally, fish supplies about 20-30kcal every day and  about 1 billion people depend on fish as their major source of protein (WHO, n.d.).

Bioaccumulation in fish

High demand for fresh water fish has led to researches on bioaccumulation of metals in fish species. Metals get into fishes’ bodies through respiration, adsorption, and ingestion. Two important factors that contribute to metal accumulation in fish due to respiration are: habitat and the rate of resorption. For example, fishes that live at the bottom of the water (benthic fish) are expected to have higher metals levels than the fish that live close to the water surface (pelagic fish) because the level of metals in the sediments of water is much higher than on the surface. The bioaccumulation of metals also vary in different species of fishes (Uysal, 2011). Concentration ratios of metals in fishes depend on the environment in which they are caught; fish species that are caught from highly polluted environment such as areas with high industrial activities tend to accumulate higher concentration of heavy metals. It was discovered that fishes caught from very polluted areas have high level of heavy metals concentration whereas low level concentration of  heavy metals were found in  those caught from less polluted areas (Jan et al., 2010).


Season is also a factor that contributes to how metals accumulate in the body of fishes. Fishes caught during the raining season tend to have less heavy metals concentration than the ones caught during dry season. In addition, size and age of the fishes also affects the level of heavy metal concentrations; for example, small and younger fishes have less heavy metal concentrations than the larger and older ones.  The way metals accumulate in various tissues and organs of the same species also varies. For example, the concentration of metals in the digestive tract, gills, skin, and liver are much higher than in muscles of fishes (Uysal, 2011), but concentrations can also vary by species. In Koycegiz Lake in Turkey, research showed that heavy-metal concentrations in the liver, gills, and muscles of three different species varied by species. The highest concentrations of heavy metal was found in the liver and gills of the different species, and the lowest concentrations was detected in the edible portion of all the species (muscles) (Yilmaz, 2009). In Porsuk Dam Lake in Turkey, concentration levels of heavy metals in the tissues and organ of different species varied in tissues of the same species (Uysal, 2011).

Health impact of heavy metals from fish consumption

When humans consume high quantities of heavy metals, they show symptoms of gastrointestinal disorders, diarrhea, tremors, vomiting, depression, paralysis, stomatitis, pneumonia, and convulsion. The accumulation of these metals can lead to neurotoxic, carcinogenic, mutagenic, teratogenic, and toxic effects on the human body (Zahra et al., 2017). In Minamata, Japan, people were exposed to serious mercury poisoning during the 1950s, which was associated with consuming fish that was contaminated with mercury. Hundreds of people that were affected experienced neurological symptoms such as ataxia, hearing loss, mental misperception, and parasthesia, which resulted in death of several individuals  (Knobeloch, Steenport, Schrank, & Anderson, 2006).  A study showed outbreaks of mercury poisoning in Japan and Iraq, which greatly affected the development of fetuses. Also, in the same study, mercury was found in an elderly woman in her early 50s. It was thought to have been caused by consuming fish that was affected by mercury (Risher, 2004).


Water pollution in developing countries

In developing countries, increase in industrialization and urbanization has led to the significant discharge of wastewater, which may be used for irrigation in semi- urban and urban agriculture. Wastewater is water that has been used or polluted from industries, homes, agricultural activities, and surface runoff; it can also be from natural source, example stormwater. Irrigation farming derives important economic activity, provides support to poor farmers, and significantly alters the water quality of freshwater bodies (Singh, Sharma, Agrawal, & Marshall, 2010).


For example, farmers in some African countries such as Kenya and some parts of Asia such as India choose to use undiluted wastewater for irrigation because it is inexpensive and gives nutrients compared to other water bodies. Small-scale farmers in these continents cultivate perishable (unpreserved) crops like vegetables using wastewater to improve their domestic income. Furthermore, 60 to 80 percent of consumed vegetable in some parts of Western Africans countries are produced in semi- urban and urban areas using wastewater ( FNB, 2004, as cited in Qureshi, Hussain, Ismail, & Khan, 2016).


Regular use of sewage and industrial wastewater for irrigation of agricultural land may lead to heavy-metal accumulation in the soil, which later drains to the water bodies. The types of heavy metals present in wastewater depend on the kind of activities that take place. Examples of heavy metals often found in wastewater include zinc, copper, lead, manganese, nickel, chromium, and cadmium  (Singh et al., 2010). Environmental contamination can be dangerous to aquatic organisms. This is because all the pollutants (land and air) finally concentrate in aquatic habitat (Zahra et al., 2017).


River Benue, a source of fish in northeast Nigeria

At a length of 1,083 km, River Benue in northeastern Nigeria is the second largest river in Nigeria. It originates in Northern Cameroon.  River Benue is prone to pollution because people depend on it for their personal use, industrial activity and irrigation farming. Wastewater from these activities enters the river because the river flows through the capital city, Yola, and the major urban center (Jimeta). Random waste disposal and agricultural (irrigation farming) activities commonly take place around the river. Solid waste is disposed in open dumps, and human waste is disposed in pit latrines and septic tanks, but the liquid part of the waste may be disposed via the main drainage and ends up in the Benue River (Hong, 2014).

Fishing activities on the river provide fish for consumption for the local communities. Agriculture and other human activities that occur near the River Benue may lead to pollution of the river with toxicants, such as metals, and this may in turn results in bioaccumulation of heavy metals in organisms in the river, such as in the tissues and organs of fishes.


In a study conducted in  River Benue of Adamawa in 1995, Northeastern Nigeria, lead concentrations were higher than the WHO’s permissible limit, and this was thought to be a result of waste discharged from the city into the river (Eromosele, Eromosele, Muktar, & Birdling, 1995).  Adamu & Nganje (2009) reported high level of heavy metals such as lead, manganese, copper, and zinc in the water sediments of Benue River in Makurdi, and this was considered associated with industrial effluents and anthropogenic activities. Another study in River Benue, North-central Nigeria, reported high level of heavy metals concentration in the gills, intestine and  tissues of tilapia and catfish species (Eneji, Sha’Ato, & Annune, 2011) . A similar study conducted in Benue River found a high level of lead in the water sediment, which was above the recommended guidelines by WHO. This was  assumed to be caused by waste discharge (solid discharge) from paints alloys, plastics made from polyvinyl chloride, and  lead batteries (Eneji, Sha’Ato, & Annune, 2012). Hence, heavy metal contamination in water and fish ought to be controlled (Yi et al., 2011).


The River Benue holds great importance to thousands of Nigerians who live near it. It provides a source of income for fishermen, source of protein for people, and water for irrigation. However, the river suffers from siltation, pollution from industrial and urban effluent, overuse by people, and climate change. Contaminated fish caught from the Benue River and sold in markets may present a public health issue. Therefore, a research was conducted to investigate heavy-metal concentrations in the tissues of fish found in Upper Benue River (wild-caught) and from commercially farmed fish, the latter serving as a control. The aim was to determine the possible health risks of consuming these fish. I compared my results with the World Health Organization’s permissible limits for heavy metals. Moreover, recommendations and findings would be shared with the Adamawa State Ministry of Health.



Null Hypothesis:

There are no differences in heavy metal concentrations among wild-caught and farmed fish in Yola-Jimeta region, Adamawa state, Northeastern Nigeria

Alternative Hypothesis:

Heavy metal concentrations are greater in wild-caught fish than in farmed fish in Yola- Jimeta region, Adamawa State, Northeastern Nigeria



  • To determine if commercially available fish in the Yola-Jimeta region, Adamawa State, Northeastern Nigeria, present a public-health threat


  • To collect fish samples from rivers and streams (wild-caught fish) and from fish farms
  • To determine if, and which, heavy metals are present in the tissues of these fish samples
  • To determine the heavy-metal concentrations in the tissues and organ of these fish samples
  • To compare the types and concentrations of heavy metals between wild-caught and farmed fish
  • To compare these measured concentrations with World Health Organization (WHO) permissible limits
  • To make recommendations to the Adamawa State Ministry of Health




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