The presence of heavy metals in food remains an issue of public health concern. The study was therefore designed to determine the heavy metal concentrations in water and fish (Clarias gariepinus and Tilapia guineensis). The study made use of Agodi lake in Ibadan. A laboratory based study was adopted. The parameters included, pH, dissolved oxygen (DO), lead, cadmium, chromium, zinc and copper. Descriptive statistics, t-test and ANOVA were used for the data analysis. The mean pH and DO levels in water were 7.20 ± 0.31 and 4.39 ± 2.08 mg/L respectively. The mean concentrations (mg/L) of heavy metals in water were: 0.01±0.01 (cadmium) ; 0.01 ± 0.01 (chromium); 0.02 ± 0.01 (copper); 0.04±0.01 (zinc); 0.06±0.04 (lead). The heavy metal with the highest concentration found in both fish species was Zinc and the one with the least concentration was Cadmium. The mean concentrations (mg/kg) of heavy metals in cat fish were, 0.73±0.25 (cadmium) ; 4.8 ± 0.34 (chromium) ; 7.51±1.8 (lead) ; 17.62±0.27 (copper) ; 119.22±0.51 (zinc) while those of tilapia fish were, 0.48 ±0.22 (cadmium); 3.57±1.13 (chromium); 7.91 ± 1.69 (lead); 17.37±0.34 (copper) ; 176.82±0.27 (zinc). All heavy metals assessed in fish were above WHO and National guideline limits. The release of heavy metals into Agodi lake has increased the risk of its use as source of water for human activities and the consumption of resident fin-fishes. The need for adoption and enforcement of appropriate monitoring and management strategies for the protection of the lake is therefore important.
1.1BACKGROUND TO THE STUDY
Metallic elements are environmentally ubiquitous, readily dissolved in and transported by water, and readily taken up by aquatic organisms (Alam et al., 2002). Heavy or toxic metals are trace metals that are at least five times denser than water. They generally do not break down further into less harmful constituent; and accumulate where they are released (Kennicutt et al., 1992). As such, they are stable (cannot be metabolized by the body) and bio- accumulative. They are sometimes passed up the food chain to humans (Fergosson, 1990). Some metals are essential to biota but could be very harmful when present in excessive concentration. Heavy metals are non-degradable and very harmful to plants, aquatic organisms and human health at certain level of exposure (Mustafa and Nilgun, 2006). At low levels, some heavy metals, such as copper or cobalt, are essential for enzymatic activity, but act as enzyme inhibitors at higher concentrations. Other metals, such as cadmium and lead, have no known essential role in living organisms, and are toxic even at low concentrations (Bryan, 1976). Heavy metals can be classified as potentially toxic; such as arsenic, cadmium, lead, mercury, etc., probably essential; such as nickel, vanadium, cobalt and essential; such as copper, zinc, iron, manganese (Kheradmand et al., 2006).
In recent years, the need for a better understanding of heavy metal concentration and dispersion patterns in aquatic environments has been highlighted following the discovery of high levels of toxic heavy metals (particularly cadmium and lead) in fish and other living organisms (McConchie et al., 2008). Heavy metal concentrations in aquatic ecosystems are usually monitored by measuring their concentrations in water, sediments and biota, which generally exist in low levels in water and attain considerable concentration in sediments and biota (Camusso et al., 1995). Heavy metals including essential and non-essential elements have a particular significance in ecotoxicology, since they are highly persistent and all have the potential to be toxic to living organisms (Storelli et al., 2005).
Fish, which is often at the top of the aquatic food chain concentrate large amount of these metals from the surrounding waters. Fishes are important and the largest groups of vertebrates in the aquatic system and heavy metals can be accumulated via both food chain and water (Gibson, 1994). Fishes have been considered good indicators for heavy metal contamination in aquatic systems because they occupy different trophic levels with different sizes and ages (Burger et al., 2002). Fish can be considered as one of the most significant indicators in freshwater systems for the estimation of metal pollution level (Rashed, 2001). In addition, fishes are widely consumed in many parts of the world by humans, and polluted fish may endanger human health.
Studies on heavy metals in rivers, lakes, fish and sediments (Özmen et al., 2004; Begüm et al., 2005; Öztürk et al., 2008; Praveena et al., 2008) have been a major environmental focus especially during the last decade. The human body has need for approximately 70 friendly trace elements including heavy metals, but there are 12 poisonous heavy metals, such as lead, mercury, aluminium, arsenic, cadmium, nickel, chromium, etc., that act as poisonous interference to the enzyme systems and metabolism of the body. It is important to continuously monitor the levels of heavy metals in aquatic environments as the presence of metal pollutants in fresh water is known to disturb the delicate balance of the aquatic ecosystem. Furthermore, fishes are notorious for their ability to concentrate heavy metals in their muscles and since they play important role in human nutrition, they need to be carefully screened to ensure that unnecessary high level of some toxic trace metals are not being transferred to man through fish consumption (Adeniyi and Yusuf, 2007). Clarias gariepinus and Tilapia guineensis are among the most commonly harvested and consumed fresh water fish species in tropical waters. There is therefore a need for regular monitoring of the quality to prevent heavy metal contamination and bioaccumulation along the food chain.
In an aquatic environment, dissolved oxygen, hardness, pH, alkalinity and temperature have been reported to influence metal toxicity (Adhikari et al., 2006). Nsikak et al., (2007) also reported that accumulation of heavy metals is influenced by the length and weight of fish while Haffor and Al-Ayed (2003) identified time of exposure to heavy metals as another factor.
1.2STATEMENT OF PROBLEM
Among environmental pollutants, heavy metals are of particular concern; due to their potential toxic effect and ability to bio-accumulate in aquatic ecosystems (Censi et al., 2006) thus, the pollution of the aquatic environment with heavy metals has become a public health problem worldwide during recent years. Heavy metals that are deposited in the aquatic environment may accumulate in the food chain and cause ecological damage as well as threat to human health (Van de Broek et al., 2002; Gagnaire et al., 2004). Studies have shown that fish accumulate these heavy metals from the surrounding water bodies thereby leaving a health risk if taken as food ( Prusty, 1994; US.DPHHS, 2005). Heavy metal intake has also been reported to be essentially due to drinking contaminated water and ingestion of contaminated food (Idodo-Umeh, 2002; FEPA, 2003; Asonye et al, 2007).
Heavy metals such as copper, iron and nickel are essential metals since they play important roles in biological systems, whereas cadmium and lead are non-essential metals, as they are toxic, even in trace amounts (Fernandes et al., 2008). However, these essential metals can also produce toxic effects when the metal intake is excessively elevated (Tüzen, 2003). Heavy metals are taken into biological systems via inhalation, ingestion and skin absorption. If they enter and accumulate in body tissue faster than the body’s detoxification pathways can dispose of them, a gradual build-up of these toxins will occur and result in toxicity. High concentration exposure however, is not necessarily required to produce a state of toxicity in the body tissue as overtime low concentrations can also reach toxic levels ( Prusty, 1994).
Humans, being the occupants of the apex of the food chain get the highest level of toxicity in their food intake. This was evident in the Serious human mass poisonings from seafood in Minamata in 1956 and 1965 in Niigata due to alkylmercury discharged from chemical manufacturing plants (Environment Agency, Japan, 1975). Similarly, the toxic threat from bio-accumulated cadmium was demonstrated by human “itai-itai” disease in Japan in 1947, where Industrial discharge of cadmium into Jintsu River area, resulted in the death of more than 100 people who consumed the contaminated water ( Hutzinger , 1980; Ramade, 1987). In Nigeria, heavy metals have been implicated in the upsurge of liver and kidney diseases and believed to be responsible for a high proportion of mortality caused by kidney and liver morbidity (Ndiokwere, 2004).The acute neurological effects of cadmium toxicity manifests itself in the form of nausea, abdominal cramps, bloody diarrhoea, vomiting, dizziness and chest pain. Also Rheumatic arthritis, muscular pain and osteomalacia in the elderly are evidence of chronic cadmium exposure (Klaassen, 1995). Long term exposure of humans to chromium can cause kidney and liver damage as well as damage to the circulatory and nerve tissues (Jackson and Morris, 1989). Lead(Pb) exposure can result in a wide range of biological effects, with developing foetus more affected than adults. The fatality rate of Pb neurotoxicity is about 25% while about 40% of the survivors have to live with neurological sequel such as mental retardation, optic atrophy and cerebral palsy (Harvey, 1975).
Regular monitoring of water bodies is an effective method of protecting aquatic life as well as humans from the toxic effects of heavy metals. However, several lakes in Nigeria have not been adequately assessed for heavy metals concentration. Hence, this study assessed the concentration of heavy metals in water and fish species from Agodi lake; a recipient of sewage effluent as well as other anthropogenic inputs and a provider of water as well as fish consumed by humans.
1.3AIM AND OBJECTIVES
The main objective of this study was the determination Of heavy metal concentrations in fish and water. The aim of this study was achieved through the following specific objectives:
1. Determination of the physico-chemical quality of water samples from Agodi lake and its tributaries.
2. Determination of the level of lead, cadmium, chromium, zinc and copper in the water samples from the lake and its tributaries.
3. Determination of the level of lead, cadmium, chromium, zinc and copper in Clarias gariepinus and Tilapia guineensis from Agodi lake.
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