CHAPTER ONE

1.1  Introduction

Heavy metals are those metallic elements with high atomic or specific gravity that is at least five times greater than that of water. Several elements have been listed in this group. The presence of heavy metals in the aquatic environment in trace concentrations is important for normal development of the organism (Kori-Siakpere and Ubogu, 2008).

They could be detected in the aqueous medium and in the bottom; some however, are completely deleterious and need to be monitored continuously in the bodies of organisms as they are capable of bioaccumulation, resulting to morbidity and often mortality of organisms  (Ayotunde et. al, 2011, 2012., and Ada, et. al., 2012). Heavy metal human health concern has stimulated a lot of research in this area, some of which include the works of Muchuweti et. al., (2006), Satarug et. al. (2000) and Adefemi et. al. (2012).

When metals enter aquatic environment, a great portion settles and is absorbed by the bottom mud (Ayotunde, 2012). They could be recycled by chemical, physical and biological processes such that some quantity remains dissolved in the water column and some part is being absorbed by the inhabitants (Rayms- Keller et. al., 1998 and Kori- Siakpere and Ubogu, 2008). Fishes are at the apex of the food chain and can bio accumulate some of these substances into their tissues (Olaifa et. al., 2004).

Kori- Siakpere and Ubogu, (2008) observed decreased haematological parameter values and heamodilution in fish exposed to sublethal concentrations of zinc for 15 days. They explained that zinc accumulates in gills of fish and it is an indication of depressive effect on respiration in tissues. It may lead to death due to hypoxia, reduced hatchability of eggs, changes in ventilator heart physiology and general change in fish behaviours such as lack of balance, agitated swimming, air gulping, and death.

Some heavy metals are not biodegradable and can continue to be accumulated in the tissue of organisms until they reach intolerable levels resulting in morbidity or mortality (Offem and Ayotunde, 2008). Man being higher in the food chain stands the risk of higher bioaccumulation. The dangers of heavy metals have been long noted, but the symptoms are poorly diagnosed and patients could be treated for some other illness thereby aggravating heavy metal pollution problems of man (Kaye et. al., 2002 and Nolan, 2003).

There is an increasing concern regarding the roles and fates of trace metals in Nigerian environment. Much of this concern arises from the low level of available information on the concentrations of these metals within the environment. The contamination of sea foods by trace metals is a potential problem to man. Aquatic organisms accumulate metals to concentrations many times higher than present in water.

The potentially toxic are lead, zinc, nickel, chromium, arsenic, selenium, vanadium, beryllium and barium. Natural and anthropogenic activities result in gaseous emissions and waste water discharge into air, water and land. When  substances in the emissions and effluent discharges in the environment are in very minute amounts or in low  concentrations, are not toxic to plants and animals and have short residence time in the environment, they are described as contaminants’ (Odiete, 1999).

Bioconcentration is the net accumulation of a substance from water into an aquatic organism resulting from the simultaneous uptake and elimination of the substance. Fish and bivalve molluscs are used in bioaccumulation tests because they are higher tropic level organisms and are usually eaten by man. Tissues such as liver, kidney, muscle, viscera and whole organisms are analyzed to determine the concentration of the metals (Dublin-Green, 1994).

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