There have been relatively few studies of marine fish, perhaps because contaminated discharges generally receive much greater dilution in salt waters than in fresh waters, a fact that probably discouraged an early search for effects in the sea. Mercury a highly toxic metal, results in a variety of adverse health effects. Due to wide use of mercury in agriculture, industrial, medical and other fields, its exposure is cannot be avoided. The toxicity of mercury depends greatly on the forms of the mercury compounds (elemental, inorganic and organic). Excessive levels of mercury in the marine environment can affect marine biota and pose risk to human consumers of seafood. Hence, mercury compounds found in the marine environment pose risk to human health through the consumption of contaminated seafood. Accordingly, it is desirable to minimize such exposure to levels that do not cause adverse effects. In an attempt to define and measure the effect of pollutants on an ecosystem, biomarkers have attracted a lot of interest. The underlying principle of the biomarker approach is the analysis of an organism’s physiological or biochemical response to pollutant exposure. Mercury induced oxidative stress; make an important contribution to molecular mechanism for liver injury, thus enzymatic biomarkers is proved to be suitable biomarkers in mercury studies. The measurement of biochemical and physiological parameters is a diagnostic tool commonly used in aquatic toxicology and biomonitoring, so Hematological and immunological parameters are suitable biomarkers in mercury studies. During stress, fish respond in a number of ways in order to regain homeostasis and two important physiological processes which are modulated when fish are exposed to stress, are hormonal status and immune function, Therefore, alterations in specific hormonal functions and consequent biochemical effects may constitute important stress biomarkers. One of the great advantages of using histopathological biomarkers in environmental monitoring is that this category of biomarkers allows examining specific target organs, which are responsible for vital functions in the fish.