5794 Journal of Applied Sciences Research, 9(11): 5794-5810, 2013 ISSN 1819-544X This is a refereed journal and all articles are professionally screened and reviewed ORIGINAL ARTICLES Corresponding Author: Hanan S. Gaber, National Institute of Oceanography and Fisheries, Cairo, Egypt. Fish health as a biomarker for the condition of Lake Nasser (Review article) Hanan S. Gaber National Institute of Oceanography and Fisheries, Cairo, Egypt ABSTRACT The present review is a part of a project that focuses on the relationship between environmental pollution and fish diseases. The persistence and ubiquitous nature of these pollutants coupled with their tendency to accumulate in organisms ultimately produce toxic reactions in aquatic biota, especially fish. This document gives a brief overview of the heavy metal. The overview deals among other issues, the toxic effects of pollutants on the aquatic biota with emphasis on fish and the public health implication. During the past two decades, a variety of histopathological alterations in fish and bivalves have been developed and used as biomarkers in pollution monitoring. Some of these have been successfully adopted in major national monitoring programs, while others, although show promise, are still in the experimental stage. This paper critically reviews the scientific basis, cause and effect relationship, reliability, advantages and limitations of 14 histo-cytopathological biomarkers [Fin erosion, Skeletal malformation, Epidermal hyperplasia, Operculum abnormalities, Liver histopathology, Gill histopathology, Kidney histopathology, Oocyte atresia, Embryonic defect, Macrophage aggregates, Histopathology of mollusks, Lysosome integrity, Lipopigment content and Peroxisome proliferation] . The usefulness and practical application of each biomarker have been evaluated against a number of objective criteria including: ecological relevance, sensitivity, specificity, dose–response relationship, confounding factors, technical difficulties and cost-effectiveness. Key words: Fish Pathology, Biomarkers; Fish Diseases; Histo-Cytopathology; Aquatic Pollution; Xenobiotics Melano-Macrophage Centers, Pigments. Nasser Lake, Egypt. Introduction According to National Water Research Center (NWRC, 2000), the River Nile from Aswan to El-Kanater Barrage receives wastewater discharge from 124-point sources, of which 67 are agricultural drains and the remainders are industrial sources. Along Damietta branch, there are Talkha fertilizer plant, Kafer saad electric power station, Delta milk, Edfina factories, besides the sewage and domestic wastes discharging from the neighboring villages along Damietta (El- Serw and Ras El-Bar cities) without any treatment into the branch (RNPD, 1990 and Abdo, 2004). The River Nile at El-Rahway drain receives all sewage of El-Gieza governorate in addition to agricultural and domestic wastes of El-Rahway village and discharged these wastes directly without treatment into Rosetta branch (Tayel et al., 2008). In aquatic ecosystem, heavy metals are considered as the most important pollutants, since they are present throughout the ecosystem and are detectable in critical amounts. Generally, metals enter the High Dam Lake (Nasser and Nubia) from a variety of sources, including: rocks and soils directly exposed to waters, dead and decomposing vegetation and animal matter, wet and dry fallout of atmospheric particulate matter and human activities, including the discharge of various treated and untreated wastes to the water body (Abo El Ella et al., 2005). Metal contamination in the environment is an ongoing problem, particularly in aquatic environments, and there has been extensive investigation of metal effects on aquatic organisms (Niyogi and Wood, 2004). However, they likely show toxic effects when organisms are exposed to higher doses than those normally required. Other elements such as Pb and Cd are non-essential for (metabolic activities and exhibit toxic properties even with trace level (FAO, 1992). Fish is an important source of protein to humans and other animals. Fish industry also offers employment opportunities to many people as well as income at household and national levels (FAO, 1996). The tilapias are freshwater fish that belong to the family Cichlidae, and they are exclusively associated with Africa and Middle East. Initially, tilapias were considered to be more resistant to bacterial, parasitic, fungal, and viral diseases compared to other species of cultured fish. In more recent years, however, tilapias have been found to be susceptible to both bacterial and parasitic diseases. The presence of the pathogen in the environment of the fish is inadequate to cause a disease outbreak. Other factors usually come in to play. This phenomenon is often precipitated by “stress” (Yanong and Francis-Floyd, 2002). Stress in fish may be induced by various a biotic environmental factors such as changes in water temperature, pH, oxygen concentration and water pollutants