Indicators of stress
Because chronic stress is not immediately lethal, it often goes undetected until its effects influence fish populations and community structure. Interest in the early detection of stress in fishes has led to increased study of biomarkers, which are cellular and subcellular indicators of environmental stress (Adams 2002). The principle behind the study of biomarkers is that stress can be detected at the subcellular and cellular level before it affects organismal or population health. Biomarkers, as well as biological indicators of stress at higher levels of biological organization, have been an active area of research.
Environmental stressors can result in the alteration ofDNA and interfere with the molecular activity of somehormones (Hodson 2002; Fil by et al. 2007). Exposure to many chemicals can result in increased levels of liver enzymesresponsible for their detoxification and metabolism, and also the induction of stress proteins (discussed earlier). Therefore, levels of thesebiochemical can be indicators of exposure to stress. Chronic stress can result in a variety of changes in cellular and tissue morphology in various organs, and biomarkers at this histopathological level are seen as good indicators because they show integrated, cumulative effects of physiological stress (Myers & Fournie 2002). Various biomarkers in the liver, spleen, skin, and musculo skeletal system seem to be the best supported by research thus far. The liver is the primary organ of contaminant detoxification, so it frequently shows signs of a fish dealing with environmental contaminants. The spleen also shows signs of environmental stress because of its important role in fish immune systems, as indicated by the presence of macrophage aggregates, also called melano macro phage centers. These have been shown to be good biomarkers of multiple environmental stressors and also can be indicators of past exposure because they remain once they have formed and accumulate with age. Several studies have supported the use of splenic macrophage aggregates as indicators of environmental stress (Wolkeet al. 1985; Blazer et al. 1987; Macchi et al. 1992; Blazeret al. 1994), and they may be able to show decreased stress in fish in areas that have undergone environmental improvement(Facey et al. 2005).
Through these and other biomarkers and bioindicators,it is becoming possible to detect stress from a variety ofagents, thereby permitting early detection of potential impacts on fish physiology, health, growth, reproductive success, and community structure.