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.
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