الفهرس 
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Abstract
Gills constitute the major organ for respiration (path way for oxygen
and carbon dioxide transfere between body tissues and water). But in some
species they are also involved in gas exchange with air (Grahame, 1976).
Gill plays an important role for ion regulation as well as osmotic balance
between internal and external media in fresh and sea water fishes.
Furthermore, other biosynthetic and metabolic functions have also been
assigned to them (Hughes and Morgan, 1973; Maetz, 1974 and Hughes,
1977).
Artificial methods of intensive fish fanning constitute predisposing
stress factors, resulting in several gill affections as well as other affections
(Sarig, 1968).
Several types of etiological agents are incriminated as a cause or gill
affections, among which parasites, bacteria, fungus and viruses. Pollution
with several types of water herbicides and heavy metals, or even the
interaction between these agents.
Pathogenic changes of affected gills including proliferation or tissues
and loss of surface epithelium, resulted in impaired respiration and hindering
of excretion of nitrogenous waste materials and disturbance or osmotic
balance. These changes have a detrimental effect on the fish health and
threaten their survival. The affected fish tries to over come this handicap by
a wider opening of its gill coverings and by increasing the frequency or