Deposition of aluminium in gills leads to chloride cell necrosis or at lower levels, to inhibition of the enzymes carbonic anhydrase and/or gill NA+, K+ – ATPase (NKA), required for seawater tolerance. Such changes impair osmoregulatory capacity and cause physiological stress due to disruption of gas and ion transport, altered blood chemistry and hormonal imbalance.
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Saprolegnia often presents as a secondary infection that is diagnosed by the appearance of white or grey cotton-like tufts that, when out of water, have a somewhat mucoid appearance. Typical disease signs are visible circular or crescent-shaped, cotton-wool like, white or grey patches of filamentous mycelia on the fish skin. The lesions appear mainly around the head and the caudal, adipose and anal fins. Lesions may spread over the body until adjacent lesions coalesce.
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