Thirty six all male Oreochromis niloticus were divided into two groups, using six glass aquaria. The treated group was fed on a toxic strain of M. aeruginosa, while the control group was fed on a non toxic strain of M. aeruginosa, in order to measure their grazing rates from both strains of M. aeruginosa and consequently its effect on general fish health represented by, clinical signs, hematological changes and biochemical parameters. Grazing rates and daily intake of M. aeruginosa showed close grazing rates between the treated and the control groups with slightly higher
intake of non toxic strain than the toxic one were the individual intake was 63,050 and 62,567 cell/ml/fish respectively. Clinical signs in O. niloticus associated with exposure to toxic cyanobacteria were investigated. The observed clinical signs were sluggish movement and reduced reflexes during the late stages of exposure period.Plasma glucose, cortisol, T4, creatinine and AST showed higher concentrations in the treated group than that in the control, indicating on increased fish stress that was reflected in its health state and consequently its performance. On the opposite side, plasma total protein concentration was higher in the control than that in treated group. The hematocrit, plasma albumin, LDH and Alkaline phosphatase concentrations did not show any significant difference in both groups. The study concluded that the
uptake of toxic strain of M. aeruginosa via oral route had deleterious effects on some clinical and biochemical parameters of the blood of Nile tilapia which led finally affected the fish health. The study recommendations include prevention and
monitoring of organic and inorganic pollution that promote the harmful algal blooms,and take the early preventive measures to avoid toxic effects of cyanobacteria on fish and consequently on humans