The purpose of this paper is to highlight the importance of blood analysis in assessing the welfare and health status of the fish. The blood is the circulating fluid that, through its dynamism, ensures the supply of all body cells with oxygen and nutrients. The peripheral blood is probably the most informative tissue because it may reflect all organism functions and can be non-lethally sampled in fish. Blood samples for hematological analysis are preferably collected by puncturing the caudal vessels or the heart, when there is no need for the fish to be sacrificed. Hematological indices, such as red blood cell counts, hemoglobin, hematocrit, leukocyte profile, differential white blood cell counts are widely known as indicators of several diseases and environmental stress in fish. The direct examination of the blood smears stained with May-Grunwald-Giemsa solution through routine hematological procedures can provide first signs of parasitosis (e.g. increase in eosinophil’s number) or infections (e.g. increase in numbers of neutrophils, monocytes) together with the identification of some pathogens, such as haemoparasites and bacteria (septicaemia). Moreover, different observations of thin blood smears regarding the shape, size and color of the formed elements (red and white blood cell) have congruent diagnostic value. Our paper includes a broad overview of the working steps in hematological analysis and blood smear examination as well as the data gathered from our practice related to fish species variability manifestations under the action of extrinsic factors. As a conclusion, blood analyses may help as first indicators in the evaluation of fish health and valuable data base of blood investigations concerning their interspecies physiological reaction must be obtained from large numbers of individuals.
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