Fish erythrocytes are ellipsoidal nucleated cells of different size (102–800 fl) and life span (13–500 days). Erythrocyte count depends on fish activity, water temperature and dissolved oxygen concentration as well as on other environmental factors and shows considerable seasonal variability. It is also dependent on age, sex, nutritional, and reproductive status, and may differ among various populations of the same species. It usually ranges from 0.5–1.5 × 10⁶/mm³ in less active species to 3.0–4.2 × 10⁶/mm³ in more active ones. Antarctic icefishes adapted to cold and well-oxygenated water lack erythrocytes. The head kidney is the main erythropoietic site in teleost fishes, and the process of erythropoiesis is similar to that in other vertebrates and involves the same precursors. Fishes show a weak barrier between hematopoietic tissue and circulating blood in which numerous immature cells are present, often comprising over 10% of all erythrocytes. As in other vertebrates, fish erythrocytes contain tetrameric hemoglobins of different oxygen affinity – lower in species living in well-oxygenated water than in those that experience hypoxia. Often several hemoglobin isoforms of different oxygen affinity are present in blood, which is considered an adaptation to variable oxygen concentration in water. Fish erythrocytes are sensitive to environmental pollution, and their morphological evaluation may be used as a bioindicator of toxicity.

Hematological analysis is commonly used to assess the physiological state of fish. It includes red blood cell parameters, white blood cell parameters, and the number of thrombocytes per blood volume unit. Hematological analysis is one of the basic tools (often accompanied by biochemical and histopathological analyses) to assess the influence of organic and inorganic substances on fish. It is, therefore, applicable in both ecotoxicology and pharmacotoxicology. The advantages of this research method are the lack of need for specialized laboratory equipment and low costs, and the limitations are the need for extensive experience among the personnel performing the tests. One of the recommended methods of supplementing routinely determined hematological parameters is assessing the cellular composition and activity of hematopoietic tissue. As there is very little scientific data available on the issue of the effects of xenobiotics on the cellular structure of fish head kidney hematopoietic tissue, filling this gap should be considered an urgent need. Therefore, we recommend conducting research with the simultaneous use of hematological and hematopoietic analysis as reliable and complementary methods of assessing the impact of toxic substances on fish.

Antibacterial agents are commonly present in aquatic environment at low concentrations. Terrestrial animal farms, human medicine and aquaculture are main sources of water contamination with antibacterials. Antibiotics were proved to be directly toxic to fish causing oxidative stress, general stress response, histopathological lesions, hematological, metabolic, and reproductive disorders, as well as immunosuppressive and genotoxic effects. Environmentally realistic low concentrations of antibiotics also disturb aquatic bacterial communities causing alterations in fish symbiotic microbiota and induce emergence of antibiotic-resistant pathogenic bacteria by exerting selective pressure on spread of antibiotic-resistance genes.