Abstract
<jats:p>One of the most important issues in ecological physiology and ichthyology is the study of the physiological and biochemical mechanisms underlying fish adaptation to changing environmental conditions. This issue is of particular significance in the context of increasing anthropogenic pressure on freshwater ecosystems. Understanding the ecological and physiological aspects of adaptation within the framework of integrated population studies is essential for developing a scientific basis for predicting the biological status of fish. Addressing this problem requires systematic investigations of the dynamics of homeostatic parameters in functional body systems, taking into account their sex-, age-, and season-related variability. In the present study, seasonal investigations were conducted to determine the hematological characteristics of taxonomically closely related bony fish species that differ in physiological activity, feeding habits, and ecology, sampled from the Aleksandar Stamboliyski Reservoir in Bulgaria. The physicochemical parameters of the water—temperature, pH, electrical conductivity, and dissolved oxygen—were within the allowable values by national and EU legislation. However, the water analyses revealed elevated concentrations of nitrate nitrogen (N–NO₃⁻) and nitrite nitrogen (N–NO₂⁻) compared to the permissible values for moderate ecological status under Regulation No. H-4. Furthermore, the concentrations of toxic metals in the water—chromium (Cr), cobalt (Co), lead (Pb), and zinc (Zn)—were also within the limits. However, the cadmium (Cd) levels were reported as &lt;0.02 mg/L (20 µg/L), exceeding the permissible value of 5 µg/L. Seasonal dynamics and species-specific patterns were observed in hematological parameters. The erythrocyte indices in the three fish species – common carp (Cyprinus carpio Linnaeus, 1758), Prussian carp (Carassius gibelio Bloch, 1782) and European perch (Perca fluviatilis Linnaeus, 1758) showed similar values during spring and winter; the highest values were recorded in common carp during summer and the lowest in Prussian carp during summer. The hemoglobin and hematocrit values were highest during winter for all three species, with the highest levels observed in common carp, followed by European perch and Prussian carp. Morphological alterations were also identified in the studied species - in common carp, changes were observed in erythrocyte nuclei; in Prussian carp, a high number of dividing erythrocytes was recorded; in European perch, the observed changes included the presence of rounded erythrocytes.</jats:p>