Seasonal Dynamics of Hematological Parameters in Three Freshwater Fish Species from Aleksandar Stamboliyski Reservoir (Bulgaria)

Abstract

Seasonal variability can complicate the interpretation of fish hematological parameters used in environmental assessment. We examined seasonal changes in erythrocyte count (RBC), hemoglobin concentration (Hb), hematocrit (PCV), and erythrocyte morphology in common carp (Cyprinus carpio Linnaeus, 1758), Prussian carp (Carassius gibelio (Bloch, 1782)), and European perch (Perca fluviatilis Linnaeus, 1758) from the Aleksandar Stamboliyski Reservoir, Bulgaria. Ten individuals of each species were sampled per season. Water temperature, pH, electrical conductivity, dissolved oxygen, nutrients, and selected metals were also measured. Season significantly affected most hematological variables, although the patterns differed among species. RBC remained comparatively stable in common carp, decreased during summer and autumn in Prussian carp, and was the lowest during autumn in European perch. Hb and PCV reached their highest values during winter in all three species. Qualitative microscopy also indicated species-specific erythrocyte morphological alterations. Most measured physicochemical variables were within the applicable environmental thresholds, whereas nitrate and nitrite nitrogen exceeded the regulatory values listed for moderate ecological status. As this is a preliminary study, contaminant concentrations were not measured in fish tissues. Moreover, since this study lacked an uncontaminated (reference) site, no causal relationship between water chemistry indices and hematological parameters could be established. However, the present findings provide baseline information on seasonal and interspecific variability in the hematological profiles of freshwater fishes, yet they require further analyses.