Microplastic (MP) contamination is increasingly recognized as a global environmental problem affecting aquatic ecosystems, food quality, and animal and human health. Farmed fish represent an important and increasing component of the human diet. Therefore, understanding potential human exposure to MPs is essential for ensuring food safety. In the current paper, we present the results of a preliminary study conducted in Bulgaria on MP contamination in the muscle tissue of rainbow trout [Oncorhynchus mykiss (Walbaum, 1792)] reared in freshwater aquaculture systems. Edible tissues were analyzed using Laser Direct Infrared (LDIR) imaging spectroscopy, a highly sensitive method enabling rapid detection and accurate identification of polymer types present in samples. MPs were detected in all examined specimens, demonstrating that these particles are bioavailable and capable of accumulating in fish muscle tissues commonly consumed by humans. Moreover, the presence of multiple polymer types suggests diverse contamination sources within aquaculture environments. Although the present findings do not allow direct conclusions about human health risks, they indicate potential risks of trophic transfer and highlight the need for improved monitoring strategies and management practices in farmed fish production. Overall, this study provides novel data on MP exposure in aquaculture species and emphasizes the preventive importance of assessing plastic pollution in fish intended for human consumption.

Microplastic (MP) contamination is increasingly recognized as a global environmental problem affecting aquatic ecosystems, food quality, and animal and human health. Farmed fish represent an important and increasing component of the human diet. Therefore, understanding potential human exposure to MPs is essential for ensuring food safety. In this study, we present the first assessment in Bulgaria of MP contamination in the muscle tissue of rainbow trout [Oncorhynchus mykiss (Walbaum, 1792)] reared in freshwater aquaculture systems. Edible tissues were analyzed using Laser Direct Infrared (LDIR) imaging spectroscopy, a highly sensitive method enabling rapid detection and accurate identification of polymer types present in samples. MPs were detected in all examined specimens, demonstrating that these particles are bioavailable and capable of accumulating in fish muscle tissues commonly consumed by humans. Moreover, the presence of multiple polymer types suggests diverse contamination sources within aquaculture environments. Although the present findings do not allow direct conclusions about human health risks, they indicate potential risks of trophic transfer and highlight the need for improved monitoring strategies and management practices in farmed fish production. Overall, this study provides novel data on MP exposure in aquaculture species and emphasizes the preventive importance of assessing plastic pollution in fish intended for human consumption.

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.

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 <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.