Abstract
Introduction
Pollution of inland and marine waters is one of the most important environmental problems, mainly due to industrial, agricultural, and human activities and their wastewater discharge into aquatic environments. In recent decades, heavy metal pollution has attracted the attention of toxicological and environmental studies due to its high persistence, high bioaccumulation, and toxic effects on aquatic organisms. In this regard, lead is one of the heavy metals that has entered aquatic environments in large quantities due to industrial activities. In the aquatic environment, lead is absorbed and accumulated by aquatic organisms, which can negatively affect all vital aspects of these organisms, such as growth, immunity, metabolism, and osmotic regulation. Grey mullet is a fish susceptible to heavy metal contamination due to their omnivorous diet and near-bed habitat. Therefore, physiological and hematological changes in this fish may reflect the impact of the environment, especially pollutants, on this species. Blood indices are widely used in toxicological studies to determine the physiological responses of fish to environmental changes. Although several hematological studies have reported heavy metal contamination in fish, little information is available on gray mullet. Therefore, considering the economic and ecological importance of gray mullet and its sensitivity to heavy metal contamination, the present study aimed to investigate the effect of environmental lead concentration on the hematological status of this species.
Materials and Methods
Mullet fish were collected from coastal estuaries of Chabahar Bay and transferred to the aquaculture department of the Off-shore Fisheries Research Center under aeration and distributed in 500 L polyethylene tanks containing seawater with a salinity of 35 g/L for a 10-day acclimation period. After the acclimation period and, after weighing the fish (28.2 ± 3.1 g), twelve 40 L tanks containing seawater (with a salinity of 35 g/L) were stocked at a density of 10 fish per tank. Fish were exposed to different doses of lead nitrate [Pb(NO3)2] at ambient concentrations of 5, 15, and 25 μg/L (MERCK Co.) for 96 h in three replicates. After 96 hours of exposure, blood samples were collected by cutting the caudal peduncle of the fish (due to the small size of the fish and to obtain the maximum amount of blood) in heparinized tubes and used for the analysing blood parameters. Hematocrit index (Hct) was measured using the microhematocrit method, in which blood samples were first centrifuged (12000 × g, 5 min) and then the volumetric ratio of the sediment phase of blood cells to the total blood volume was calculated. Hemoglobin (Hb) concentration was measured using the methemoglobin method at a wavelength of 540 nm. A hemocytometer was also used to determine the number of red and white blood cells under a light microscope.
Results and Discussion
The fish were exposed to environmental concentrations of lead nitrate (Pb(NO₃)₂) of 5, 15, and 25 μg/L for 96 hours and their blood parameters were determined. According to the results, there was no significant change in blood parameters between the control group and the group exposed to 5 μg/L of lead (p>0.05). However, at higher concentrations (15 and 25 μg/L), significant decreases in hematocrit (Hct), red blood cell count (RBC), hemoglobin (HB), mean corpuscular hemoglobin (MCH), mean corpuscular volume (MCV), and mean corpuscular hemoglobin concentration (MCHC) were observed (p<0.05). In contrast, the white blood cell (WBC) count significantly increased in fish exposed to 15 and 25 μg/L lead, indicating a stress response (p<0.05).
Conclusion
The results of the present study indicated that lead exposure may impair immune function and oxygen transport in fish and make them more sensitive to environmental stressors. Also, these results suggested that hematological parameters can be biomarkers of heavy metal pollution in aquatic environments.
|
