Understanding trace elements dynamics and environmental health: insights from Fish Mortality in Rajshahi, Bangladesh

Introduction

This study investigates the combined impact of trace metal contamination and environmental factors on fish mortality in aquaculture ponds in the Rajshahi region of Bangladesh. The primary objective is to assess how physicochemical parameters and trace metal concentrations interact with climatic conditions to influence fish health and survival.

Materials and Methods

Water, sediment, and fish tissue samples were collected and analyzed for arsenic (As), mercury (Hg), chromium (Cr), cadmium (Cd), and lead (Pb) using Atomic Absorption Spectrometry (AAS-240FS). Key physicochemical parameters such as pH, electrical conductivity (EC), total dissolved solids (TDS), and dissolved oxygen (DO) were measured. Climatic variables, including relative humidity and temperature, were recorded to evaluate their relationship with DO levels and fish mortality events.

Results and Discussion

Mean values of pH, EC, and TDS were 6.77, 433.4 µS cm⁻¹, and 209 mg L⁻¹, respectively. Trace metal concentrations in water followed the order Pb > As > Cr > Hg > Cd; in sediment, Cr > Pb > As > Hg > Cd; and in fish tissue, Cr > Pb > As > Hg > Cd. A significant inverse correlation was found between high relative humidity (90–95 %) and DO levels, which decreased from 4.00 mg L⁻¹ to 1.08 mg L⁻¹ well below the critical threshold of 5–6 mg L⁻¹ required for fish survival. Additionally, TDS exhibited a negative correlation with DO and a positive correlation with sedimentary arsenic, indicating that siltation from heavy rainfall contributes to oxygen depletion. The highest fish mortality, recorded on September 1, 2020, coincided with overcast conditions, elevated temperatures, and reduced photosynthetic activity, all of which exacerbated hypoxic stress.

Conclusion

The findings highlight the complex interplay between environmental stressors and trace metal pollution in aquaculture ponds. Regular water quality monitoring, pollution control, and adaptive management strategies are essential to reduce fish mortality risks and ensure the long-term sustainability of aquaculture in the region.