Ecological Assessment of Fish Diseases as Bio-Indicators of Water Quality in Freshwater Reservoirs

Abstract

Water quality plays a crucial role in maintaining the health and productivity of freshwater fish populations. The present study was conducted to evaluate the occurrence of fish diseases in relation to water quality parameters in a freshwater reservoir of Mirzapur District, Uttar Pradesh, India. Seasonal sampling of water and fish was carried out during pre-monsoon, monsoon, and post-monsoon periods. Physico-chemical parameters including temperature, pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), and turbidity were analyzed following standard methods. Simultaneously, fishes were examined for the presence of common diseases such as bacterial infections, parasitic infestations, and fungal lesions. The results revealed clear seasonal variations in water quality. Water temperature ranged from 24.6°C to 30.8°C, while pH remained slightly alkaline (7.2–8.3). Dissolved oxygen values varied between 4.8 and 7.2 mg/L, with the lowest levels observed during the monsoon. BOD and COD showed increased values during the monsoon season (BOD: 3.9–6.4 mg/L; COD: 18.5–32.7 mg/L) indicating higher organic pollution. Fish disease prevalence was highest during the monsoon (28.4%) and lowest in the post-monsoon season (12.6%). Parasitic infections and bacterial gill diseases were the most frequently observed conditions. Statistical analysis showed a positive correlation between disease occurrence and increased BOD, COD, and turbidity, while dissolved oxygen exhibited a negative correlation. These findings suggest that fish diseases can serve as reliable bio-indicators of deteriorating water quality in freshwater reservoirs.

Introduction

Freshwater ecosystems provide essential ecological, economic, and social services, including water supply, fisheries, biodiversity conservation, and livelihoods. However, increasing urbanization, industrialization, agricultural runoff, and other human activities have caused significant deterioration of freshwater quality through pollution, habitat degradation, and eutrophication. Water quality parameters such as temperature, pH, dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), and turbidity strongly influence fish health by affecting metabolism, immunity, growth, and disease resistance.

Traditional physicochemical monitoring methods provide information on environmental conditions but may not capture the long-term biological impacts of pollution. Therefore, biological indicators, particularly fish, are increasingly used to assess ecosystem health because fish integrate the effects of multiple environmental stressors over time. Fish diseases are sensitive indicators of environmental degradation, as poor water quality weakens fish immunity and increases susceptibility to bacterial, fungal, and parasitic infections.

The study highlights the importance of integrating water quality assessment with fish disease monitoring for evaluating freshwater ecosystem health. The research was conducted in a reservoir of Mirzapur District, Uttar Pradesh, India, with seasonal sampling during pre-monsoon, monsoon, and post-monsoon periods. Water quality parameters were measured, and commercially important fish species such as Labeo rohita, Catla catla, and Cirrhinus mrigala were examined for disease occurrence.

Results showed significant seasonal variations in water quality. The monsoon season exhibited the poorest environmental conditions, with increased temperature, turbidity, BOD, and COD, along with reduced dissolved oxygen levels due to runoff and organic pollution. Fish disease prevalence was highest during the monsoon (28.4%), followed by pre-monsoon (19.8%) and post-monsoon (12.6%). Common diseases included parasitic infestations, bacterial gill infections, and fungal infections.

Statistical analysis revealed strong relationships between deteriorating water quality and disease occurrence. Disease prevalence showed positive correlations with BOD, COD, and turbidity, while dissolved oxygen showed a strong negative correlation. These findings indicate that increased organic pollution and reduced oxygen availability create favourable conditions for pathogen growth and weaken fish resistance.

Overall, the study demonstrates that fish diseases can serve as effective bio-indicators of freshwater ecosystem degradation. Combining biological monitoring with conventional water quality assessment provides a more comprehensive approach for reservoir management, pollution control, fisheries sustainability, and freshwater conservation. The findings support the development of integrated biomonitoring strategies for maintaining healthy aquatic ecosystems.

Conclusion

The present study demonstrated a significant relationship between seasonal variations in water quality and the occurrence of fish diseases in a freshwater reservoir of Mirzapur District, Uttar Pradesh. The findings revealed that deterioration in physicochemical parameters, particularly reduced dissolved oxygen and elevated biochemical oxygen demand, chemical oxygen demand, and turbidity during the monsoon season, was associated with increased prevalence of bacterial, fungal, and parasitic infections in freshwater fishes. Statistical analysis further confirmed a strong positive correlation between disease prevalence and organic pollution indicators, whereas dissolved oxygen exhibited a strong negative correlation with disease occurrence. These observations indicate that fish health responds sensitively to environmental changes and reflects the cumulative impact of multiple stressors acting within aquatic ecosystems. The study therefore establishes that fish diseases can serve as reliable bio-indicators for evaluating freshwater ecosystem health and complement conventional physicochemical water quality assessment. Integrating fish disease surveillance with routine environmental monitoring can facilitate early detection of ecological degradation, support sustainable fisheries management, and contribute to the conservation of freshwater biodiversity. The findings are consistent with previous studies highlighting the usefulness of fish-based ecological assessment for monitoring aquatic ecosystem integrity (Levin et al., 2019; Argillier et al., 2012; Singh et al., 2024).

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Copyright

Copyright © 2026 Brij Bali Singh, Yogesh Mishra. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.