Agricultural pesticides are major contaminants of freshwater ecosystems and pose significant risks to non-target aquatic organisms, particularly fish. The present study investigated the haematological and metabolic effects of sublethal exposure to two widely used pesticides, methyl parathion (organophosphorus) and endosulfan (organochlorine), in the freshwater fish Oreochromis mossambicus. Adult fish were exposed to 1/10th of the 96 h LC₅₀ concentrations of each pesticide for 10, 20, and 30 days, followed by a 30-day recovery period in pesticide-free water. Haematological parameters including red blood cell count (RBC), haemoglobin (Hb), packed cell volume (PCV), mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH), mean corpuscular haemoglobin concentration (MCHC), and erythrocyte sedimentation rate (ESR) were analyzed, along with metabolic performance assessed through oxygen consumption rate.

Both pesticides induced significant, time-dependent haematological alterations characterized by marked reductions in RBC, Hb, PCV, and MCV, accompanied by increased MCH, MCHC, and ESR, indicating the development of microcytic and hypochromic anaemia and inflammatory stress. Oxygen consumption decreased progressively with exposure duration, reflecting metabolic depression and impaired respiratory efficiency. Endosulfan exerted more pronounced haematological and metabolic toxicity than methyl parathion, consistent with its higher persistence and bioaccumulative potential. Recovery experiments demonstrated partial restoration of altered parameters, suggesting physiological resilience in O. mossambicus, although complete recovery was not achieved, particularly after endosulfan exposure.

The findings confirm that haematological indices and oxygen consumption are sensitive and reliable biomarkers for assessing sublethal pesticide toxicity in fish. The study highlights the ecological risks associated with continued use of organophosphorus and organochlorine pesticides and underscores the need for improved pesticide management and monitoring strategies to protect freshwater ecosystems.