Physiological Stress Response of Nile Tilapia (Oreochromis niloticus) to Sub-lethal Concentrations of Petroleum Hydrocarbons Under Controlled Conditions

Understanding the sub-lethal physiological effects of petroleum hydrocarbon exposure on commercially important fish species provides critical data for setting water quality standards and evaluating ecological risk in oil-producing regions. This study investigated the dose-dependent physiological stress responses of Nile tilapia (Oreochromis niloticus) to sub-lethal concentrations of crude oil water-accommodated fraction under controlled laboratory conditions. Juvenile tilapia were exposed to five crude oil WAF concentrations (0, 0.1, 0.5, 1.0, and 5.0 milligrams per litre) for 14 and 28-day exposure periods. Haematological parameters including packed cell volume, red blood cell count, white blood cell differential, and haemoglobin were measured at Days 0, 7, 14, and 28. Liver histopathology was conducted using haematoxylin and eosin staining. Oxidative stress biomarkers including superoxide dismutase, catalase, malondialdehyde, and glutathione-S-transferase activities were measured in liver and gill tissue. CYP1A gene expression was quantified using quantitative PCR. Concentration-dependent anaemia, leucocytosis, and elevated hepatosomatic indices were observed at concentrations of 1.0 milligrams per litre and above. Histopathology revealed hepatocellular vacuolation, necrosis, and proliferative bile duct changes at 5.0 milligrams per litre. Oxidative stress markers increased significantly at all sub-lethal concentrations from Day 7 onwards. CYP1A expression was upregulated up to 12-fold at the highest concentration. A no-observed-adverse-effect level of 0.1 milligrams per litre was established. The study recommends revision of Nigerian water quality standards for petroleum hydrocarbons to reflect sub-lethal toxicity endpoints. Keywords: petroleum hydrocarbons, Oreochromis niloticus, oxidative stress, CYP1A, sub-lethal toxicity.