Temporal Toxicodynamics of Lead Nitrate in African Catfish (Clarias gariepinus) Fingerlings: Behavioural responses, probit-derived toxicity thresholds and environmental risk characterization

Lead contamination of freshwater ecosystems remains a major environmental challenge with serious implications for fish health, aquaculture production, and ecological sustainability. The present study evaluated the acute toxicodynamics of lead nitrate in African catfish (Clarias gariepinus) fingerlings through a 96-hour laboratory bioassay. Fish were exposed to graded concentrations of lead nitrate (0.00, 39.96, 79.93, 159.85 and 319.70 mg L⁻¹), and cumulative mortality, behavioural alterations, and Probit-derived toxicity thresholds were determined. Mortality increased progressively with both concentration and exposure duration, reaching 66.7% at the highest concentration after 96 hours. Correspondingly, the median lethal concentration (LC₅₀) declined from 9.987 mg L⁻¹ at 24 hours to 4.174 mg L⁻¹ at 96 hours, representing approximately a 58% reduction in toxicity threshold over time. Behavioural responses, including increased air gulping, erratic swimming, abnormal opercular movement, impaired equilibrium, abnormal tail beating, skin peeling, body discolouration, and loss of reflex, intensified progressively with increasing exposure. A Behavioural Toxicity Index (BTI) developed in this study demonstrated a clear concentration-dependent increase in behavioural impairment. Estimated safe exposure concentrations derived using the Sprague and National Academy of Sciences/National Academy of Engineering (NAS/NAE) application factors also declined with increasing exposure duration, indicating progressively reduced environmental safety margins. Collectively, the findings demonstrate that lead nitrate toxicity is strongly influenced by exposure duration and highlight the value of integrating behavioural biomarkers with Probit-derived toxicity thresholds for environmental risk assessment. The study provides baseline toxicological information that will support freshwater environmental monitoring, ecological risk assessment, and sustainable aquaculture management in regions affected by heavy metal contamination.