Time-Dependent Toxicity and Behavioural Alterations in Oreochromis mossambicus Exposed to Monocrotophos: A Quantitative Assessment

Monocrotophos (MCP), an organophosphate insecticide widely used in agriculture, poses significant ecological risks to non-target aquatic organisms through runoff contamination. Standard 96-hour LC50 bioassays inadequately represent chronic exposure scenarios typical of pesticide-contaminated watersheds, potentially underestimating ecological risks. Static-renewal bioassays were conducted on Oreochromis mossambicus (Mozambique tilapia) over 7-day and 14-day exposure periods using nominal MCP concentrations ranging from 0–10.0 ppm (7-day) and 0–4.5 ppm (14-day). Mortality was recorded daily, and LC50 values were calculated using Probit analysis. Behavioural alterations were assessed through systematic observational monitoring, documenting erratic swimming, loss of balance, increased mucus secretion, and reduced response to stimuli. The percentage of fish displaying each behavioural alteration was quantified for each treatment group. Probit analysis yielded LC50 values of 3.6 ppm (95% CI: 3.2–4.1 ppm) for 7 days and 2.4 ppm (95% CI: 2.1–2.7 ppm) for 14 days, representing a statistically significant 33% reduction in LC50 with prolonged exposure. Regression slopes of 4.2 ± 0.6 (7-day) and 5.1 ± 0.8 (14-day) indicated sharper dose-response relationships over time. Behavioural alterations emerged sequentially: initial hyperactivity (24–48 h), followed by loss of equilibrium, erratic swimming, rapid opercular movements, frequent surfacing, and excessive mucus secretion (days 7–14). At 3.0 ppm, 70% of fish displayed erratic swimming and 60% showed loss of balance by day 7, increasing to 85% and 75%, respectively, by day 14. The time-dependent decline in LC50 values reflects bioaccumulation and sustained acetylcholinesterase (AChE) inhibition characteristic of organophosphate toxicity. Quantitative behavioural assessment revealed concentration-dependent and time-dependent patterns of neurotoxic impairment at concentrations overlapping with those reported in agricultural runoff (0.01–5.0 ppm). These findings emphasise the importance of extended bioassay protocols incorporating behavioural endpoints for ecologically realistic risk evaluation of monocrotophos in aquatic environments. Future studies incorporating AChE activity measurements and histopathological analysis of target tissues would further elucidate the mechanistic basis of the observed neurotoxic effects.