Asian Journal of Research in Zoology

Behavioural responses can provide sensitive indicators of contaminant stress in fish, including exposure to organophosphate pesticides. This laboratory study characterised the time course of behavioural responses in the freshwater air-breathing fish Channa punctatus exposed to nominal concentrations of Rogor (30% EC dimethoate) corresponding to fractions of a previously reported 96-h LC₅₀ of 0.74 mL/20 L. Fish were assigned to four groups: control (A), 1/5 LC₅₀ (0.148 mL/20 L; B), 1/3 LC₅₀ (0.247 mL/20 L; C), and 1/2 LC₅₀ (0.370 mL/20 L; D), with three replicate aquaria per group and 10 fish per aquarium. Opercular movement, surfacing frequency, feeding response, qualitative locomotor abnormalities, and stress-related behaviours were recorded at 48, 72, and 96 h and at 10, 20, 30, and 40 days. The reported observations showed relatively transient changes in Group B and more persistent respiratory, feeding, locomotor, and stress-related abnormalities in Groups C and D. No mortality occurred in Groups A or B; two deaths occurred in Group C and four in Group D during the prolonged exposure. Interpretation of treatment effects is necessarily cautious because exposure concentrations were nominal rather than analytically verified and the recorded mean water temperature was higher in the control group than in the treated groups. Within these limitations, the findings indicate that prolonged exposure to LC₅₀-derived fractions of Rogor was associated with progressively greater behavioural disturbance and mortality at the higher nominal concentrations. Long-duration behavioural observations may therefore complement conventional acute toxicity endpoints when supported by rigorous exposure verification and experimental-unit-aware analysis.