Acute Behavioural Responses and Mortality of Juvenile Clarias gariepinus Exposed to a Commercial Glufosinate-Ammonium Herbicide Formulation

Abstract

Agricultural herbicide formulations may affect non-target aquatic organisms, but toxicity responses vary with species and product composition. A 96-h laboratory exposure examined visible behavioural responses and mortality in 120 juvenile African catfish (Clarias gariepinus) allocated to an untreated control group and five nominal concentration groups (1, 2, 4, 8, and 16 mL of commercial glufosinate-ammonium formulation per litre of water), with two tanks of 10 fish per group. Behaviour and deaths were recorded at 24-h intervals. No deaths or abnormal behavioural signs were recorded in the control group. The exposed groups showed concentration-related changes in movement, equilibrium, air gulping and reflex responsiveness; visible skin-surface changes were noted only at the highest concentration. Cumulative 96-h mortality was 10%, 20%, 30%, 60%, and 100% at 1, 2, 4, 8, and 16 mL/L, respectively. Eighteen of the 20 fish in the 16-mL/L group died within the first 24 h, and all had died by 48 h. The observed 50% mortality response was bracketed by the 4- and 8-mL/L treatments; a model-based LC₅₀ was not established from the available tank-level information. The reported mean dissolved oxygen concentration was lower in exposed water than in control water. These observations identify short-term adverse outcomes associated with high nominal concentrations of the tested formulation, while the absence of measured exposure concentrations and tank-level water-quality records limit mechanistic and environmental extrapolation.