Acute Toxicity of Water Extracts of Barks of Yellow Oleander, Thevetia peruviana (Persoon), and Neem Plant, Azadirachta indica (Lodd) to Mormyrids, Hyperopisus bebe occidentalis (Gunther)

Abstract

Fingerling of mormyrids, Hyperopisus bebe occidentalis (Gunther), mean weight 13.02±0.54 g, were exposed in 45 × 25 × 20 cm3 glass aquaria to 0.00,0.05,0.25,0.50, and 1.00 mg/L concentrations of freeze-dried water extracts of barks of yellow oleander, Thevetia peruviana, and neem plant, Azadirachta indica, for 96 hours in a static bioassay test. The 96-hour LC50, opercular ventilation rates, and threshold values (maximum dose at which mortality does not occur) were determined. The 96-hour LC50 of the water extracts of bark of yellow oleander and the neem plant were 0.05 and 0.125 mg/L, respectively. Extracts of yellow oleander were observed to be more toxic than the extracts of the neem plant. Opercular ventilation increased markedly with exposure to both extracts, then declined as the fish became fatigued. The threshold value for the yellow oleander was observed to be lower, 0.047mg/L than that of the neem plant, 0.124 mg/L. Mortality is believed to be due to fatigue and direct toxic effects of the plant extracts on the fish tissues.