Toxicity of aqueous extract of white hoary pea, Tephrosia candida (Papilionoideae) on Nile tilapia, Oreochromis niloticus (Cichlidae) fingerlings

Abstract

Fish poisoning using Tephrosia candida, which is an exotic plant to Sri Lanka is taking place in streams in the boundary of the Sinharaja forest, a tropical forest range, designated as a world heritage site by UNESCO in 1988. T. candida is a source of flavonoids and rotenoids including rotenone, tephrosin, and deguelin. Fishermen add large amounts of grounded plant matter to kill almost all the fishes in the stream within a short period of time. This method of unregulated fishing may have a long term negative effect on fish diversity and abundance in the area. A 96 h static renewal toxicity bioassay was carried out in the laboratory to determine the median lethal concentration (LC50) of aqueous extract of T. candida leaves on Oreochromis niloticus fingerlings. Experimental fish were exposed to test water in 20 L glass aquaria with concentrations of plant extract of 5, 7.5, 10, 15 and 20 mg L-1. All five treatments aquaria and the control aquaria without plant extract were triplicated. Fish exposed to plant extract showed symptoms of toxicity including, initial inactivation, agitated swimming, turning movement, air gulping, increased opercular movement followed by erratic swimming, loss of reflex, slow opercular movement, setting at the bottom motionless and knockdown before death. The gills of the dead fishes were damaged, swollen and external bleeding were observed. Lower concentrations of the extracts had sub lethal effects which manifested as zigzag movement, air gulping, increased opercular movement and some fish gathered near the air stones. The LC50 values at various exposure periods were 10.83 mg L-1 for 24 h; 8.61 mg L-1 for 48 h; 7.26 mg L-1 for 72 h and 6.43 mg L-1 for 96 h. It could be concluded that the application of T. candida extract causes lethal toxic effects on fish even at very low concentrations.