Abstract
The toxicity of varying concentrations of T. alnifolia extract on P. geometra (leeches) was investigated. Aqueous crude extract of roots and stem bark of T. alnifolia plant was obtained and concentrations of 5%, 10%, 15% and 20% of extract were made. Twenty-five juvenile C. gariepinus fish were distributed into five tanks of A-control (0%), B (5%), C (10%), D (15%) and E (20%) of herb extract with two replicates all held in static renewal bioassays. Twenty leeches were introduced into each tank and six hourly observations show that leeches were negatively affected by the herb extract. The extract elicited reduction in swimming activity, caused weakness, paleness and death of the leeches. The concentration- response relationship of P. geometra and T. alnifolia extract shows a mortality of 25%, 40%, 70% and 80% and a median lethal time (LT50) of 42h, 30h, 18h and 12h for 5%, 10%, 15% and 20% at 24h exposure respectively. The resulting sigmoid curve had an arithmetic median lethal (LC50) value of 113mg/l and a logarithmic median lethal (LC50) value of 1.72mg/l. Probit mortality of 4.33, 4.75, 5.52 and 5.84 were observed for 5%, 10%, 15% and 2% concentrations. The herb extract did not have any noticeable effect on the fish. Dissolved oxygen was significantly (P<0.05) higher in control than in treatment tanks. The herb extract had a significant effect (P<0.05) on the rate of mortality of P. geometra at 24h exposure time indicating that T. alnifolia extract may be effective in the control of leeches in fish culture.
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