Abstract

Oreochromis niloticus juveniles (mean body weight, 13.99±0.38 g and length, 8.32±0.93 cm) were exposed in a daily renewal bioassay to different concentrations of Lepidagathis alopecuroides (0.00, 1.0, 1.5, 2.0, 2.5 and 3.0 mg/l), for 96 h to assess its toxicity. The opercular (obf), tail beat (obf) frequencies and mortalities were monitored during the exposure period. The pattern of response of OBF/min and TBF/min decrease with time of exposure, but increased with increase in concentration of the toxicant. Opecurlar beat frequency (OBF) per minute was more variable from the 24^th h, whereas TBF were less variable throughout the duration of the exposure. The cumulative mortality of the fish increased with time and concentrations of the toxicants. Less than 50% mortality was recorded for 1.0 and 1.50 mg/l by the 96^th h, while the threshold concentrations that killed 100% of exposed fish were 3.0, 2.5 and 2.0 mg/l at the 48^th and 72^nd h, respectively. Wide variations were recorded in OBF and mortality at the various times of exposure and concentrations. The relationship between OBF, TBF and time of exposure was negative and significant at (p< 0.01) but positive with mortality and insignificant. The 24 and 96 hrLC₅₀ were 2.27 and 0.88 mg/l and their safe concentration were 0.23 and 0.09 mg/l, respectively. The mean lethal time decreased with increase in the concentration of L. alopecuroides. The results suggest that aqueous extracts of the leaves of L.alopecuroides has piscicidal property and are highly toxic to O. niloticus.Hence, care should be exercised in the use of the toxicant under culture conditions except for pond cleansing of unwanted fauna before stocking.

Key words: Opercular beat frequency, tail beat frequency, mortality, Lepidagathis alopecuroides and Oreochromis niloticus.