Abstract

Emmanuel K. Mbaru, Thomas K. Mkare and Joseph O. Rasowo

This study reports tolerance levels and lethal concentrations of yolk sac and free swimming fry of African catfish (Clarias gariepinus) to different therapeutic doses of formalin. Comparative treatments were up to 2000 ppm for 15, 30 and 60 min exposure times and up to 150 ppm for 24 h exposure time for the second experiment on lethal concentrations. In both fry stages, higher survivals were recorded at short exposure times and at lower concentrations. The LC50 value for the yolk sac and free swimming fry was 130 and 90 ppm with total mortalities at concentrations beyond 150 and 125 ppm respectively. Although other concentrations and exposure times gave best performance in terms of survival, we recommend 15 min bath dip at 500 ppm for the yolk sac fry and 15 min at 600 ppm for the free swimming fry for routine use by rural fish farmers to reduce fry mortalities.