An experimental elvaluation of the use of formalin to control trichodiniasis and other ectoparasitic protozoans on fry of Cyprinus carpio L. and Oreochromis mossambicus (Peters)

Mortalities amongst Cyprinus carpio L. and Oreochromis mossambicus (Peters) fry occurred at the Lowveld Fisheries Research Station, Transvaal, South Africa, owing to severe infestations by Trichodina spp., Chilodonella hexasticha Kiernik, 1909 and Ichthyophthirius multifiliis Fouquet, 1876. The protozoans involved were specifically identified, the levels of infestations studied and relative species composition determined. Under laboratory conditions, the infested fry were exposed to different formalin concentrations ranging from 15 to 110 mg/l. The activity of trichodinids was used as a measure for determining the effect of the formalin concentration. It was found that 25 mg/l formalin effected complete eradication of trichodinids on C. carpio, whereas 45 mg/l was required to effect eradication on O. mossambicus fry within 24 h. Fry survived concentrations of formalin as high as 100 mg/l for three days. It is suggested that the prescribed dosage to eradicate trichodiniasis on fry of these two species should be between 45 and 50 mg/l administered directly to the fish ponds.