Parasiticidal treatment against the fish-pathogenic protozoan Trichodina sp., an ectoparasite on the body surfaces of cultured barfin flounder, Verasper moseri, 1: Freshwater tolerance of Trichodina sp. as determined by a short-term incubation of epidermal mucous secretions from infected fish

The purpose of this study was to develop a new and clean protocol for the control of Trichodina sp. infections on cultured barfin flounder, which could replace the use of a formalin bath. Firstly, in order to examine the environmental tolerance of Trichodina sp. in detail, short-term incubation of Trichodina sp. was established. Epidermal mucous secretions harbouring parasites were collected from the body surface of infected barfin flounder, smeared on a slide glass, and immersed in filtered seawater. Incubations were done at 3 different temperatures (5 °C, 15°C and 20°C), each of which yielded a high rate of live parasites (42.5-83.3 %) after 8 hours, suggesting that Trichodina sp. could readily survive under these culture conditions. Secondly, using this culture system, the effects of immersion of parasites in fresh water or in 25% or 50% sea water on vitality of Trichodina sp. were examined. The rate of surviving parasites after incubation in fresh water decreased sharply and reached 0 % by 30 minutes of incubation; immersion in partial sea water did not cause a distinct reduction in vitality of parasites. Indeed, successive observations on the morphology and vitality of parasites immersed in fresh water indicated that immersion for at least 30 minutes was essential for inducing the parasite to burst and die. Accordingly, it is suggested that a freshwater bath of infected barfin flounder for 30 minutes may prove very suitable as a new parasiticidal method against Trichodina sp. infection in an aquaculture environment.