Parasiticidal treatment against the fish-pathogenic protozoan Trichodina sp., an ectoparasite on the body surfaces of cultured barfin flounder, Verasper moseri, 2: Parasiticidal effects of fresh water on Trichodina sp. and the tolerance of juvenile barfin flounder to freshwater exposure

Short-term incubation of Trichodina-harbouring epidermal mucous secretions from barfin flounder in fresh water for over 30 minutes was previously shown to be effective for killing of this protozoan ectoparasite. In the present study, the in situ parasiticidal effects of freshwater baths on Trichodina sp. were further investigated. Infected barfin flounder were bathed in fresh water for different durations, i.e., 0 (control), 15 and 30 min, and the parasite density on the body surface was monitored until 28 days after bathing. In groups exposed to freshwater for 15 min and over, parasite densities decreased to near-zero the following day, whereas high parasite loads were seen in control groups throughout the experimental term. In the group bathed in fresh water for only 15 min, the parasite density increased gradually from 14 days post-exposure. The density eventually reached a very similar value to that observed before bathing, because a few parasites could survive the freshwater bathing regime. In contrast, in the group bathed in fresh water for 30 min, the parasite density remained at 0% even by 28 days after exposure, and re-establishment of Trichodina sp. never occurred. These results show that freshwater exposure should last for at least 30 minutes to ensure that all Trichodina sp. on the body surface are killed, which coincides with previous culture experiments. Such treatment was further shown to not affect the survival of juvenile barfin flounder when experimentally reared in fresh water for 6 hours, although mortality linearly increased beyond that duration. These results thus prove that freshwater baths for 30 min of duration, sufficient to eliminate on Trichodina sp., is not harmful to barfin flounder. Therefore, the use of freshwater baths for 30 min is an effective and clean method for preventing cultured barfin flounder from Trichodina sp. infection.