In northern Japan, juvenile chum salmon Oncorhynchus keta (Walbaum) are released from hatcheries to enhance the fishery resource. Infections with ectoparasitic protozoans, particularly the flagellate Ichthyobodo salmonis and the ciliate Trichodina truttae , occasionally cause severe mortality among hatchery‐reared juveniles. This study examined the susceptibility of the two parasites to wide‐ranging UV irradiation (experiment...

The present study performed three experiments to establish a practical prevention strategy for the ectoparasitic flagellate Ichthyobodo salmonis and ciliate Trichodina truttae in hatchery‐reared juvenile chum salmon Oncorhynchus keta using dietary supplementation with oregano essential oil. Experiment 1 showed that a diet supplemented for 3 weeks with 0.02% oregano essential oil significantly prevented infection with I. salmon...