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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. salmonis and T. truttae in juveniles reared in small tanks. Experiment 2, in outdoor hatchery ponds, demonstrated that the oregano treatment completely prevented I. salmonis infection for 52 days and T. truttae infection for 38 days. Oregano-treated juvenile mortality attributable to infection with these protozoans also decreased to 7.6% of control juvenile mortality, confirming the utility of this treatment in cultured O. keta. Physiological analyses of the oregano-treated juveniles elucidated the treatment's safety in relation to their metabolism, osmoregulation, natural immunity and olfactory responses and also detected carvacrol (a major component of oregano essential oil which shows antimicrobial activity) on the skin. In experiment 3, exposure of the two protozoans to oregano essential oil revealed a weak antiparasitic action on the body surface of the juvenile O. keta. The overall results demonstrate that dietary oregano supplementation is a practical prevention strategy for I. salmonis and T. truttae in hatchery-reared juvenile O. keta and suggest the possibility that its anti-parasitic action is attributable to a component of the oil that emerges onto the skin of the body of the fish.

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Infestations of the ectoparasitic flagellate Ichthyobodo salmonis and the ciliate Trichodina truttae have caused acute mortalities of hatchery-reared juvenile chum salmon Oncorhynchus keta in Hokkaido, northern Japan. This study examined the epizootiology of I. salmonis and T. truttae on wild chum salmon as a possible infection source of the 2 parasitic protozoans in hatcheries. Infestations by both ectoparasites were detected on freshwater-adapted adult and juvenile chum salmon in all 4 rivers examined. This is the first study of an anadromous Pacific salmonid to report infestation of I. salmonis and T. truttae in adults returning for spawning. Among the marine-inhabiting phase of chum salmon, infestation with I. salmonis, but not T. truttae, was observed on adults and juveniles. The 2 protozoans were experimentally transmitted at the same time from wild to hatchery-reared chum salmon juveniles, and caused a high rate of mortality in the hatchery fish. In freshwater, the proliferation rate of T. truttae was greater than that of I. salmonis. These observations show that the euryhaline ectoparasite I. salmonis can infest chum salmon throughout their life cycle, in both river and ocean habitats, whereas T. truttae is able to infest these salmonids only in freshwater. Furthermore, wild chum salmon were shown to be a potential infestation source for both T. truttae and I. salmonis in hatchery fish. KEY WORDS: Hatchery fish · Ichthyobodosis · Infestation experiment · Pacific salmon · Trichodinosis Resale or republication not permitted without written consent of the publisherDis Aquat Org 126: [99][100][101][102][103][104][105][106][107][108][109] 2017 ever, to interrupt the infestation route as a preventive measure, it is essential to identify the infestation sources of I. salmonis and T. truttae. In Hokkaido, T. truttae infests a variety of wild salmonids, including adult chum salmon, masu salmon O. masou, rainbow trout O. mykiss, white-spotted char Salvelinus leucomaenis, and dolly varden S. malma. Moreover, hatchery-reared chum salmon juveniles have been experimentally infested with T. truttae from wild masu salmon (Mizuno et al. 2016). In comparison, I. salmonis can infest chum salmon, but apparently not masu salmon, from which Ichthyobodo parasites have been isolated and described as a new species (Urawa et al. 2014). Additionally, the source of I. salmonis in hatcheries remains unclear since infestations have occurred in ponds supplied with unsterilized spring or well water, in which residual wild fish were absent (Urawa 1992c). In northern Japan, adult chum salmon mi grate upstream for spawning from September to January, while juveniles migrate from the river to the sea from March to May (Salo 1991). Adults used for artificial fertilization are stocked in hatchery ponds until ovulation or spermiation, from September to December, and the fertilized eggs, hatched larvae, fry, and juveniles are cultured in hatcheries from September until the next May (Kobayashi 1980). Therefore, either or both adult ...

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 1) and then investigated whether UV disinfection of the rearing water using a commercial device was useful for preventing infections among juveniles in a small‐scale rearing system over a 28‐day period (experiment 2). In experiment 1, parasite mortality reached 100% with UV irradiation doses of ≥9.60 × 105 μW s/cm2 for I. salmonis and ≥8.40 × 105 μW s/cm2 for T. truttae. In experiment 2, disinfection of the rearing water at a UV irradiation dose of 2.2 × 106 μW s/cm2 succeeded in complete prevention of both parasites in the juvenile salmon. These results elucidate the minimum dose of UV irradiation for inactivation of I. salmonis and T. truttae, and demonstrate the usefulness of water disinfection using a commercial UV irradiation device to prevent infections by these parasites in hatchery‐reared juvenile chum salmon.

A previous study detected the major soluble antigen (msa) gene and its corresponding mRNA from Renibacterium salmoninarum (R.s.) in the kidney tissue and surface mucus of asymptomatic chum salmon fry and juveniles by nested PCR (nPCR), quantitative realtime PCR (qPCR) and a reverse transcription nested PCR assay (RT-nPCR) (Suzuki et al., 2017a). The chum salmon had been transferred from a hatchery in Hokkaido at the fry-stage (0.5 g) and reared at a wetlaboratory in the Salmon and Freshwater Fisheries Research Institute in a fresh water environment for 10 months (Suzuki et al., 2017a). In January 2017, the same group of asymptomatic chum salmon juveniles and another group of asymptomatic chum salmon juveniles, which were transferred at the fertilized-egg stage from another hatchery in Hokkaido to the wet laboratory and reared until juvenile, began to die. Most of the dead fish showed the clinical signs of bacterial kidney disease (BKD). The purpose of the present study was to examine the outbreaks of BKD in chum salmon juveniles subclinically infected with R.s. Materials and Methods Fish Two groups of asymptomatic chum salmon (hereafter referred to as Group-A and Group-B) were examined. Group-A fish were the progeny of returning chum salmon female adults in 2015 at Site A (Suzuki et al., 2017a, 2017b) and transferred from Site A at the fry-stage weighing 0.5 g in March 25, 2016, and reared until juveniles at the wet laboratory in the Salmon and Freshwater Fisheries Research Institute (Suzuki et al., 2017a). BKD had not previously been reported at this institute. The fish were confirmed by nPCR, qPCR and RT-nPCR for the presence of the msa gene and msa mRNA to indicate subclinical infection with R.s. seven times (Suzuki et al., 2017a). Group-B fish were transferred from another hatchery in Hokkaido at the fertilized-egg stage in October 8, 2015, to the wet laboratory and reared until juvenile. Group-B fish were confirmed by nPCR and qPCR to be subclinically infected with R.s. twice.

We investigated the seasonal distribution and migration patterns of hatchery‐reared masu salmon Oncorhynchus masou in the area surrounding Hokkaido, the northern island of Japan. From 1993 to 1999, we tagged and released 313,000 yearling masu salmon smolts into the coastal waters of the Sea of Japan adjacent to southwestern Hokkaido. From May of the release year through July of the following year, a total of 1,693 fish were recaptured in the coastal waters of northern Japan. Just after their release, masu salmon juveniles had two migration routes; one group moved northward along the western side of Hokkaido in the Sea of Japan, while the second group passed through the Tsugaru Strait and then moved eastward into the Pacific Ocean. It was estimated that both groups reached the Sea of Okhotsk at 30–40 d postrelease. The summer distribution of tagged fish was unknown because none were recaptured. Young fish appeared in the Sea of Okhotsk again in autumn and then migrated southward, retracing the above two routes. They reached the Tsugaru Strait and Pacific coastal waters ranging from the northeastern Shimokita Peninsula in Honshu to the Iburi district of Hokkaido. Tagged masu salmon juveniles were recaptured in these areas until the following March, which indicates that these sites were the most important overwintering areas. Adult fish rapidly returned to the area of release from April to June. Our results suggest that hatchery masu salmon migrated in the coastal waters of northern Japan within a relatively short period.

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