Epizootiology of the ectoparasitic protozoans Ichthyobodo salmonis and Trichodina truttae on wild chum salmon Oncorhynchus keta

Abstract

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 ...

Effects of dietary supplementation with oregano essential oil on prevention of the ectoparasitic protozoans Ichthyobodo salmonis and Trichodina truttae in juvenile chum salmon Oncorhynchus keta

“…For artificial transmission from infected to uninfected animals by means of cohabitation, it is generally considered that all groups are not subjected to the same pathogenic pressure, because the infected animals do not all have the same infection intensity; this condition may therefore influence the accuracy of the results. Even so, previous studies on artificial transmission of I. salmonis and T. truttae in O. keta juveniles suggested that transmission through cohabitation of ≥20 infected fish and ≥500 uninfected fish could ensure relatively consistent experimental results (Mizuno et al ., , , ).…”

“…These considerations suggest that the oregano-supplemented juveniles released from the hatchery could reach the ocean without heavy mortalities due to T. truttae infection. In addition, the oregano-supplemented juveniles would survive in the ocean, regardless of an episode of T. truttae infection in the river, since the protozoan cannot survive in seawater (Khan, 1991;Mizuno et al, 2017b). Consequently, T. truttae infection among oregano-supplemented juveniles (such as in experiment 2) would not pose a serious problem for O. keta stock enhancement programmes.…”

“…The schedule for rearing and sampling the juveniles is shown in Figure 1. One control group was used as the positive control exposed to both I. salmonis and T. truttae and the other as the negative control that was exposed to either para- (Mizuno et al, 2016(Mizuno et al, , 2017a(Mizuno et al, , 2017b…”

Experimental evidence on prevention of infection by the ectoparasitic protozoans Ichthyobodo salmonis and Trichodina truttae in juvenile chum salmon using ultraviolet disinfection of rearing water

“…All sampled fish were examined for infection with either I. salmonis or T. truttae in accordance with a previously established method (Mizuno, Urawa, Miyamoto, Saneyoshi et al, 2017). In brief, to detach ectoparasitic protozoans from the skin of the fish, juveniles were individually anaesthetized in 1 g/L unbuffered tricaine methanesulfonate (MS-222; Callahan & Noga, 2002).…”

“…In Hokkaido salmon hatcheries, the infection sources of I. salmonis and T. truttae are considered to be wild chum salmon and various other wild salmonids, respectively (Mizuno, Urawa, Miyamoto, Saneyoshi et al., ). Once separated from a host fish, each of these parasites can easily invade hatchery ponds via the river water used for aquaculture.…”