Since the late 20th century, the biomass of Pacific salmon Oncorhynchus spp. has increased. Hokkaido, northern Japan, is one of the main areas of chum salmon O. keta production in the North Pacific and intensive hatchery programs support the recent high abundance. However, proper management of naturally spawning populations is necessary to conserve healthy stocks of this species. In 2008, we started a program to assess the naturally spawning chum salmon populations in Hokkaido. Of the total of approximately 1,500 rivers in Hokkaido, 238 rivers with lengths of longer than 8 km (excluding those rivers used for hatchery broodstock collection) were surveyed in 2008 and 2009. The number of nonenhanced rivers found to contain naturally reproducing chum salmon was 59 (31.4% of surveyed rivers) and 50 (37.6% of surveyed rivers) rivers in 2008 and 2009, respectively. Including the rivers where hatchery broodstock were collected and rivers shorter than 8 km that contain naturally spawning chum salmon, chum salmon ascended at least 191 and 175 rivers in Hokkaido in 2008 and 2009, respectively. Repeated foot surveys indicated that the run timings of naturally spawning chum salmon may be affected by coastal commercial fisheries. This study showed that naturally spawning chum salmon remain in many rivers in Hokkaido where hatchery programs have been intensively conducted.

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This book that contains 38 research articles on marine ecology of Pacific salmon and steelhead trout, is dedicated to the International Year of the Salmon and is the first scientific publication that contributes to the library of the project.

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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 ...