Nobuhisa Koide

Changes in seawater adaptability and blood thyroxine concentrations during smoltification of the masu salmon, Oncorhynchus masou, and the amago salmon, Oncorhynchus rhodurus

Abstract The development of hypo-osmoregulatory ability, assessed by monthly seawater-challenge tests and changes in serum thyroxine levels, was studied during smoltification of hatchery-reared masu, Oncorhynchus masou , and amago salmon, Oncorhynchus rhodurus . In masu salmon, the best hypo-osmoregulatory performance was obtained from smolts collected in late May, and coincided with the peak of smoltification assessed by external appearance. Analysis of serum samples from March to August in 1981 showed a distinct surge in serum thyroxine levels; the highest value was obtained in fish undergoing smoltification collected in early May, and it coincided with the new moon. In amago salmon, the maximum development of seawater adaptability occurred in smolts collected during December and January, and this coincided with elevated plasma thyroxine levels. Highest levels were observed in smolts collected in late November, coincident with the new moon. Thus, the elevated blood thyroxine levels observed during smoltification in these two species of salmonids, which smoltify during different periods of the year, may indicate that this hormone plays an important role in the process of parr—smolt transformation. In addition, the peak of blood thyroxine levels, coincident with the new moon in both masu and amago salmon, suggests that the lunar phasing phenomenon is a generalized characteristic of Oncorhynchus species.

Stimulatory Effects of Insulin-Like Growth Factor 1 on Expression of Gonadotropin Subunit Genes and Release of Follicle-Stimulating Hormone and Luteinizing Hormone in Masu Salmon Pituitary Cells Early in Gametogenesis

Abstract Insulin-like growth factor-I (IGF-I) has been shown to be involved in pubertal activation of gonadot-ropin (GTH) secretion. The aim of this study was to determine if IGF-I directly stimulates synthesis and release of GTH at an early stage of gametogenesis. The effects of IGF-I on expression of genes encoding glycoprotein α (GPα), follicle-stimulating hormone (FSH) β, and luteinizing hormone (LH) β subunits and release of FSH and LH were examined using primary pituitary cells of masu salmon at three reproductive stages: early gametogenesis, maturing stage, and spawning. IGF-I alone or IGF-I + salmon GnRH (sGnRH) were added to the primary pituitary cell cultures. Amounts of GPα, FSHβ, and LHβ mRNAs were determined by real-time PCR. Plasma and medium levels of FSH and LH were determined by RIA. In males, IGF-I increased the amounts of all three subunit mRNAs early in gametogenesis in a dose-dependent manner, but not in the later stages. In females, IGF-I stimulated release of FSH and LH early in gametogenesis, whereas no stimulatory effects on the subunit mRNA levels were observed at any stage. IGF-I + sGnRH stimulated release of FSH and LH at all stages in both sexes, but had different effects on the subunit mRNA levels depending on subunit and stage. The present results suggest that IGF-I itself directly stimulates synthesis and release of GTH early in gametogenesis in masu salmon, possibly acting as a metabolic signal that triggers the onset of puberty.

Estimation of heritability and genetic correlation of number of abdominal and caudal vertebrae in masu salmon

Ten sibship masu salmon Oncorhynchus masou masou families were produced to estimate the heritability of the number of abdominal vertebrae (AV) and caudal vertebrae (CV), and the genetic correlation between the two traits were calculated. The AV and CV of both the parents and their offspring were counted separately, and the heritability was estimated by performing parent-offspring regression analysis. The mean vertebral number of the offspring differed among families although they were reared under identical water temperature conditions and had the same parental vertebral number type in crossbreeding. The heritabilities were estimated as 0.65 for AV and 0.84 for CV. The genetic correlation between the number of AV and CV was calculated as −0.92. These results suggest that AV and CV are governed by genetic components even when the fish are reared under identical environmental conditions, and the total vertebral number is expected to converge within a certain range. It is necessary to analyze separately the variations in the number of AV and CV when investigating the vertebral number in masu salmon.

Cryopreservation diluents for spermatozoa of Sakhalin taimen Hucho perryi

To develop a suitable cryopreservation diluent for spermatozoa of the endangered Sakhalin taimen Hucho perryi, all possible combinations of cryoprotectants (glycerol, dimethyl sulfoxide [DMSO], methanol) and extenders (fetal bovine serum [FBS], 300 mM glucose solution [GS], artificial seminal plasma for masu salmon) were examined by observing sperm motility 10 s after thawing. Spermatozoa cryopreserved with diluents such as mixtures of 10% glycerol plus 90% FBS, 10% DMSO plus 90% FBS, and 10% methanol plus 90% GS showed the highest motility. The maximal post-thaw motility was observed at 10% among all concentrations (0, 5, 10, 15 and 20%) of these three cryoprotectants. No significant difference among three diluents was observed in motility at 10 s. Mixtures of 10% glycerol plus 90% FBS, 10% DMSO plus 90% FBS, and 10% methanol plus 90% GS are suitable cryopreservation diluents for Sakhalin taimen spermatozoa.

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

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 treatments 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.

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

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.