Yoshitaka Sasaki

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.

Migratory timing of masu salmon and Dolly Varden smolts exiting the Uebetsu River near the Shiretoko World Heritage Site, Hokkaido, Japan, and potential angling effects

Numbers of masu salmon and Dolly Varden in streams in the Shiretoko Peninsula, location of the Shiretoko World Heritage Site in north eastern Hokkaido, appear to be declining. One concern is whether masu salmon smolts continue to migrate during July, after the existing May to June angling closure established to protect migrating smolts in eastern Hokkaido. Smolt timing and relative abundances were monitored in the Uebetsu River near the World Heritage Site from May to July during 2005–2007. Each year some masu salmon smolts emigrated during July, especially in 2005 when 82% of smolts captured were caught that month. In contrast, most anadromous Dolly Varden smolts emigrated prior to early June when stream temperatures <8°C. Mean fork lengths of smolts in both species decreased during the runs. The current fishing closure period (May to June) may be insufficient to protect masu salmon smolts in this unique area from angling.

Estimation of heritability of vertebral number in chum salmon Oncorhynchus keta

Hereditary causes of variation in the vertebral number of chum salmon were analyzed to estimate heritability for populations in the Chitose and Shikiu Rivers, Hokkaido Island, northern Japan. A total of 70 families were produced by diallel cross mating and then heritability was estimated using two statistical tests: a two-way ANOVA and a parent-offspring regression. Heritability estimates ranged from 0.26 to 1.91 and showed a strong correlation between the values from the two different tests. Estimates from female components were substantially higher than those from male components, which can be attributed to the maternal effect. Heritability in the Shikiu River population was shown to be higher than that in the Chitose River population. These results indicate that the heritability of vertebral number in chum salmon is generally high, and also that vertebral number is controlled by hereditary causes.