Takahashi Hiroshi

Viral edema of carp is caused by Carp edema virus and is a major carp disease listed on the WOAH list of emerging infectious diseases. However, an effective experimental infection method that can control virus copy number has not been established. In this study, we compared three experimental infection methods：immersion infection using gill suspensions from diseased fish (immersion method), infection by rearing water from diseased fish (rearing water method) and co-infection method. Cumulative mortality in the immersion groups ranged from 40 to 60%, while that in the rearing water infected and co-infected groups ranged from 80 to 100%. The rearing water method can be used to control the viral DNA copy number and to perform experimental infections when frozen storage is possible. Thus, we performed the same experimental infection using frozen rearing water and found no disease or mortality, indicating that stable experimental infection using the rearing water method is difficult. The immersion method has a lower mortality rate than the other two methods, and it requires a large amount of gill tissue because of the low number of virus DNA copies that can be obtained from the gill tissue. However, it is possible to control the number of virus DNA copies and to preserve the gill tissue suspension by freezing. Therefore, the immersion method was considered the most suitable for stable experimental infection under the same conditions,

Growth of immature Lates japonicus in eelgrass (Zostera japonica) beds of the Tomouchi River located at around 4.3 km upstream from the river mouth of the Gokase River, Kyushu Island, Japan was studied by the mark and recapture method and scale reading from September 1998 to March 2004. Newly occurring 0-year-old L. japonicus specimens of 13.9–78.0 mm SL (standard length) in Z. japonica beds of the Tomouchi River in August or September reached sizes ranging 58.8–125.7 mm SL by April and 203.4–232.8 mm SL between October and November of the following year. Monthly changes in specific growth rates of individuals demonstrated that rapid growth months of immature fish were limited to the period of high water temperature (22–28℃) from June to October. Also, it was supposed that the large-sized fish of the 0-year-old fish migrated away from Z. japonica beds after December, however a few fish remained in Z. japonica beds until the following November.

Viral edema of carp (VEC) caused by the carp edema virus (CEV) causes economic losses for Japanese koi farms. In this study, we investigated the infectivity and pathogenicity of a domestic CEV isolate (genogroup IIa) in koi carp, common carp and goldfish. The challenge test consisted of 9 groups (n = 15):3 groups each of koi carp, common carp, and goldfish, at 15, 20 and 25℃. These groups were challenged with CEV (3.0×10^3 copies/µL) in duplicate. All koi carp died in the 15 and 20℃ groups, but all survived in the 25℃ group. The surviving koi carp in 25℃ groups showed high PCR positive rates of 66.7 and 73.3%, with VEC histopathological changes observed. For the common carp, 1 and 2 fish died in the 20℃ groups, but no deaths or VEC symptoms were observed in the 15 and 25℃ groups. In all common carp groups, PCR-positive fish were observed along with histopathological changes. For all goldfish groups, no deaths or VEC symptoms were observed. As with the common carp, PCR-positive fish were found in all goldfish groups, yet no VEC histopathological changes were detected. These results demonstrate infectivity of this CEV strain in koi carp, common carp, and goldfish, but low pathogenicity in common carp and goldfish.

The Japanese lates, Lates japonicus is an endemic fish to the Pacific coast of southern Japan, and commonly found in estuaries and coastal waters of Miyazaki and Kochi Prefectures. The larvae, juveniles and young fish stay in the eelgrass, Zostera japonica beds in brackish water areas. Effects of water temperature and salinity on feeding and growth of immature L. japonicus were studied using combinations of seven levels of temperatures (16 to 28℃) and 6 different salinities (9 to 34 ppt) over two experimental periods each of 30 days. At each of these conditions, ten fish were reared in aquaria for periods of 30 days. Based on values of the daily feeding rates, specific growth rates and feeding efficiencies, we found that immature L. japonicus were euryhaline and at salinities ranged from 9 to 34 ppt showed similar feeding and growth rates. The suitable water temperature range was 22 to 28℃, and the optimal water temperature range was 26 to 28℃ for feeding and growth for immature L. japonicus.