Abstract
Overwintering ayu Plecoglossus altivelis (n = 23) were captured in Sho-river, Toyama Prefecture in April, 2007, and kept under a crowded condition (120 g fish/L). When the fish became moribund or dead, bacteria were isolated from the kidney of the fish using a modified Cytophaga agar plate, and the isolates were examined by the PCR-RFLP method using primers targeting gyrA, gyrB and PPIC genes of Flavobacterium psychrophilum. The isolates obtained from nine individuals of the fish were identified as F. psychrophilum and classified into two genotypes of A/R/QR and A/S/QR. An isolate from each genotype was virulent to ayu. These results showed that overwintering ayu carry virulent F. psychrophilum, suggesting a source of infection for newcomers of ayu in this spring.
