A Survey of Flavobacterium psychrophilum Infection in Ayu Plecoglossus altivelis in the Kagami River

We examined the amphidoromous, land-locked, and released (farmed) ayu in the Kagami River for Flavobacterium psychrophilum infections using TaqMan real-time PCR targeting the PPIC gene. We collected 524 ayu in 2014-2015 at 4 stations in the lower reaches and 5 stations in the upper reaches of Kagami Dam. The prevalence of infection in apparently healthy individuals did not differ between amphidromous and released fish in the lower reaches or between land-locked and released fish in the upper reaches. However, most dead fish were released ayu. The most commonly isolated F. psychrophilum genotype from ayu was A/G-C type. Ayu carrying F. psychrophilum were found in mid-June 2014, before ayu fishing season started, at two stations in the upper reaches of the dam. F. psychrophilum with genotype A/G-T was detected in 2014-2015 at one of these two stations, suggesting that F. psychrophilum reservoirs may be present and part of the overwintering population that contributes to an increase in infection rates in the next year. It is possible that land-locked ayu larvae and juveniles are F. psychrophilum reservoirs, because the PPIC gene (over 105 copies/L) were detected in the river water collected at the station where eggs were laid in the 2015 spawning season.