Outbreaks of bacterial cold-water disease (BCWD), caused by Flavobacterium psychrophilum, are widespread in Japan, especially among ayu Plecoglossus altivelis. There are few investigations of F. psychrophilum in river water, and its seasonal distribution has not been clarified. We aimed to identify the spatiotemporal dynamics of F. psychrophilum and ayu to provide information that is useful for establishing a countermeasure for BCWD. Quantitative analysis of environmental DNA (eDNA) was used to clarify the year-round dynamics of ayu and F. psychrophilum. We sampled river water from the Nagara and Ibi rivers in Japan, and conducted monthly water sampling and eDNA quantification. Changes in the eDNA concentration of ayu were consistent with the known life histories of the fish. There was a strong negative correlation between the eDNA concentration of F. psychrophilum and water temperature, suggesting a strong dependence of F. psychrophilum dynamics in the river on water temperature. Furthermore, relatively high eDNA concentrations were recorded for both organisms in early summer and fall, suggesting that ayu is infected with F. psychrophilum during these seasons when experiencing up- and downmigration, respectively.

在日本，由嗜冷黄杆菌引起的细菌性冷水疾病（BCWD）暴发十分普遍，特别是在Ayu Plecoglossus altivelis中。在河水中对嗜热镰刀菌的研究很少，其季节性分布尚不清楚。我们旨在鉴定F. psychrophilum和ayu的时空动态，以提供有助于建立针对BCWD的对策的信息。使用环境DNA（eDNA）的定量分析来阐明香鱼和精神嗜热菌的全年动态。我们从日本的Nagara和Ibi河中抽取了河水，并每月进行水采样和eDNA定量。香鱼eDNA浓度的变化与该鱼类的已知生活史一致。F. Psychrophilum的eDNA浓度与水温之间存在很强的负相关性，表明河流中F. Psychrophilum的动力学对水温的依赖性很大。此外，在夏初和秋季，两种生物均记录到相对较高的eDNA浓度，这表明在这些季节中，当ayu分别经历向上和向下迁移时，它们感染了F. psychrophilum。