Environmental factors affecting Edwardsiella ictaluri‐induced mortality of riverine ayu, Plecoglossus altivelis (Temminck & Schlegel)

We analysed the predisposing factors for Edwardsiella ictaluri infection in the riverine ayu Plecoglossus altivelis on the basis of environmental and epidemiological data obtained in a tributary to and the lower reaches of the Tama River, Japan, in July and August 2011–2015. Mortality of ayu due to E. ictaluri infection was observed only in the tributary in August 2012 and 2013; both periods were unusually hot. During these mortality events, daily average water temperatures rose approximately 3–4°C over 4–8 days, reaching the optimum temperature for E. ictaluri infection (>25°C) and approaching the upper tolerable limit for ayu (30°C). Diurnal water temperature ranges (DWTRs) in the tributary during the mortality events exceeded 6°C, which was 1–2°C greater than in the lower reaches. Experimental infection of ayu with E. ictaluri resulted in higher mortality when exposed to 6°C DWTR than to 4°C DWTR. Furthermore, water levels in the tributary were generally low in August 2012 and 2013 because of low rainfall. From these results, we conclude that unusually high‐water temperatures combined with high DWTRs and low water levels drove riverine ayu mortality from E. ictaluri infection.