Infection Status of Ayu and Other Wild Fish with Flavobacterium psychrophilum and Edwardsiella ictaluri in the Tama River, Japan

抄録

The Tama River is an urban river flowing throughout the Tokyo Metropolis. The water quality has been improved and a large number of wild ayu Plecoglossus altivelis migrate upstream in recent years. We conducted an epidemiological survey on two fish pathogens, Flavobacterium psychrophilum and Edwardsiella ictaluri, among ayu and other wild fish species based on PCR and culture methods at four sites (lower-, middle-, upper-reaches and tributary) in the Tama River basin from May 2011 to November 2012. F. psychrophilum was detected in 15 fish species including ayu throughout the survey period, and the occurrence of F. psychrophilum infection (bacterial cold-water disease) among ayu was observed in upper-reaches and tributary areas between mid- and late June of both years. On the other hand, E. ictaluri was detected in 7 fish species mainly in summer and late fall of both years, and E. ictaluri infection of ayu with mass mortality occurred in the tributary area in late August 2012 only. It is speculated that the unusually high water temperature during August 2012 (approximately 6°C higher than that of 2011) served as a predisposing factor in this mortality. Disease occurrence in riverine ayu might be attributable to the subclinical infection status of fish and stress conditions imposed by the environment.