An investigation on the distribution of Flavobacterium psychrophilum in the Umikawa river

Abstract

An investigation on the distribution of Flavobacterium psychrophilum was conducted at the Umikawa River in Niigata Prefecture in 1999. Wild ayu (Plecoglossus altivelis), released ayu and other feral fishes (Oncorhynchus masou, Trybolodon hakonensis, Cottus kazika, Chaenogobius urotaenia, Rhinogobius brunneus, Tridentiger obscurus) were caught every month from May to December. In November newly-hatched ayu were caught in the river. In addition to fish, we collected water samples and small algae growing on the surface of stones on the river bed, which ayu lived on. By nested PCR and/or IFAT, the bacterium was detected in the gills and/or kidney tissue samples of both wild and released ayu as well as of four (Oncorhynchus masou, Trybolodon hakonensis, Chaenogobius urotaenia, Rhinogobius brunneus) of the six other fish species. The prevalence of the carrier fish was generally high in June and October. The eggs taken from wild mature ayu and the larvae collected in the river were revealed also to be carriers. Algae collected in May, November and December, and a water sample in December were found to be positive for F. psychrophilum by PCR. These findings suggest that non-ayu fishes as well as released ayu may serve as sources of infection for wild young ayu which migrate from sea to the river. It is also suggested that the coldwater disease may be transmitted from spawners to larvae.