An Interim Report on the Etiology of So-Called Dropsy of Juvenile Japanese Char, Salvelinus leucomaenis

Abstract

An outbreak of so-called dropsy occurred among juvenile Japanese char, Salvelinus leucomaenis, in the hatchery of Okutama Branch, the Tokyo Metropolitan Fisheries Experiment Station in April 1990. More than 90% of 20, 000 juvenile char showed dropsy, and numerous bacterial colonies were observed on the gill surface of the fish under microscope. Examination of Bouin or formalinefixed samples revealed that such bacterial colonies were also associated with outbreaks of dropsy in juvenile char in other places in April 1988 and February 1990. We found that 1% salt water bath for 1 hour is effective to eliminate the bacteria from the gills of diseased fish, and a continuous treatment for 8 days with 1 % salt water bath is effective to cure the dropsy. These results suggest that the bacteria observed on the gills might be the causative agent of the dropsy of juvenile Japanese char, but all attempts of infection trials failed to substantiate it. Further works concerning the relationships between bacterial attachment and dropsy will be required.