| Summary: | In the previous paper, we reported that Aeromonas salmonicida was isolated from mature chum salmon (Oncorhynchus keta), pink salmon (O. gorbuscha) and masu salmon (O. masou) without signs or furunculosis. In the present study, we determined the prevalence of A. salmonicida in immature chum salmon without signs or furunculosis captured in rivers and on the coast of Hokkaido. A. salmonicida was not isolated from 480 immature chum salmon were examined from September 1919 to October 1990. We compared the prevalence of A. salmonicida in the kidney of immature and mature chum salmon which ascended the Shibetsu, Nishibetsu and Ishikari rivers. The prevalence of A. salmonicida in the immature fish was lower than that in the mature fish. A. salmonicida was neither detected from the kidneys of chum and masu salmon fry, nor isolated from the surface of chum salmon eggs in the hatcheries. From these results, we considered that these salmonids were not infected with A. salmonicida until they entered the rivers.
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