Red‐spotted masu trout ( Oncorhynchus masou ishikawae ; hereinafter, referred as amago trout), is a freshwater salmonid fish species endemic to Japan. From September to October 2023, chronic mortality was observed on an amago trout farm in eastern Japan. To identify the cause of death, bacterial isolation, predominant bacterial species identification, histopathology and experimental challenge test were performed. As a result,...