Open-jaw syndrome in chinook salmon (Oncorhynchus tshawytscha) at a hatchery

Nearly 0.5% of the yearling spring chinook salmon (Oncorhynchus tshawytscha) at a national fish hatchery were observed with mouth agape, the condition occurring in two of 16 ponds. X-radiographs and histological preparations indicated that the articular bone of the lower jaw was malformed and dislocated dorsal and posterior to its normal point of attachment. The bone appeared to be embedded in the mandibular muscle and surrounded by an extensive fibrous tissue network. Genetic aberration, environmental interaction, and teratogenic substances are discussed as possible causes of the anomaly.