Abstract
In the process of artificial production of Ayu fry (Plecoglossus altivelis), considerable mortalities of the fry (aged one to two months) were frequently found. Each of these dead fish had a white girdle along the spinal cord, apparently showing the characteristic of the disease.
Fourty one of the diseased fish were histopathologically investigated by the present authors. All the diseased fish erroneously swallowed the diets through the pneumatic duct into the swim bladder, without transferring into the alimentary canal. In 39 fish, the lumen of the swim bladder showed hyperplasia or necrosis due to bacteria, which probably multiplied abruptly in the organ. In 2 fish, a great number of fungi were observed in the diet found in the swim bladder; some organs (kidney and body musculature) near the swim bladder were remarkably affected by the fungi.
From the present work, it is considered that the erroneous-swall-owing of diet causes the mortalities of Ayu fry.
