76 American fisheries Society. 



and, where the numbers of sick fish were large, the appearance 

 of the excrement suggested that the seat of the trouble was in 

 the intestines rather than in the gills. This proved to be the 

 correct clue. On further search it was found that the fish in 

 the entire hatchery were quite generally infected with the in- 

 testinal protozoan, Octomitus. The sick and dying fish were so 

 heavily parasitized as to leave no room for doubt that Octomitus 

 was the chief cause of the trouble. Further observations, there- 

 fore, were directed toward the study of this organism and its 

 effect upon the fish in producing the disease. 



SYMPTOMS OF THE DISEASE AND DIAGNOSIS. 



So far as observed there are no definite external lesions, 

 though badly infected fingerlings show somewhat abnormal 

 coloring and a characteristic behavior. The dwarfed emaciated 

 specimens often appear blackish, while those of an older growth 

 present a pale or faded appearance. Balance seems easily lost 

 and the fish turn over repeatedly with a "whirling" or "cork- 

 screw" motion. Weakened by the disease they are unable to 

 stem the current of the water which wafts them toward the foot 

 of the troughs where they congregate in the corners, nose along 

 the sides and near the surface and eventually die on the screens. 

 As the sick fish turn over on their backs the gills are in feverish 

 action and appear distended, a symptom which has doubtless 

 given rise falsely to the diagnosis "gill trouble." 



The examination of the intestine gives the more dependable 

 diagnosis, the wails of which in bad cases of infection appear 

 translucent, yellowish or whitish and somewhat inflated. The 

 contents are watery and easily run away from the vent. In 

 the earlier stages of infection the motile organisms are most 

 abundant in the fore-intestine, but later they are more generally 

 distributed in the lower intestine and the rectal region. In 

 severe cases of infection among the young fingerlings the entire 

 lumen of the fore-intestine, then devoid of food contents, may be 

 filled with a mucous fluid in which the motile parasites are ex- 

 ceedingly numerous. By transferring a drop of this fluid to a 

 microscope slide for examination they may be observed, even 

 with medium powers, as minute, watery, pear-shaped objects, ex- 

 tremely active, moving forward through the fluid with a revolv- 

 ing or cyclic motion. 



