240 U. S. BUREAU OF FISHERIES 



gills of fry hatched in s^Dring water. Consequently, there can be lit- 

 tle doubt that the annual spring mortality at this station has been due 

 to gill disease. 



Later in the season there was an outbreak of gill disease in a lot 

 of 15,000 rainbow fingerlings that were being held in a small, spring- 

 fed pond. The disease made its appearance A^ery suddenly, and in 

 a few days nearly all of the fish were dead. Apparently they were 

 infected with a virulent strain of the bacteria, inasmuch as in all 

 previous outbreaks among fingerlings the mortality increased grad- 

 ually and there was no sudden onset of the disease, as in this case. 



The j)ond containing the rainbow fingerlings drained into a similar 

 pond containing 33,000 brook-trout fingerlings, which were in excel- 

 lent condition prior to the outbreak of the disease in the upper pond. 

 Although these fish were removed immediately, it was too late, and 

 nearly all died of the disease within a few days. The disease ap- 

 peared in several other lots of brook-trout fingerlings also, but in 

 these it was checked before it had caused such heavy losses as in the 

 other lots mentioned. It also appeared in several lots of yearling 

 trout, and while the losses were not as great as among the fingerlings, 

 it is evident that, if not controlled, it is quite capable of causing 

 serious mortality among older fish. 



Until recently the disease was definitely known to occur only at 

 the Holden hatchery, but during 1927 it caused heavy losses in 

 a hatchery near Barneveld, N. Y. ; and there is evidence that it is 

 prevalent in New England, New York, and Pennsylvania. For- 

 tunately, in the case of fingerlings and older fish it can be controlled 

 readily if proper precautions are taken before the fish become badly 

 infected. Further experience with the copper-sulphate treatment 

 has confirmed the results of last year, when it was found that two 

 treatments with a 1 to 2,000 solution of the sulphate will completely 

 destroy all bacteria on the fish. The fish should be placed in the 

 solution for one minute and then be transferred quickly to running 

 water. If the treatment be carried out properly there should be 

 very little loss, unless the fish have been weakened already by the 

 disease. Unfortunately, the disease usually develops gradually and 

 becomes firmly established before its presence is suspected. The 

 stricken fish exhibit no characteristic symptoms, and the only certain 

 way to diagnose it is to demonstrate the presence of bacteria on the 

 gills. This requires considerable experience, because the bacteria 

 are transparent and difficult to distinguish. 



Studies of fin disease have shown that it is evidently identical 

 with the so-called " fin rot " or " tail rot," which recently caused 

 considerable mortality at a number of widely separated hatcheries. 

 It has been confused sometimes with the fin disease caused by Gyro- 

 dactylus, a small flatworm that occurs on the fins and body. Fin rot 

 evidently is of bacterial origin, and although the specific organism 

 has not yet been isolated there is good reason to believe that a long, 

 rod-shaped bacterium is the cause of the disease. This bacterium 

 has been found in large numbers in all cases of fin rot from several 

 localities, and apparently it is the only organism uniformly present. 

 Fin rot is characterized by disintegration of the fins, which often are 

 destroyed entirely. In most cases the dorsal fins are affected first, 

 the others being involved later, but this is not the invariable rule. 



