Moore. — Nezv Species of Parasite in Trout. 77 



Lesions are produced in the walls of the intestine and in 

 the glandular tissue of the caeca, * which provide sufficient ex- 

 planation for the cause of death. These lesions are occasioned 

 by the activities of the parasite in passing through the various 

 stages of its life history. They will be considered in detail later 

 in the paper. 



SUSCEPTIBILITY AND MORTALITY. 



All species of trout fingerlings thus far examined are sus- 

 ceptible to the disease — brook, brown, rainbow, steelhead and 

 lake trout — approximately in the order named. The suscep- 

 tibility of brook trout over other species is pronounced and may 

 help to explain the decline in brook trout production at certain 

 well known and long established hatcheries. 



Mortality is highest among the smaller fingerlings. During 

 the single epidemic which has been observed, the larger number 

 of deaths occurred among fingerlings of 1^ inches to 1^ inches 

 in length. Infection begins soon after artificial feeding com- 

 mences and progresses rapidly for about 6 to 8 weeks, when a 

 decline sets in. These periods would probably vary with local 

 conditions. There may be recurring waves of the disease during 

 the season with relatively less mortality among the larger finger- 

 lings. It may be that in cases of epidemics all contract the 

 disease and that those affected lightly develop immunity. It is 

 certain, from the large numbers examined throughout the season, 

 that most of the fingerlings which survive are to a large extent 

 carriers. Even adult trout about the hatcheries have been found 

 to be carriers, though apparently unaffected by the presence of 

 the organism. 



The gradient of mortality, showing the approximate number 

 a trough can carry without loss from the disease, could not be 

 completely established this season, but it was found that in 

 troughs under special observation carrying 500, 300 and 100 

 fingerlings, 2}4 to 3 inches in length, there were no losses after 

 the second day following removal from troughs in which the 

 disease was rampant. 



* Dr. H. S. Davis, Pathologist of tlie U. S. Bureau of Fisheries, investi- 

 gating simultaneously the cause of an epidemic among fingerling rainbow trout 

 at Wlaite Sulphur Springs, W. Va., ascribes, in his unpublished researches, the 

 cause of the disease to the organism in question and finds evidence of an In- 

 vasion of the glandular tissue of the intestinal and caecal epithelium. 



