•746 Diseases of Fishes 



two things concerning this parasitism among the trout of Yel- 

 lowstone Lake : First, the abundance of parasitized trout in 

 the lake; second, the migration of the parasite into the mus- 

 cular tissue of its host. The argument cannot be well sum- 

 marized in as short space as the requirements of this paper 

 demand. It is sufficient to say that what appear to me to be 

 satisfactory explanations are supplied by the peculiar condi- 

 tions of distribution of fish in the lakes of this national park. 

 Until three or four years ago, when the U. S. Fish Commission 

 stocked some of the lakes and streams of the park, the condi- 

 tions with relation to fish life in the three principal lakes were 

 as follows; Shoshone Lake, no fish of any kind; Heart Lake, 

 at least three species, Saliiio clarki, Lettcisais liiieatiis, and 

 Catostoniiis ardciis; Yellowstone Lake, one species, Salnio clarki. 

 Shoshone and Yellowstone Lakes are separated from the river 

 systems which drain them by falls too high for fish to scale. 

 Heart Lake has no such barrier. The trout of Yellowstone 

 Lake are confined to the lake and to eighteen miles of river 

 above the falls. Whatever source of parasitism exists in the 

 lake, therefore, must continue to affect the fish all their lives. 

 They cannot be going and coming from the lake as the trout 

 of Heart Lake may freely do. If their food should contain 

 eggs of parasites, or if the waters in which they swim should 

 contain eggs or embryos of parasites, they would be continually 

 exposed to infection, with no chance for a vacation trip for 

 recuperation. To quote from my report ; 



"'It follows, therefore, from the peculiar conditions sur- 

 rounding the trout of Yellowstone Lake, that if there is a cause 

 of parasitism present in successive years the trout are more 

 liable to become infested than they would be in waters where 

 they had a more varied range. Trout would become infested 

 earlier and in greater relative numbers, and the life of the para- 

 sites themselves — that is, their residence as encysted worms — 

 must be of longer duration than would be the ,rule where 

 the natural conditions are less exceptional. . . . There are 

 probably not less tlian one thousand pelicans on the lake the 

 greater part of the time throughoutthe summer, of which at 

 any time not less than 50 per cent, are infested with the adult 

 form of the parasite, and, since they spend the greater part of 



