3 50 Diseases of Fishes 



almost every case, had attained such a size that they far ex- 

 ceeded in bulk the entire viscera of their host. 



" From the fact that the examples obtained were of compar- 

 atively the same age, it may be justly inferred that the period 

 of infection to which the fish are subjected must be a short one. 

 I did not discover the final host, but it is almost certain to be 

 one or more of the fish-eating species of birds which visit that 

 region, and presumably one of which, in its migrations, pays 

 but a brief visit to this particular locality. This parasite was 

 found only in the young suckers which inhabit a warm tributary 

 of Witch Creek. They were not found in the large suckers of the 

 lake. These young Catostomi were found in a single school, 

 associated with the young of the chub (Leuciscus lineatus), 

 in a stream whose temperature was 95 F. near where it 

 joined a cold mountain brook whose temperature was 46 F. 

 We seined several hundred of these young suckers and chubs, 

 ranging in length from 6 to 19 centimeters. The larger suckers 

 were nearly all infested with these parasites, the smaller ones 

 not so much, and the smallest scarcely at all. Or, to give con- 

 crete examples: Of 30 fish ranging in length from 14 to 19 cen- 

 timeters, only one or two were without parasites; of 45 speci- 

 mens averaging about 10 centimeters in length, 15 were infested 

 and 30 were not; of 65 specimens averaging about 9 centimeters 

 in length, 10 were infested and 55 were not; of 62 specimens 

 less than 9 centimeters in length, 2 were infested and 60 were 

 not. None of the chubs were infested with this parasite. 



"The conditions under which these fish were found are 

 worthy of passing notice. The stream which they occupied 

 flowed with rather sluggish current into a swift mountain 

 stream, which it met almost at right angles. The school of 

 young chubs and suckers showed no inclination to enter the 

 cold water, even to escape the seine, but would dart around 

 the edge of the seine, in the narrow space between it and the 

 bank, in preference, apparently, to taking to the colder water. 

 When not disturbed by the seine they would swim up near to 

 the line which marked the division between the cold and the 

 warm water, and seemed to be gazing with open mouth and 

 eyes at the trout which occasionally darted past in the cold 

 stream. The trout appeared to avoid the warm water, while 



