LIFE HISTORY OP A TROUT PARASITE. 
353 
food, nor suffer from having a part of their food supply appropriated by another 
species, nor receive partial immunity from sudden death by having their co-species 
furnish a part of the food of their common enemies. 
Indeed I am not sure that the proj)ortion of parasitized trout in Yellowstone Lake 
is so overwhelmingly greater than it is in Heart Lake. While I did not find any flesh 
parasites among the Heart Lake trout, I found that a great many of them had the 
parasites in the peritoneal cavity, both encysted and free among the pyloric cceca. 
Pelicans were seen frequently during our stay on Heart Lake and their breeding 
ifiace at the south end of Yellowstone Lake is only 10 or 12 miles away. It would 
be very strange, therefore, if the trout of Heart Lake were found to be free from 
these parasites. 
Neither am I at all certain that the parasites of the Heart Lake trout never pene- 
trate the flesh of their hosts. However, after examining a great many trout from this 
lake without finding any penetrating the flesh, it may be worth while to consider this 
question: Why should the parasites of the Yellowstone trout have the habit of bur- 
rowing into the flesh of their hosts, while those of Heart Lake seldo^n or never do % 
I am not sure that I can give an altogether satisfactory answer to this question. 
It appears to me, however, that the reason for this difference is to be found in the 
peculiarly isolated and circumscribed situation of the trout in Yellowstone Lake. 
In Heart Lake the trout have at least two species of fish besides their own to feed 
upon ; in Yellowstone Lake the trout, if they are to eat fish at all, are obliged to resort 
to cannibalism. This is certainly done to some extent. In a few cases I have found 
evidence of it in the stomachs of the Yellowstone trout. In each such case the canni- 
bal was a trout above the average size. Mr. Elwood Hofer had also observed this fact 
and stated as the result of his observation that “occasionally a cannibal is met with, 
and when it is, it is sure to be a big fish.” 
The Yellowstone Lake trout are confined to the lake and the river above the 
upper falls. It is true that this species is found in the river between the falls and be 
low the lower falls, being quite abundant in the Yellowstone Eiver and its tributaries. 
It is not likely that large fish could be carried over the lowerfalls and live ; small fish 
and ova, however, might be so transported uninjured. Whatever may be the truth 
with regard to the passage of fish over the falls, it is certain that no fish could return 
to the lake after having once made the descent. The trout of the lake, therefore, are 
compelled to pass their whole life within the limits of the lake and its tributaries, or 
if they do leave, the door is shut behind them with no hope of its ever opening for 
their return. In Heart Lake the case is different ; not only can the trout leave the 
lake at will, either by tributary streams or by the outlet, but having left they can 
return again. 
To what extent these fish migrate with the changing seasons and diminishing 
food supply, I have no exact knowledge. Whatever may be their habits in this 
regard, the trout of Yellowstone Lake are forced to limit their migrations to the lake 
and its tributaries, and to the less than 20 miles of river between the outlet and 
the upper falls. The trout, being thus circumscribed in their range, if from any reason 
their food supply should fail, must suffer the consequences. They can not seek new 
feeding-grounds. If their food should be of such a kind as to produce peculiarities of 
flavor, or if it should contain the germs of disease or parasitism, they will be continu- 
Bull. U. S. F. C. 89 23 
