LIFE HISTORY OF A TROUT PARASITE. 
355 
It is probable that other small islands in the southern end of the lake, for exam- 
ple, Peale’s Island in the south arm, may also be breeding-places of the pelican. We 
saw them on our return, roosting on the south end of Frank’s Island, but we did not 
pass near enough to ascertain whether it was a breeding-place or not. 
It is easy to see from the foregoing why the fish of Yellowstone Lake should be 
greatly infested by these parasites. There are probably not less than one thousand 
pelicans on the lake the greater part of the time throughout the 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 their time on or over the water disseminate 
millions of tape-worm eggs each in the waters of the lake. It is known that eggs of 
other Dibothria hatch out in the water, where they swim about for sometime, looking 
much like ciliate infusoria. Donnadieu found in his experiments on the adult Dibothria 
of ducks, that the eggs hatched out readily in warm water and very slowly in cold. 
If warm water, at least water that is warmer than the prevailing temperature in the 
lake, is needed for the proper development of these ova, the conditions are supplied 
in such places as the shore system of geysers and hot springs on the west arm of the 
lake, where for a distance of nearly 3 miles the shore is skirted by a hot spring 
and geyser formation with numerous streams of hot water emptying into the lake, and 
large springs of hot water opening in the floor of the lake near shore. Trout abound 
in the vicinity of these warm streams, presumably on account of the abundance of food 
there. They do not love the warm water, but carefully avoid it. Several persons with 
whom I talked on the subject while in the iiark assert that diseased fish, that is to say, 
those which are thin and affected with flesh-worms, are more commonly found near 
the warm water, that they take the bait readily, but are logy. I frequently saw peli- 
cans swimming near shore in the vicinity of the warm springs on the west arm of the 
lake. It would appear that the badly infested or diseased fish, being less active and 
gamy than the healthy fish, would be more easily taken by their natural enemies, who 
would learn to look for them in places where they most abound. But any circum- 
stances which cause the pelican and the trout to occupy the same neighborhood will 
multiply the chances of the parasites developing in both the intermediate and final 
host. The causes that make for the abundance of the trout parasite conspire to 
increase the number of adults. The two hosts react on each other and the parasite 
profits by the reaction. About the only enemies the trout had, before tourists, ambi- 
tious to catch big strings of trout and photograph them with a kodak, began to frequent 
this region, were the fish-eating birds, and chief among these in numbers and voracity 
was the pelican. It is no wonder, therefore, that the trout should become seriously 
parasitized. 
It may be inferred, from the foregoing statements, that the reason why the para- 
site of the trout of Yellowstone Lake migrates into the muscular tissues of its host 
must be found in the fact that the life of the parasite within the fish is much more 
prolonged than is the case where the conditions of life are less exceptional. 
VIII.— REMEDY. 
A natural inquiry following the discovery of the true nature of this parasite will 
be : What remedies, if any, are proposed ? 
A very effective remedy, and one which suggested itself immediately to Captain 
Boutelle, he being an enthusiastic lover of the gentle sport of fishing, is to extermi- 
