Diseases of Fishes 347 



their time on or over the water, disseminate milKons 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 some time, looking much like ciliated infusoria. Don- 

 nadieu 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 of the lake, is needed for the 

 proper development of these ova, the conditions are supphed 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 three 

 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 springs, pre- 

 sumably on account of the abundance of food there. They do 

 not love the warm water, but usually avoid it. Several persons 

 with whom I talked on the subject while in the park 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 fre- 

 quently saw pelicans swimming near the 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 circumstances which 

 cause the pelican and the trout to occupy the same neighbor- 

 hood 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, ambitious 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, there- 

 fore, that the trout should have become seriously parasitized. 



