194 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



numbers and had at least one breeding-place on some small islands in the southeastern 

 arm of the lake. 



The case is of interest here because the unusual conditions at Yellowstone Lake 

 are, in great degree, parallel to those which exist in an artificial pond ; that is to say, 

 the natural enemies are diminished in number and the geographical range of individ- 

 uals is limited. In the case of the fish of Yellowstone Lake the effects of parasitism 

 were more marked and the instances more numerous than they were in Heart Lak, 

 where usual conditions of food, enemies, and geographical range prevail. So, in 

 any confined area, such as a fish pond or even lake, if conditions favorable to para- 

 sitism exist, the cases of parasitism will, in all probability, be more numerous and more 

 serious than they will be in an ordinary stream. A knowledge of the life-history of 

 the parasite in question will be of the greatest value. This will help us to understand 

 the importance of what we usually call purely scientific work. 



As fish-culture becomes more extensive, there will naturally develop a condition 

 of things in a degree paralleled by what we see in the case of the domestic animals. 

 As civilization advances, the carnivorous enemies of the domesticated animals are 

 exterminated, or at least driven out, and are no longer a source of loss to the sum 

 total of herbivorous animals. Further, a link in the chain of an important group of 

 animal parasites is thereby broken, and occasional cases of infection, to a degree 

 which might prove fatal to the herbivorous host while the carnivores were ranging 

 the country, would be impossible under the conditions imposed by civilization. 



So the cultivation of useful food-fish should lead naturally to the extermination 

 of such enemies as fish-eating birds, mammals, and fish which are not of economic. 

 value. Thus one source of parasitism would be destroyed. 



In those cases, however, where the food-fish is the final host, the intermediate host 

 being an invertebrate which is a necessary source of food for the fish, no link in the 

 chain of parasitic existence is broken, and the extermination of such parasites seems 

 to be altogether impossible. Something can be done, possibly, by instructing fisher- 

 men to burn or bury fish which are not in good condition and the viscera of fish, and 

 not to throw them back into the water. Especially should this be insisted on where the 

 fishing is done in the smaller lakes. It should be remembered that the destruction of 

 a single adult cestode worm destroys immediately many thousands and even millions 

 of eggs and prevents many thousands more from developing for each month which the 

 worm might continue to live. 



For some general considerations on this subject, as well as upon some phases of the 

 economics of parasitism, reference is here made to an article prepared for the World's 

 Fisheries Congress held in Chicago in 1893, and published in the Bulletin of the 

 United States Fish Commission for 1893, pages 101-112; especially Sections III and 

 IV of that article. In order, as far as possible, to avoid repetition, I shall continue 

 the discussion under headings corresponding to the several natural orders or groups 

 which furnish the majority of cases of parasitism among fish. 



It does not come within the proposed scope of this paper to discuss vegetable 

 parasitism among fishes. Reference may be made, however, to an article in the U. S. 

 Fish Commission Bulletin for 1893, by G. P. Clinton : Observations and Experiments 

 on Saprolegnia infesting Fish, pp. 103-172, with a bibliography. 



A list of authorities, for the most part found in publications of the U. S. Fish Com- 

 mission and National Museum, is appended, and will be referred to by number. More 

 extended reference to the literature of the subject will be found in these publications. 



