1G8 liEPOKT OF COMMISSIONER OF FISH AND FISHERIES. 

 DISEASES AND PARASITES OF PISHES. 



Fishes arc subject to a variety of diseases, some of wliicli are local 

 and uniiiiportaiit, liaviiig little or no effect upon the general condition 

 of the lisli, Avbile others assume a very serious (character, becoming even 

 epidemic in their course and causing the mortality of immense numbers 

 of individuals. But little attention has been paid to the nature or 

 ]>athology of su(!h diseases or to their treatment, and an important 

 liekl is, therefore, open for investigations in this direction. It is well 

 known that a large percentage of the disorders which have been ob- 

 served among lishes is due to i)arasitism of one sort or another, and as 

 biologists have long been interested in the natural history of the para- 

 sites so concerned, much information has been secured regarding the 

 structure and the development of these organisms. This branch of 

 research must, in fact, precede the more special consideration of the 

 relations of the parasites to their hosts and their effects upon the 

 latter; but the time has come Avhen not only the amount of material 

 collected seems ample to begin upon a full investigation of this more 

 practical phase of the subject, but also when the demands for accurate 

 information regarding it have become sufficiently urgent to necessitate 

 its being taken up without delay. Dr. Kevere R. Gurley has recently 

 been assigned to the study of these problems. 



The parasites which infest fishes belong to both the animal and 

 vegetable kingdoms, and some fishes even are parasitic on others. The 

 grouijs of crustaceans and worms furnish the greatest variety of known 

 parasitic forms, the former occurring generally on the exterior of the 

 body or in the mouth cavity, the latter in or among the viscera and in 

 the tissues. A majority of these, however, seem to be entirely liarm- 

 less, but many .i)roduce a diseased condition of greater or less extent, 

 and some at least must eventually prove fatal. The protozoan para- 

 sites, called X)Sorosperms, give rise to large excrescences on the exterior 

 of the lish, making it very unpresentable in appearance, and undoubt- 

 edly soon causing death. Among fresh- water lishes most harm is 

 ])robab]y effected by low forms of plants, wliich often result in a very 

 widespread mortality, as in some of the large northern lakes. Tlu>ir 

 attacks are not confined to the adults, but extend also to the younger 

 stagiis an<l the embryos as well as to the eggs, and in the artificial 

 haU'liing ai)paratus they often cause much destruction. A great mor- 

 tality also occurs among fishes, which has not been traced to i)arasit- 

 ism, and of the true nature of which we are still ignorant. One instance 

 of this character is noticed else\Yhere in tliis report, in the account 

 of Prof. S. A. Forbes's observations at Lake Mcndota, Wisconsin. 



The long-ijontinued investigations by the Fish Commission on the 

 Atlantic coast of tlie United States, especially in the vicinity of Woods 

 Hull, Mass., liave alfordcd the means of collecting and studying the 

 crustacean and worm parasites of marine fishes under exceptionally 



