AN ECONOMICAL CONSIDERATION OF FISH PARASITES. 



BY EDWIN LINTON, PH. D., 

 Professor of Biology, Washington and Jefferson College. 



It is not the purpose of this paper to attempt more than a brief sketch of the 

 subject. Before an exhaustive discussion of the economics of parasitism of fish could 

 be profitably undertaken it would be necessary for us to know a great deal more 

 than we do about the life- histories of the forms which infest fish. All that I shall 

 attempt to do, therefore, will be to gather together in brief space such points as have 

 come under my notice which seem to me to bear on the general thesis of parasites of 

 fish economically considered. 



The literature of parasitism as aifecting fish, mainly systematic or morphological, 

 is widely scattered through a great variety of publications and in many languages; and 

 on account of the great amount of pioneer work which needs to be done for only a 

 comparatively small number of species of the fishes of North America have been 

 examined for parasites with care no compilation is yet possible for the parasites of 

 fishes which could be of such permanent utility as the excellent ones which are being 

 prepared for the Department of Agriculture by Dr. C. W. Stiles relating to the parasites 

 of the domestic animals. 



I think it must be acknowledged also, for the present at least, that by far the 

 greater number of species of parasites infesting fishes are of interest to the zoologist 

 alone and do not concern the practical fish-culturist, except as he may be interested 

 in questions which have not yet emerged from the comparatively limited field ot 

 scientific investigation into the broader field of practical application. And yet even 

 here it may not be wise to despise the day of small things. Under conditions 

 incident to the work of fish-culture the natural interworking of bionomic relations may 

 be so far disturbed as to give an otherwise insignificant parasite all the importance 

 which attaches to the efficient cause of an epidemic. It is quite within the bounds 

 of possibility for damaging cases of parasitism to arise among the fish of a given fish 

 pond which owe their origin to the casual visit or brief sojourn of a fish-eating bird. 



An unusual, though altogether natural, condition of this kind exists in Yellow- 

 stone Lake, which has been much written about. It is sufficient to say here that the 

 lake when first discovered contained but a single species of fish the Rocky Mountain 

 trout which, it is thought, made its way across the great continental divide by way 

 of a bifurcating stream on Two-Ocean Pass. A considerable percentage of the trout 

 of the lake were found to be infested with a parasitic flesh-worm. Upon a careful 

 examination it was found that this worm, although more commonly occurring in cysts 

 in the body cavity, very frequently left the cyst, and, migrating into the flesh of its 

 host, there developed until it was, in extreme cases, a foot or more in length. This 

 worm was plainly a serious drain on the vitality of its host and doubtless caused the 

 death of large numbers of the trout. The very probable source of infection in this 

 case was shown (No. 12) to be the pelican, which in 1890 frequented the lake in large 



193 



F. C. B. 189713 



