Parasites of Fishes 



Fishermen are frequently disturbed when they find small yellow or 

 black spots on the fishes they catch. These spots are caused by one or 

 more of a large variety of parasites, with which almost all fresh-water 

 fishes are infected more or less. In fact, it is unusual to find fishes 

 which do not harbor some parasites. With a single exception (the broad 

 fish tapeworm) these parasites are not harmful to human beings, but 

 they can cause extensive damage among fish populations. 



Some parasites are external ones which live on the skin or gills; 

 others are internal parasites living in the intestine or in and among 

 the various organs. Many larval forms are found as cysts in the muscles 

 and skin. 



Many adult parasitic worms live in the digestive tract and are usual- 

 ly not discovered by the fisherman who discards the viscera without 

 exploring the contents of the intestine. The parasites most often de- 

 tected are the larval forms of worms encysted in the body or among 

 the viscera. The most common or conspicuous are the several larval 

 flukes — the yellow grubs which live just under the skin and the black 

 grubs which live in the skin. Some tapeworm larvae appear as yellow 

 cysts in the flesh. Most of the flukes and tapeworms of fishes have 

 complicated life histories, the adults living in one species and the 

 larvae living in one or more other species. 



A wide variety of parasites attack the surface of the gills and skin 

 of fishes. The largest of these are the eelhke lampreys (p. 67) , low 

 fishlike vertebrates which measure six to fifteen inches in native Min- 

 nesota forms and several feet in the big marine lamprey found in the 

 Great Lakes. Lampreys attach themselves by their funnellike suckers 

 to the skin of a fish and rasp a hole one-half to several inches in 

 diameter. They retain their hold until gorged with blood, leaving the 

 fish in a weakened condition and with a large gaping wound which 

 often becomes infected by fungus and bacteria. These lampreys should 

 not be mistaken for leeches, which are commonly called blood suckers 

 in many regions. 



Many, but not all. species of leeches attack fishes. Some of the 

 smaller leeches attach themselves to the gills and the fining of the 

 mouth and opercle. Others may fasten themselves to the outer skin. 

 They suck blood until gorged, and if numerous, may cause consider- 

 able damage. 



Tiny copepods belonging to the Crustacea may be found crawling 

 over the gills and skin of fishes. Some species burrow into the fins and 

 the gills. These parasites, which are sometimes called fish lice, can do 



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