COMMON PARASITES OF FISHES 



Glenn L. Hoffnnan, Bureau of Sport Fisheries and Wildlife, Leetown, W. Va. 



and 

 Carl J. Sindermann, Bureau of Comnnercial Fisheries, Boothbay Harbor, Maine 



Fish taken conamercially, for sport, or 

 raised by fish fanciers are sometimes found 

 to be abnormal due to injury, deformity, 

 disease, or the presence of parasites. This 

 paper is concerned with connnnon parasites 

 of fishes and their recognition. Life cycles 

 and biology are mentioned briefly, and 

 references to more detailed literature are 

 included. 



Parasites nnay be defined as "animals 

 that live on or in another animal, the host, 

 at the expense of that animal." There are 

 many kinds of parasites which may be 

 found on the surfaces of fish, including the 

 gills, as well as in the flesh and internal 

 organs (fig. 1). Those on the surface are 

 known as external parasites, and those that live 

 inside are called internal parasites. Several 

 types may occur together in a single fish. 

 Some parasites are fairly large (up to 

 several inches) and can be seen easily, 

 but many are microscopic. Scientists who 

 work on parasites describe the organisms, 

 study the life cycles (some have several 

 intermediate hosts), survey the kinds and 

 numbers found in fish, study damage done 

 to fish, and attempt to control them. Re- 

 search is carried on by the U. S. Fish 

 and Wildlife Service and some colleges and 

 universities. 



Under natural conditions in both fresh 

 and salt water, nnost of the parasites pro- 

 duced are lost to enemies or to the expanse 

 of water before they can infect fish. Ap- 

 parently a light parasitic infection does 

 little harm to the host. Under crowded 

 conditions or inadequate water and oxygen 

 supply, however, fish may become heavily 

 parasitized; in such cases, more 

 damage is done and the fish may even 

 die. Such conditions sometimes occur 

 in hatcheries, causing heavy loss of 

 fish. 



Much of the damage to fish appears to be 

 mechanical. Parasites may injure tissues 

 and blood vessels by their burrowing, or 

 block blood vessels entirely with their 

 bodies or their eggs. Some actually eat 

 skin, other tissues, mucus, or body fluids. 

 Some parasites are known to release toxic 

 materials in other hosts, but this has never 

 been demonstrated in fish. Probably fish 

 weakened by parasites are easily captured 

 by predators. 



Fishermen who find parasites when they 

 clean their catch frequently discard the 

 fish. This is an unnecessary waste. Although 

 worms and other parasitic fornns are un- 

 sightly, none of them can possibly harm 

 humans if the fish is thoroughly cooked. 

 Freezing and hot smoking of the fish will 

 also kill nnost parasites, but sonne may 

 remain alive in brine for a month. A few 

 parasites may develop in man if the fish 

 containing them is eaten raw. Best known 

 in this category is the broad fish tapeworm 

 (Diphyllobothrium latum), whose larvae may be 

 found in the muscles and among the viscera 

 of pike and perch, particularly in our north- 

 ern lakes. Most fish parasites will not live 

 in man. 



The commercial fishing industry suffers 

 great losses each year because of parasit- 

 ized fish which are condemned for human 

 consumption and can be sold only for 

 animal feed at reduced price. In Minnesota, 

 for example, tullibees (ciscos) are infected 

 with tapeworm cysts (Triaenophoras) \vhichdo 

 not infect humans but are unsightly. The 

 annual catchof 2-l/2 to 3 million pounds, if 

 uninfected, would find ready markets for 

 human use, but as much as 1,500,000 pounds 

 have been condemned by governmental 

 agencies. Sinnilar losses occur in marine 

 fisheries. 



