224 American Fisheries Society 



tissue they give rise to troubles of a serious type. At all 

 events extensive fatal epidemics among fish, both in fish 

 ponds and in nature, have been traced to their presence. 1 

 do not find that in this country any such direct association 

 has yet been determined, but I have seen many instances in 

 which they had caused serious injuries and the tables show 

 that they are subject to most striking variations in number 

 in individual cases. 



One finds leeches often on the skin of fishes and in some 

 cases the number is sufficient to be injurious to the fish. 

 Lake trout and whitefish are not infrequently taken with 

 large numbers of these worms on the surface of the body. 



The Crustacea are as characteristic external parasites as 

 are the worms internal parasites. Numerous types of this 

 great group are familiar to the fish culturist. The flattened 

 scale-like fish lice, or Argulidse, occur on fresh-water fish. 

 In France at least they have been shown to be the cause of 

 great damage to fish in artificial ponds, where they increase 

 more easily than in nature. Thus far no effective means of 

 disposing of them have been discovered. The numbers of 

 such forms may be reduced to the minimum by keeping sur- 

 face-feeding fish in the fish ponds, since the young forms of 

 the parasites are free-living surface swimmers and are 

 eagerly sought out and devoured by plankton- feeding fish. 



The parasitic Copepoda, or Siphonostoma, are sometimes 

 found on the skin but more frequently attached to the gill 

 bars or gill covers inside the gill chamber. They present 

 in the full grown condition an irregular, shapeless appear- 

 ance that renders it difficult to recognize their close rela- 

 tionship to the group of free-living Copepoda which forms 

 so important an element in the food of fishes. They are not 

 so numerous in fresh-water fish as in the marine species and 

 when the number of these ectoparasites found on a single 

 host is not large, they are probably of little influence on it. 

 Any considerable increase in numbers is accompanied by the 

 death of the infected fish. 



