220 American Fisheries Society 



only member of its family, in which flukes were not found. 

 This is all the more striking in that flukes are abundant in 

 more fish shown in the table than most other types of para- 

 sites, being markedly so in fourteen fish listed, as against 

 sixteen records of marked abundance in a given host species 

 for tapeworms, 7 for roundworms and 8 for spiny-headed 

 worms. In the catfishes the only very abundant group of 

 parasites is the flukes and that occurs in extra measure in a 

 single species, while other types of parasites are distinctly 

 infrequent. In this same group of the catfishes, however, 

 very few fish were found to be free from parasites, indicat- 

 ing thus a steady infection but of low rate. On the other 

 hand the sunfishes illustrate an irregular, casual infection, 

 often becoming strikingly large. An inspection of the table 

 for this group shows that most individuals do not shelter any 

 trematodes, but those that do are relatively heavily infected. 

 The frequency of infection in a given host is indicated by a 

 comparison of the number of hosts infected with the num- 

 ber free, and the degree of the infection is shown by the 

 average number of parasites present in those hosts that are 

 infected. Finally the extreme number of any type of para- 

 site found in a single individual is some indication of the 

 possibility of extreme infection. Thus only one steelhead 

 was infected with trematodes but that one sheltered 142 of 

 these parasites; and one bluefin whitefish contained 303 tape- 

 worms. Numerous similar instances occur in the sunfish 

 family, where half or more of the individuals were free from 

 one or more types of parasite and heavily infected with some 

 other parasitic form which in turn was absent in the next 

 fish examined. Among the perches also parasitic invasion 

 was usually very light, but in a few cases distinctly heavy. 

 These as well as other interesting relations appear on ex- 

 amination of the data given. 



It is important to call attention to the necessity of care 

 in interpreting the figures found in the table. On the face 

 of the data given there the stickleback was not heavily in- 

 fected, since one-third of the total number of lish recorded 



