ultimate losses were 87 percent in the Berlin 

 trout and 51 percent in the Bellefonte fish. 



In the second experiment brook trout 

 from Bellefonte, Beaver Creek and Berlin were 

 marked by fin clipping and mixed in equal num - 

 bers. Lots were infected by injecting several 

 fishes in each group with a pure culture of H. 

 piscium . Furunculosis broke out spontaneously, 

 and the fish suffered from a mixed infection . 

 Observations were continued for 205 days. Ul- 

 timate losses expressed as percentages were 

 as follows: Berlin trout 75, Bellefonte trout 63, 

 Beaver Creek trout 49 . 



Brook trout from Berlin, N. H., Belle- 

 fonte, Pa., and Erwin, Tenn . , were compared 

 in the third experiment. The latter are con- 

 sidered to be a local race and are called 

 Appalachian brook trout. (These trout were ob- 

 tained through the courtesy of Dr. R. E. Lennon 

 of the U.S. Fish and Wildlife Service.) 



Somewhat less than half of the fish of all 

 three strains were marked and equal weights 

 (not numbers) per trough were kept together. 

 The remaining fish of each strain were kept 

 separately. All fish were infected by the addi- 

 tion of a mixture of fresh culture of H . piscium 

 and A. salmonicida to the diet fed during the 

 first two days of the test. The experiment was 

 terminated after six weeks. 



The test with mixed fish was run in quad- 

 ruplicate: 



Total 123 136 119 130 508 



After six weeks the percentage mortalities 



were: 



The test with the strains kept in separ- 

 ate troughs were run in duplicate . The original 

 numbers per trough and the percentage mortal- 

 ities after six weeks were: 



The results of this experiment will be 

 considered in greater detail elsewhere. Mr. 

 C. E. Dunbar of this laboratory collaborated in 

 the study . 



In the light of the results presented 

 above and the general agreement that disease 

 resistance is a function of heredity, there is 

 justification to accept as true populations or 

 subpopulations strains of fishes which display 

 inheritable and significant differences in sus- 

 ceptibility to various diseases . It is a well 

 established fact (Burnet, 1953) that strains of 

 animals resistant to certain diseases originate 

 by natural selection if the disease has an endemic 

 character. It is possible to accelerate the pro- 

 cess of the establishment of disease -resistant 

 strains of animals and plants by artificial selec- 

 tion and breeding. It has been found however, 

 that if disease -resistant strains are left under 

 natural conditions, free to mix with susceptible 

 populations, the resistance may gradually dis- 

 appear in the absence of the pathogen which 

 would destroy the susceptible individuals in such 

 an offspring. 



Therefore a practical conclusion can be 

 deduced from the above discussion: Fishes re- 

 sistant to certain diseases can remain resistant 

 unless exposed to crossing with susceptible in- 

 dividuals. 



The assistance of Dr. K. E. Wolf of 

 the Microbiological Laboratory, in the prepara- 

 tion of this manuscript, is gratefully acknowledged. 



127 



