heavy losses (Davis 19^6). Davis (1946) states that among mature trout 

 the disease is usually rare except during and shortly after the spawning 

 season vfhen the fish are particularly susceptible to infection. However, 

 Plehn (1924) states that fry contract the disease rarely in Germany, 

 and Blake and Clark (1931) in Great Britain found that of twenty-five 

 brown trout (S. trutta ) which had died of furunculosis in rivers and 

 submitted to the laboratory for examination, all were at least 3 years 

 old. Similar results have been found by Blake and Clark (1931) in the 

 case of furunculosis in hatcheries, trout of 2 years of age and older 

 being most frequently affected. Among thirty-seven infected brown 

 trout from a hatchery examined by these workers, ten were 2-year-old 

 and twenty-seven were 3 to 5 years. Laboratory experiments by Blake and 

 Clark (1931) support the view that in Great Britain at least 

 susceptibility increases with age. Table 11 shows their results. 



Table 11. — Susceptibility of S. trutta at different ages. S» trutta 

 were variously exposed to infection (B. salmonicida added 

 to food and to water, water from infected fish, and 

 contact with in.fected fish). 



. of fish Number exposed Number 



to infection infected 



Hatchery fry. 180 



Hatchery yearlings. 24 5 



Two-year-old. LA 38 

 Three- years, wild Loch 



Leven. 14 12 



The conditions under which trout of different ages were exposed 

 to infection were as far as possible the same, and the same strain of 

 B. salmonicida was used throughout the series of experiments tabulated. 

 In support of the findings of Blake and Clark (1931) in Great Britain, 

 the Furunculosis Committee (1933) also found that finnock (S. truttae) 

 showed a lesser susceptibility than adult sea trout. 



Other fish 



Surbeck (1909) reported furunculosis among grayling ( Tl'xymallus 

 vulgaris) in Europe and Duff (1932) reported it among grayling 

 ( Thymallus signif er ) in the Elk River, British Colimbia. Duff and 

 Stewart (1933) also found it in Rocky Mountain whitefish ( Prosopium 

 -yglliamsoni ) in British Columbia. Arkwright (1912) found it among 

 chub and dace in the TN^e River (Great Britain). 



39 



