CARCINOMA OF THE THYROID IN SALMONOID FISHES. 451 



1944 showed no visible evidence of thyroid disease until the fourth year, though living 

 under the conditions which produced the disease. Lot 2017, hybrids of the brook trout 

 and landlocked salmon, when it consisted of 1,553 yearling fish, showed not a single fish 

 with a visible process. In the second and fourth years a few visible growths appeared. 

 A lot (2133) of rainbow trout, which develops frequent and large tumors at some hatch- 

 eries, were held two years at the Craig Brook station without acquiring any visible 

 tumors and only a small percentage of red floors. 



As previously referred to, rainbow trout at the Caledonia hatchery in New York 

 appeared to have a very low incidence of the disease, about one-half of i per cent of the 

 fish each year showing well-developed tumors, this covering a period of approximately 

 25 years. As stated, no fresh blood during that period was added to this lot of fish. 

 Tumor fish found each year were destroyed and a probable original resistance of the lot 

 was protected and perhaps added to by this form of selection, so that, as we have pointed 

 out, in the epidemic at the Bath fish hatchery covering a pyeriod of two years these fish 

 remained practically immune, only i or 2 fish in a lot of 75 adult fish being found witii 

 tumors in the course of the two years' epidemic. Exactly the same state of affairs 

 existed in a lot of 20,000 German brown trout which had also been held without the 

 addition of fresh blood at the Caledonia hatchery. These were represented among 

 others by some 200 of their offspring, which as 4 and 5 year old fish went through the 

 epidemic without a single tumor. This was not the case with some 4,000 young German 

 brown trout which were sent from the Caledonia hatchery to the Bath hatchery as 

 young fish, and which toward the end of the epidemic, as 2-year-old fish, developed a 

 considerable incidence of the disease. 



That the rainbow and brown trout retained so .nany years at the Caledonia hatchery 

 really possessed a definite immunity against the disease is shown by the fact that during 

 this time attempts to rear brook trout to adult age and maintain them resulted in a high 

 incidence of thyroid tumors in this species. That a given lot of fish from one hatch may 

 possess at a certain age an almost complete resistance to the disease, while another lot 

 from another hatch of the same species and of the same age, kept under the same condi- 

 tions, may show a high incidence of the disease, is possibly explainable on the basis of 

 blood relationship. The manner in which spawn and eggs are taken and fertilized 

 would easily bring about the presence in any given lot of a large number of fish with the 

 same parents. With the exception possibly of the hybrids, some of the lots of which 

 were small, no lot of fish which we have studied could possibly be entirely of the same 

 parentage; but as a large number of individuals in each lot are certainly of the same 

 parentage it is explainable that the high degree of immunity or susceptibility in a given 

 lot may be due to this fact. Such a supposition is in accord with the now well-known 

 facts of family predisposition in both goiter and cancer in human beings. 



The fish offer a remarkable opportunit)' for the careful study of this phase of 

 immunity. It will be easily possible to obtain in any hatchery in which the disease 

 is endemic, on the one hand from parents both ha\'ing \-isible tumors, or, on the other, 

 from parents showing distinct immunity, a sufficient number of eggs to hatch several 

 hundred fish. A number of fish could be reared from such lots sufficient to demon- 



