CARCINOMA OF THE THYROID IN SALMONOID FISHES. 443 



degree, with the accompanying anemia referred to above. They were constantly suc- 

 cumbing to the disease, and the slightest handling, as during the manipulations incident 

 to transportation, greatly increased the death rate. Before the following summer all 

 were dead. The reaction of the disease in hybrids does not afford a typical clinical 

 picture and is not a criterion from which to infer its virulence or its analogies with other 

 disease processes. Most hybrids are not successful fish-culturally, and the salmon hybrids 

 referred to are especially defective and lacking in vigor. 



Restricting morbidity to those showing macroscopic evidence of disease, such as 

 red floors or visible tumors, the morbidity rate is widely variant among the various 

 species and hybrids, and under the various conditions of age and surroundings. Our 

 observations show, for brook trout yearlings, a rate from a minimal one-half of i per 

 cent to 28 per cent; for 2-year-olds, 20 per cent to 65 per cent; and for older fish from 5 

 to 91 per cent. Considering visible tumors only, there appears one-half of i per cent 

 to 7 per cent for yearUngs, 3 per cent to 38 per cent for 2 -year-olds, and i per cent to 

 34 per cent for older fish. Landlocked salmon have shown visible evidence of disease in 

 from 2 per cent to 7 per cent of yearlings, and 5 per cent to 37 per cent of older fish. 

 With rainbow trout our experience is very limited, and we have not seen more than 6 

 per cent of adults affected. 



Hybrids of the brook trout and saibling react much like the brook trout, but hybrids 

 of the brook trout and landlocked salmon seem, from experience with one lot only 

 (no. 2017), to have a considerable degree of immunity, showing no involvement until 

 the second year, and at 4 years only 5 per cent were affected. The salmon hybrids of the 

 genus Oncorkynchus usually show extreme susceptibility. 



On the other hand, the same hybrid with the sexes reversed (lot 1995, male blueback 

 and female humpback), was much less susceptible. It was held at first under the same 

 conditions, in trough 99, and consisted in April of 312 yearling fish, only 1.2 per cent 

 bearing tumors. In the following August there were 146 fish left, of which 5.5 per cent 

 had tumors. The clean fish were then transferred to pond 4. At 2 years of age there 

 were 77 fish left, of which 13 per cent were affected and 9 per cent had visible tumors. 

 At 3 years of age 27 fish were left, and of these 37 per cent showed red floors without 

 any visible tumors. 



The direct and indirect mortality rates can not be stated numerically. The process 

 presumably does not kill, whether directly or indirectly, save in the later stages when 

 the tumor is visible, and the only available data on the loss among fish in any stage of 

 thyroid disease is so complicated with losses from other causes — even causes having 

 nothing to do with disease, that any accurate statement is impossible. The mortality 

 rate is to all appearances not uniform, but varies from a slow fish-culturally unimportant 

 loss to occasional epidemic virulence, as in the active progress of the disease and rapid 

 loss of fish at the State hatchery at Bath, N. Y. Ordinarily and in the common exhibi- 

 tion of the disease the death rate may be said to be low. 



In the many studies of the distribution of goiter among human beings none is so 

 striking or apparently so directly comparable to the conditions found in the study of 

 the disease in fish hatcheries, as the observations of McCarrison, 1906, of endemic goiter 



