CARCINOMA OP THE THYROID IN SAEMONOID 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-cultura.ll3% 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 
obseiA'ations show, for brook trout 3'earlings, 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 yearlings, 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 w’ere affected. The salmon hybrids of the 
genus Oncorhynchus 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 \’isible tumors. 
The direct and indirect mortality rates can not be stated numericall}^ 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 obseiwations of McCarrison, 1906, of endemic goiter 
