442 
BULLETIN OF THE BUREAU OF FISHERIES. 
and other pure trout species the tumors grow progressively with the growth of the fish 
to maturity. At the spawning season gravid fish are often thrown out and destroyed 
on account of the presence of large thyroid tumors, and doubtless eggs from such fish 
not infrequently find their way into the hatchery. As far as we know they may be fer- 
tilized and hatched like the eggs of other trout. 
Onl)^ the larger tumors are evident to a casual inspection of the fish. To determine 
whether a fish has a thyroid tumor it is necessary to examine the thyroid region from all 
sides after spreading wide the gills and opening the mouth. A considerable proportion 
of the trout in fish-cultural ponds may have developed visible tumors without the knowl- 
edge of the fish culturist. By such an examination 31 per cent of the 2-year-old brook 
trout at one hatchery were found to have visible thyroid tumors, while 33 per cent 
showed the red floor indicative of an earlier stage of thyroid disease. 
Thus 64 per cent were visibly affected, although the fish culturist knew of the exist- 
ence of this disease only from having occasionally seen a dead trout with a swollen gill 
region. 
The first obvious effect of the tumor is the mechanical interference with breathing 
and eating as the tumor grows larger. By filling the mouth it obstructs the passage of 
food, and by its growth downward and outward it spreads the gill arches, limits their 
natural movement, and pinches off the vessels until in portions the circulation is stopped. 
Large tumors often show across their surface a pale atrophied series of nonfunctional gill 
filaments. Yet the fish succeeds in breathing and eating in spite of a surprising degree 
of interference. 
Tumored trout, like healthy normal trout, ordinarily have the blood sterile to com- 
mon culture media. This is shown by negative results from numbers of attempts to 
obtain cultures from the heart blood of freshly killed trout with tumors, and also from 
tumor trout dead of disease. That tumor trout are more subject to terminal infection 
than other trout is to be expected and is probably the case, though some meager observa- 
tions made by us at a State hatchery tend rather to support the contrary view. Ter- 
minal blood infection in the living as well as the dead trout has, however, been observed, 
and of course the tumors are frequently infected. That tumor trout frequently die 
without showing a general infection, or evidence of intercurrent disease, indicates that 
the thyroid process itself is fatal in a certain proportion of cases. This intrinsic death 
rate is probably low, but can not easily be separated from the rate of mortality among 
tumor fish due to all causes. 
Hybridization seems greatly to increase the susceptibility of the salmonoids to the 
incidence of tumors and to its effect on their bodily economy. In some of these the 
anemia is readily recognizable by mere inspection, and is so extreme in some individuals 
that the blood scarcely appears red. A case in point is the lot (no. 1994) of yearling 
salmon (Oncorhynchus) hybrids of the female blueback with the male humpback. In 
April they numbered 1,043, of which 16.7 per cent had visible tumors. By the following 
August they had suffered a loss of 57 per cent. Of the 594 remaining 92.5 per cent 
had visible tumors. These showed distinct emaciation, many of them to an extreme 
