CARCINOMA OF THE THYROID IN SALMONOID FISHES. 
441 
HYBRIDIZATION. 
Hybrids of the brook trout with the American saibling (Sunapee trout) and with 
the landlocked salmon do not differ materially from the pure species in reaction to 
thyroid disease. Lot 2017, for instance (female brook and male landlocked salmon), 
as yearlings showed no visible process, and experienced very little involvement during 
four years. Lots 2036, 2037, and 2038 (brook plus saibling) were little affected as 
yearlings, but considerably at three years. The appearance and character of the growth 
is substantially the same as in the brook trout. These crosses, within the genus Salve- 
linus and the closely related Salvelinus plus Salmo, are more or less successful, and 
the fish resulting are hardy, and grow' to maturity and reproduce. 
The hybrids of the Pacific salmon, though all contained in the same genus {Onco- 
rhynchus ) , are greatly inferior in vigor to their constituent species, and probably could 
not maintain themselves. They do not do well under fish-cultural conditions, are not 
hardy, and easily succumb to unfavorable conditions. Hybrids of the silver and hump- 
back salmon are subject from the embryo to deformity of the spinal column in the 
region of the caudal peduncle. They are readily susceptible to thyroid disease. One 
of these hybrid lots showed as yearlings the highest incidence of visible tumors yet 
observed in any large homogeneous brood of fish (p. 67). One lot, however (silver 
plus humpback, 1988A), consisted at the first examination of but 17 fish, all of which 
were tumored; and all the Pacific hybrids showed a high percentage of tumors. The 
general gross appearance of these growths is markedly different from that of the trout 
tumors. The hybrid tumors have a marked symmetry, most apparent on the floor of 
the mouth. Here the growth as it vegetates into the mouth usually occupies the median 
bridge, and spreads equally over the arches so that the right and left halves of the 
tumor are alike, and a distinct and sometimes almost perfect bilateral symmetry appears. 
The surface of the growth is unusually smooth. The benign impression which results 
is belied by the structure, which in these growths in hybrid salmon is among the most 
malignant of the thyroid tumors in fishes. Likewise the cachexia observed among 
tumor fishes is most extreme in these fish. (Fig. 4a). 
CLINICAL COURSE. 
MORBIDITY AND MORTALITY. 
There is no very definite symptom picture among the affected fishes. The disease 
usually runs a slow' chronic course. The earliest external evidences may doubtless 
occur in very young fish only a few months old, but rarely do fish of this age show any 
outward signs of disease. The beginning of the process is without clinical symptoms 
until the red floor or an evident tumor appears. The earliest gross tumor we have seen 
is in a brook trout about 5 months old. (Fig. 72.) Not many tumors are seen until the 
fish reach the yearling stage, when the growth is usually still small and not causing much 
interference. In certain hybrid salmon, however, and occasionally in brook trout, the 
growths in yearling fish have already reached a relatively great size, sometimes almost 
their maximum. These hybrids die rapidlv and do not grow to maturity. In brook 
