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- 

 linns 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, igSSA), 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 

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