CARCINOMA OF THE THYROID IN SALMONOID FISHES. 



431 



Pond 5 in August, 1909, contained lot 2017, consisting of 729 yearling hybrids 

 (female brook trout and male landlocked salmon), all clean fish. These were the 

 survivors of 1,553 fish f the previous April, all of which were clean at that time. This 

 lot was almost immune for two years. Only 13 red floors and 2 tumors were produced 

 in one year. The next year all of the remaining fish were clean, and finally in 1912 

 there were 10 tumored fish among the 220 remaining. 



In pond 6 were placed, in August, 1909, 469 yearling chinook salmon. This pond 

 previously contained lot 1994, badly affected hybrid salmon. During the two years 

 in which these chinook salmon were kept under observation no visible thyroid process 

 was produced. This species (Oncorhynchus tschawytscha) is a Pacific salmon, an 

 anadromous marine fish. In no adult marine salmon have we seen thyroid disease. 

 Humpback salmon yearlings in troughs of fresh water at the hatchery acquired a heavy 

 tumor involvement. 



Ponds 7 and 8 contained in July, 1910, 147 clean yearling rainbow trout (lot 2133). 

 No visible tumors have been produced. After one year 3 per cent had red floors and 

 after two years only 2 fish of the 31 remaining had the incipient process. 



Pond 9 held, in August, 1909, 97 clean 2-year-old landlocked salmon. They 

 remained clean during the first year. In July, 1911, there were 10 per cent of red floors, 

 but no visible tumors. In August, 1912, there were but 32 fish left, of which i had a 

 red floor and i a tumor and red floor. 



Pond 10 is the lowermost of the first row of ponds and is almost entirely supplied 

 by previously used water. In August, 1909, the 673 clean, 2 -year-old landlocked salmon 

 remaining after the removal of 219 tumored fish from lot 1950 were placed in this 

 pond. The loss was not excessive on this lot. By July, 1911, 7.7 per cent had visible 

 tumors while 37 per cent were affected, the fish having reached 4 years of age. Thus 

 the tumor fish of this lot recovered under more natural conditions in pond i , while the 

 remainder continued to develop tumors under the intensive conditions of pond 10. 



Pond 12 receives 30 gallons per minute of unused brook water only. In August, 

 1909, 7 tumored fish were removed from the 2-year-old brook trout of lot 1939 A 

 and the 223 clean fish were left in pond 12. In July, 1910, but 80 fish remained, of 

 which 5 per cent were affected, including but one tumor. In 1911, 5 per cent had 

 tumors and 25 per cent were affected. In 1912 the remaining 58 fish had a slight 

 increase in both respects. 



Pond 13 contained cyprinoid fish, which do not, so far as observed, acquire thyroid 

 tumors or any notable thyroid changes, and are not considered in the present study. 

 Pond 14 contained in August, 1909, 167 clean 2-year-old brook trout remaining after 

 the removal of 16 tumored fish from lot 1937. During the next year the thyroid process 

 progressed rapidly, 15 per cent acquiring tumors and 31 per cent becoming affected. 

 During the next year there was a very definite regressive process, and during the last 

 year, the fish reaching 5 years of age, there was again a slight progression. Pond 15 

 had similar susceptible fish, the process continuing slowly at first and then more rapidly. 



Ponds 17 and 18 contain Scotch sea trout, which are of especial interest and impor- 

 tance. These ponds receive almost entirely previously used brook water, and the 



