430 BULLETIN OP THB BUREAU OF FISHERIES. 



Ill this pond system trout and salmon and hybrid trout and salmon, of various 

 adult ages and sizes and of various species, were held under ordinary fish-cultural con- 

 ditions, varied to a slight extent for experimental purposes, during some three years. 

 Nearly all were examined each summer for four consecutive years. Thyroid disease 

 was seen among them in all degrees of severity up to large visible carcinoinata, and with 

 greatly varying incidence from all but complete immunity to macroscopic involvement 

 of more than 90 per cent of the individuals of a given pond lot. 



The physical conditions in these ponds were those common to intensive breeding 

 of trout. All the factors of crowding, artificial food, reduced water supply, and accumu- 

 lation of waste products, as compared with conditions under which trout live in a wild 

 state, were present in greater or less degree. But the reaction of the fish to these con- 

 ditions, as expressed by the macroscopic evidence of the thyroid disease, was by no 

 means uniform but varied within wide limits. 



It is of interest to consider more in detail the external evidence of thyroid growth 

 in these fish over a term of years. The identity of a given lot of fish is preserved by a 

 number. Visibly affected fish are those showing any macroscopic indication of abnormal 

 thyroid growth from the red floor upward. Visible tumors refer to those showing 

 definite tumors. It will be noticed that the loss on the lots from year to year is in most 

 cases considerable. This of course is not to be charged entirely to thyroid disease, 

 either directly or indirectly. Besides intercurrent disease, many fish are lost by depre- 

 dations of predatory birds and animals, and it is impossible for obvious reasons to 

 apportion the total loss quantitatively among these separate factors. 



Table iv summarizes the condition of each lot at the summer examination during 

 three or four years, and gives the percentage of loss. The tumor fish were pennanently 

 removed at each examination, unless otherwise stated. The fish with red floors only 

 were not so removed. 



In pond I were placed, in August, 1909, 119 2-year-old landlocked salmon (1950 T), 

 all bearing visible tumors. They were taken from lot 1950. They were fed proteid 

 artificially until February, 1910, and thereafter received only such food as the pond 

 naturally afforded. A large measure of recovery occurred (p. 87). After one year 

 nearly half the visible tumors had disappeared. The next year only two fish were 

 left and the experiment was abandoned. In August, 1909, 28 2-year-old clinically 

 clean brook trout were present in pond 11, and the annual remnant has remained clean 

 ever since. The conditions in these two ponds, which are separate from the rest of the 

 series, were discussed on page 68. The only water supply of pond 1 1 came from pond i , 

 which contained at the beginning only tumored fish, and the fish in pond 1 1 regularh- 

 received artificial food. 



Ponds 2, 12, and 19 receive only unused brook water, while nearly all the remaining 

 ones receive the greater part of their supply from the next pond above and the lesser 

 part direct from the brook. Pond 4 had in August, 1909, 146 yearling hybrid salmon, 

 lot 1 995 (female humpback and male blueback) with 5 per cent of visible tumors, which 

 were left with the clean fish. The next year there were 9 per cent and the following year 

 none, though the total affected fish had reached ^y per cent. 



