18 American Fisheries Society 



under such feeding have developed such tumors, one 

 must be conservative in assuming that fish as food will 

 absolutely protect the trouts. This matter of the bal- 

 anced ration for trout in fish culture is fundamentally 

 important and rather difficult, as it is in human and 

 animal feeding. It is not more demanded as a preventive 

 of thyroid disease than against fish disease in general, 

 for when the diet is as exactly adapted as possible to the 

 needs of fish, the latter have a better defense against all 

 their diseases and parasites. 



Thyroid tumor disease does not impress most trout 

 culturists as a very serious enemy to their operations, 

 and in this they have much justification, for even enor- 

 mous relative enlargements of the thyroid are yet so 

 small as not to force themselves on the attention. The 

 very large tumors are not common, and it is exceptional 

 when the mortality is high at any stage. The fish cul- 

 turist does not and need not give himself any particular 

 worry over thyroid disease, though we may assume he 

 would rather not have any of it in his ponds. That a 

 gland should become enlarged to hundreds, perhaps 

 thousands of times its normal size, is, of course, not desir- 

 able and indicates something is wrong. One can say in 

 general that the situation can at least be greatly im- 

 proved by diluting the liver and other mammalian food 

 materials used, with flour and especially with fish, 

 shrimp or any natural food available. 



Since a thyroid tumor in a trout is a form of fish can- 

 cer, and there has always been some talk of the con- 

 tagiousness of cancer, it may be well here to insist on 

 making some common sense distinctions. Infectious 

 diseases are those caused by microbes, that is, micro- 

 organisms or minute parasites, and most of them may be 

 easily "caught" or passed from person to person. But 

 some of them do not transfer readily from person to per- 

 son, that is, they are not readily contagious. One can 

 scarcely "catch" them. Now cancer has not even been 

 shown to be an infectious, that is, a parasitic disease. 

 It can only be transferred from one subject to another, 



