PROGRESS IN BIOLOGICAL, INQUIRIES 31 



and were fed on a diet of sheep liver and beef heart to which 1.5 per 

 cent of cod-liver oil and 2 per cent of dried brewer's yeast had been 

 added. The results of this experiment showed that with brook trout 

 no practical benefit is derived from the addition of cod-liver oil and 

 yeast, cither individually or in combination. In some cases ap- 

 parently an increased growth followed the addition of these acces- 

 sories, but as mortality also increased in such cases it is probable 

 a selective action may have been in effect. It is logical to assume 

 that the survivors were the larger, more vigorous fish, which would 

 account for the higher average individual weight. 



These results are in striking contrast with those obtained during 

 the previous two seasons with rainbow fmgerlings, where the bene- 

 ficial results from the addition of oil and yeast were clear and unmis- 

 takable. The explanation of such diverse results with the two species 

 of trout is not obvious, and furthur investigations are necessary 

 before the matter can be cleared up. However, there is some evi- 

 dence that in the case of the rainbow trout the beneficial effects of 

 the cod-liver oil may be associated with an obscure disease that is 

 prevalent among the rainbow fmgerlings at the hatcheries where 

 these experiments were conducted. This disease is characterized by 

 the accumulation of crystals and casts in the kidney tubules. The 

 water at these hatcheries is rich in calcium and magnesium bicarbon- 

 ates, and it is not impossible that the disease is the result of impaired 

 calcium metabolism, in which event the reason for the beneficial 

 effect of cod-liver oil is apparent. For some unknown reason brook 

 trout do not appear to be as susceptible to this disease as the rain- 

 bows. The lack of similar results from the addition of yeast to the 

 diet of the brook trout may be due to the fact that in many cases 

 the yeast was found to develop, for a short time at least, in the 

 intestines of the fish. No such growth has been observed in the 

 rainbows. 



The reason for the failure of the experiment was not as clear in 

 the case of oil as with the yeast, and further experiments with oil are 

 planned for the coming season. The results of some experiments con- 

 ducted for the bureau at the Connecticut State hatchery at Burling- 

 ton, Conn., in which 1.5 per cent of cod-liver oil was added to beef 

 liver fed to brook-trout fmgerlings, indicate that the addition of the 

 oil was beneficial. 



As in previous experiments, beef heart was found to be a more desir- 

 able diet for young fmgerlings than sheep liver, both as regards mor- 

 tality and rate of growth. As the fish grow older, the advantages of 

 the heart diet become less, and after two or three months the liver-fed 

 fish may reach a larger size than those kept on a heart diet. 



For the purpose of testing the effects of stale food on fingerling 

 trout, 2,000 brook-trout fmgerlings were divided into two lots of 1,000 

 each. One lot was fed "fresh" liver, the*, other "stale" liver that 

 had been kept in tne ice box for a week or more. This liver was 

 noticeably stale or sour but had not begun to putrefy. From the 

 very beginning of the experiment, which was continued from July 9 

 to August 11, the fish fed "stale" liver showed a much higher mortal- 

 ity and slower growth than the controls, which were fed fresh liver. 

 The total mortality in the former lot was 41.2 per cent, and the aver- 

 age individual weight increased from 0.412 grams to 0.722 grams. 



