PROGRESS IN BIOLOGICAL IXQUIRIKS, 19.33 371 



The poorest jrrowth was iiiade by a jxi'o'ip f^'^^ ^"^ '^ 'li^'t of boef 

 livor alono. Tho host «rr()wth Avas (JbtaiiuHl from the "rroup fed on a 

 diet composed of equal parts of cottonseed meal, skim milk, and 

 salmon-e<rjr meal, supplemented with 15 percent of fresh liver. 

 White-fish meal can be substituted for the cottonseed meal and will 

 produce similar results, both as re^^ards p:rowth and economy. Since 

 the best jrrowth resulted from the diet containing; 15 ])ercent fresh 

 beef liver, this combination seems adequate for a period of at least 

 6 months. 



In analyzin<2: the results of these experiments with commercial 

 feed combiiuitions, the efficiency of conversion has been calculated to 

 show the number of ojrams of feed required to produce 1 o;ram gain 

 in weight of trout during the 4-month period, and also the cost of 

 feed re(fuired to produce 1 pound of trout. It is furthermore of 

 interest to compare the ability of different trout species to convert 

 feedstuffs to body tissues, especially w^iere there is a constant water 

 temperature. 



Analysis of the results shows that the dry feed required to produce 

 1 unit by weight of trout ranged from an average of 2.69 units, using 

 a diet of skim milk, cottonseed meal, and raw beef liver, 1:1:2, to 

 as high as 5.35 units, on a diet of skim milk. 29 percent; cottonseed 

 meal, 28 joercent; raw beef liver, 15 percent; and white-fish meal, 28 

 percent. The trout are somewhat more efficient than these figures 

 shoAv, however, because the losses of feedstuffs in the water are cal- 

 culated as feed consumed. 



In these studies the mean value for food conversion among the 

 different species of trout are: Lake, 3.06; rainbow, 3.47; brook, 4.73; 

 and brown, 5.14. These results to date indicate that the species that is 

 most efficient food conversion in water with a temperature of about 

 47° F. is the lake trout, while the least efficient is the brown trout. 



Experiments were undertaken to develop a biological method of 

 assaying the potency of various growth factors in prepared food 

 materials. In vitamin assay experiments with rats, it is a common 

 practice to deplete their stores of a given vitamin until they cease to 

 grow. At this point the substance to be assayed for its vitamin 

 potency is fed at various levels to the animals that have been de- 

 pleted. The growth response serves to measure the potency of the 

 vitamin preparation. Attempts were made to establish such assay 

 methods with trout, but thus far no standard procedure has been 

 developed. We have no knowledge of the relative requirements of 

 fish for vitamins compared to the higher animals. 



Two lots of experimental fish, which showed the usual marks of 

 failure upon totally dried diets, Avere changed to a diet of dry skim 

 milk, supplemented with fresh liver, preserved in 5 percent by weight 

 of calcium hypochlorite. U])on this new diet the growth rate be- 

 came normal and the mortality rate declined, indicating that trout 

 can ingest meat preserved in hypochlorite without apparent injury. 



Another lot fed on a dry diet until the growth rate had fallen 

 and mortality sharj^ly increased was transferred to a diet of dried 

 skim milk, suj)plemented with one-twentieth of its weight of liver 

 dried in a current of nitrogen. The growth and mortality curves 

 confirm earlier findings that liver Avill retain some of its potency in 

 factor H if dried at a low tem])erature in inert gas. Such experi- 



