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PROGRESS IN BIOLOGICAL INQUIRIES, 1939 85 
Different results, however, were obtained with each diet. The conclu- 
sion was reached that the diet must include in excess of 20 percent 
fresh meat in order to revive brook trout after they have suffered 
Factor H depletion. 
Factor H depletion was accompanied by a decreased erythrocyte 
count, and led to the production of severe anemia. The addition 
of beef liver to the synthetic diet caused an increase in the red-cell 
content of the blood. A count of red blood cells may provide an 
indication of the presence of anemia, even before the onset of heavy 
mortality. 
The effect of diet on condition—The condition factor of brook 
trout, which is an index of the relative heaviness of a fish, in terms 
of a mathematical expression, increases with the length. Trout of 
the same size fed on the same diet show little variation in condition. 
The value of the condition factor showed a positive relationship to 
the protein content of the diet. 
Protein requirements of trout—Experiments conducted by Dr. 
McCay and associates, in 1931, proved that the protein requirements 
of trout was 14 percent or slightly higher. ‘The experiments were 
resumed in 1939 to define more precisely the protein requirements 
of growing trout. Groups of trout were fed diets that contained 
5, 10, 12, and 16 percent protein for 24 weeks. The trout were 
weighed periodically during the course of the experiment. and showed 
the following average gains in weight for 4-week periods: 5 percent 
protein, 7 percent gain; 10 percent protein, 17 percent gain; 12 
percent protein, 20 percent gain; and 16 percent protein, 29 percent 
gain. The mortality decreased as the percentage of protein was 
increased. With a diet containing 5 percent protein, 9 percent was 
converted into body protein; a 10 percent diet resulted in the con- 
version of 15 percent; a 12 percent diet gave 17 percent conversion ; 
and a 16 percent protien diet resulted in 22 percent conversion. 
Practical diets —Experiments were conducted to test and compare 
the cost and efficiency of diets for practical use. Mixtures of dry 
foods bound together with salted spleen were tested. The diet. con- 
sidered most useful consists of equal parts of dried skim milk, cot- 
tonseed meal, wheat-flour middlings and fish meal combined with 
4 percent of their weight of salt ‘and an equal weight of ground 
frozen spleen. Diets that contained salmon- -egg meal “and seal meal 
produced considerable mortality, accompanied ‘by a slimy condition 
of the body and a gill disease. Until more is known of the toxicity 
of salmon-egg meal and seal meal they should not be used in fish 
diets. 
INVESTIGATIONS IN INTERIOR WATERS 
Dr. M. M. ELLIS, in charge 
Pollution studies—The pollution studies of the Bureau have been 
centered at the University of Missouri, at Columbia, where a generous 
provision of laboratory space and other facilities has been made. A 
portion of the investigations has been conducted at a subsidiary 
laboratory located at ‘Fort Worth, Tex. The pollution problems 
throughout the United States, both specific and general, are so mani- 
fold that their permanent solution necessitates a continuation of a 
program of laboratory research designed to analyze thoroughly the 
