84 U. S. BUREAU OF FISHERIES 
parts per hundred. Then the vertebrae assumed no color but the 
rib bones became definitely pinkish. At the end of 20 weeks the 
rib bones retained little color but the mandible had a distinct pink 
color. Since the size of the bone may have some effect on the absorp- 
tion of madder, the experiments are to be continued. Purified 
chemicals and other brands of madder root also will be tried. 
Growth of three strains of brook trout.—Growth studies were con- 
ducted using 3 strains of brook trout, 2 from commercial trout 
hatcheries and 1 from the Bureau’s Craig Brook (Me.) Station. They 
were fed the same diet, similar amounts of food in terms of percentage 
of the body weight, and were held in troughs at the same weight per 
cubic foot of water. Although the fry did not start to feed at the 
same time, during a period of 32 weeks, the growth curves for each 
strain were surprisingly similar. The average percentage gain in 
weight by 4-week periods for the entire duration of the experiment 
was approximately the same for each population. 
Source of protein as a factor in growth—Protein is the most 
costly nutrient of the diet and may be derived from either plant or 
animal tissues. Diets were compared that consisted of 50 percent 
pig spleen, 2 percent salt and raw starch, and various percentages 
of the following ingredients, individually and in combination: Dried 
skim milk, cottonseed meal, whitefish meal, and wheat flour mid- 
dings. The amounts of the ingredients were so adjusted that in 
combination with the pig spleen the total protein content of each 
diet was 29 percent. The greatest and an almost equal percentage 
gain in weight was obtained by the use of the three diets that con- 
tained dried skim milk, whitefish meal, or a combination of the four 
ingredients, in addition to the pig spleen, salt, and raw starch. The 
diets that contained cottonseed mea! and wheat-flour middlings as 
the variable source of protein gave results somewhat inferior to the 
other three diets, but which were practically identical. 
Comparative growth of six groups of trout.—To improve the 
technique for conducting feeding experiments, and to determine the 
probable variability between troughs, similar diets were fed to 6 
groups of brook trout during a period of 24 weeks. Only 1 group 
of the 6 showed a statistically significant difference from the mean 
growth of all 6 groups. The growth during the first 12 weeks 
averaged 36 to 39 percent; during the second 12 weeks, 18 to 22 
percent; and during the entire 24-week period, 28 to 31 percent. 
Assay method for “Factor H.’—In order to develop an assay 
method for “Factor H,” brook-trout fingerlings were fed a synthetic 
diet of raw corn starch, dextrin, casein, yeast, mineral mixture, and 
cod-liver oil for a period of 4 weeks when the accelerated condition 
of mortality indicated Factor H depletion. The group was divided 
into 12 lots, and each lot was placed in a separate trough. Three 
lots were fed the same diet. The various diets contained 10 percent, 
15 percent, and 20 percent beef liver in addition to the synthetic 
diet, and the synthetic diet alone. 
After 2 weeks, 40 percent of the fish fed on the synthetic diet 
alone had died. The remainder were then fed 100 percent beef 
liver. At the end of 16 weeks the fish on the all-liver diet had 
stopped drying and had resumed normal growth. The inclusion of 
only a percentage of beef liver in the diet failed to arrest mortality. 
