314 BULLETIN OF THE BUREAU OF FISHERIES. 
of 2.5 c. c. and digested them in 23 hours. Taking asa basis for calculation Muttkowski’s 
(1918) estimate of the number of chironomid larve in shallow water and recent studies 
of deep-water fauna by Birge and Juday, there are about 474,750,000,000 in Lake 
Mendota. The number of perch they would support may be computed roughly: A 
perch of medium size if eating nothing but chironomid larve would average about 
4.2 c. c., or 78 individuals of various sizes, per day, which would amount to about 1,533 
c. ¢., or 28,470 individuals per year. On such a basis the chironomid larve could support 
16,675,447 perch per year. 
Such methods of estimating are highly speculative at present but give some gross 
approximation as to the number of perch that might possibly live in Lake Mendota. 
After studies have been completed which are now being carried on by the Wisconsin 
Geological and Natural History Survey concerning the animal population of the various 
lake habitats and the chemical composition of animals which may serve as fish food, and 
after the writers have made more extensive experiments on the rate of consumption and 
digestion of different foods, it will be possible in a few years to speak with more authority 
~ concerning the productive capacity of lakes. Piitter (1909) has made careful studies of 
the food requirements of the smelt and herring, which he expresses in terms of copepods. 
He states that the smelt needs the following numbers of copepods daily during its first 
season: May 6 to 29, 124 (1 mg.); May 29 to July 29, 248 (2 mg.); July 29 to September 
25, 496 (4 mg.). The herring needs: July 13 to 30, 3,300 (26.6 mg.); July 30 to Sep- 
tember 20, 6,080 (49.9 mg.); September 20 to November 15, 4,470 (38.8 mg.). To keep 
in good condition the smelt would require 100 to 500 copepods daily during its growth 
period, and the herring 3,000 to 6,000. 
Little is known concerning details of digestive processes in fishes. Denis (1912) 
has measured the amount and composition of urine given off by sharks and goosefishes. 
Knauthe (1898) states that the amount of nitrogen given off increases as the temperature 
of a fish rises. Greene (1914) has made interesting studies of the utilization of fat by 
the salmon during its migration. Piitter (1909) found that a goldfish would change the 
contents of its intestine more often if peristalsis was artificially stimulated by suspending 
fine sand in the water. He also analyzed the substance in the carp and found it to 
contain: Water, 78.85 percent; dry substance, 21.16 per cent; albumen, 17.38 per cent; 
fat, 2.57 per cent; ash, 1.22 per cent; nitrogen, 2.91 per cent. Kmnauthe (1898) carried 
out extensive feeding experiments with carp. He states that, when no protein was fed, 
carp slowly became unable to digest pure starch, 10 times the usual amount of nitrogen 
being given off in the excreta. However, if protein was fed after a long period of exclu- 
sive carbohydrate diet, starch was again normally digested. Old fish did well when fed 
nothing but rice meal, and Knauthe believed this was because the proteins in the gonads 
were utilized. Fish died on an exclusive diet of lean meat. Ability to digest starch 
was lowered by deficiency of minerals in the diet. Ptitter (1909) states that it was 
impossible to rear smelt, herring, or carp when they were fed nothing but small crus- 
tacea. About the only generalization that can be made from the facts reviewed is that 
some variety is apparently necessary in fish diet. 
In the present work no essentially new contribution has been made to digestive 
processes in fishes, except for the points already reviewed relating to amounts consumed 
and the rate of digestion at different temperatures. These observations agree with those 
of Knauthe (1907) and Fibich (1905). One other interesting fact was noted. During 
