418 
BULLETIN OF THE BUREAU OF FISHERIES. 
Many of the observations just mentioned refer to European rivers and lakes. 
They show, however, that the question of the food of fishes must be studied sepa- 
rately for each species. Besides, the various constituents of the plankton, even that 
belonging to one taxonomic group, differ greatly in their nutritive value. Brandt 
(1898) determined that the dried substance of Copepoda contains protein, 59 per 
cent; chitin, 4.7 per cent; fat, 7 per cent; carbohydrates, 20 per cent; and ash, 9.3 per 
cent. Knauthe (1907) analyzed two common water fleas — Sida and Bosmina — and 
found their dried substance to have the following composition: Sida, 53.3 per cent 
protein, 7.6 per cent fat, and 21.5 per cent ash; Bosmina, 72.4 per cent protein, 8.2 
per cent fat, and 17.4 per cent ash. 
The recent investigation of C. Juday (1922) shows that crude protein constitutes 
more than 50 per cent of the dry weight of plankton algae., whereas in large aquatic 
plants its amount varies from 10 to 20 per cent. In animals the crude protein con- 
stitutes from 36 to 64 per cent of the dry weight of the plankton Crustacea and from 
35 to 69 per cent in the larger forms, the maximum percentage being noted in the 
leeches. 
It is obvious that the value of the various plankton constituents as nutritive 
material is very different. If the estimation of the productive capacity of a given lake 
should be based on the determination of its resources of food available for fishes and 
other animals of commercial importance, the solution of the problem would require 
extensive special investigations. The amount of food material required for each 
separate species of fish should be determined, as well as the chemical composition of 
various plankton and bottom organisms. Evidently such an estimation is next to 
impossible at the present stage of our knowledge. 
The average content of plankton in the water, however, may be regarded as an 
indicator of the productive capacity of a lake or pond even if the plankton-eating 
fishes are entirely absent. Phytoplankton and zooplankton form the middle links 
of the chain of food relations existing in the water. At one end of this chain are 
gases and mineral salts dissolved in water and at the other end are found fishes, mus- 
sels, and other organisms forming the food for carnivorous aquatic animals. Even if 
the plankton in a given case is not consumed by the adult fishes it constitutes the 
principal food of bottom organisms, and consequently the fish resource in that lake 
depends, though indirectly, on the amount of plankton. Moreover, the food of 
young fish as a rule is composed of plankton. 
A possible error in using the quantitative study of plankton for determining the 
productive capacity lies in the method itself. The plankton samples collected by 
filtering the water through bolting silk do not represent the total amount of organ- 
isms suspended in water. A considerable part is lost by leakage through the silk. 
Kofoid (1903) found that catches made by filtering the water through filter paper 
show the presence of an average amount of plankton 3.3 times greater than the vol- 
ume of the catches taken by the silk net. The use of filter paper instead of bolting 
silk, does not provide a satisfactory volumetric method because of the great increase 
in the proportion and quantity of silt found in filter-paper catches. The collecting 
of plankton by centrifuging can be applied to the study of the microplankton only 
and is invalid for collecting Rotifera and Crustacea because of the small volume of 
water that can be studied. Therefore, up to the present time filtering through bolt- 
ing silk remains the best method, and in spite of its defects is widely used in limno- 
logical investigations. 
