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BULLETIN - OF THE BUREAU OF FISHERIES. 
ing the waters. Therefore, measurements of the total amount of plankton have often 
been applied to determine the food resource in the pond where propagation of carp 
and other fishes was under consideration. 
Many such determinations of the productive capacity of ponds were made in 
Germany and in the western part of European Russia, where the propagation of 
carp was very common. Consequently, the great part of the observations in the 
matter pertain to carp ponds. A special scale for estimating the productive capacity 
of ponds has been worked out by Walter (1905). According to his scheme the esti- 
mation of the productive capacity of a pond must be based on the determination 
of the volume of zooplankton. Walter recognizes the three following types of 
ponds: (1) Ponds of low productive capacity, in which the amount of zooplankton 
does not exceed 5 cm. 3 per cubic meter. (2) Ponds of medium productive capacity, 
in which the amount of zooplankton varies from 5 to 15 cm. 3 per cubic meter. (3) 
Ponds of high productive capacity, with zooplankton content from 15 to 50 cm. 3 
per cubic meter. 
It has been observed that the Rotifera and Copepoda are more abundant in 
ponds of low productive capacity, whereas the Cladocera are more numerous in 
ponds rich in plankton. 
The practical experience of fish-culturists dealing with the artificial propaga- 
tion of fish in ponds has discovered various methods which may be used to increase 
the productive capacity of ponds. It has been found that the production of plank- 
ton can be considerably increased if the pond is drained and its bottom allowed to 
overgrow with vegetation. When several months later the pond is again filled 
with water the zooplankton develops in greater abundance. Another method, 
successfully applied to increase the production of zooplankton, consists in throw- 
ing various kinds of soil fertilizers into the ponds. Knauthe and Zuntz (see Knauthe, 
1907) have made many laboratory experiments and field observations in studying 
this question and have proved that the amount of plankton in the ponds consider- 
ably increases after adding to the water a certain quantity of different fertilizers. 
The determination of the amount of plankton has thus been applied as a basis 
for estimating the productive capacity of ponds. Obviously the problem is com- 
paratively simple when one deals with a small pond where the fish population 
consists of one species. All the factors, such as the number of fishes, their feeding 
habits, their average size, the amount of plankton, and other conditions, can be 
easily observed and taken into consideration. 
In a natural lake, however, we are dealing with many factors that can not be 
accurately determined. First, the fish population consists of various species with 
different feeding habits, some of them being carnivorous while others feed largely 
upon vegetable matter. Second, all features of the lake, such as depth and character 
of bottom and shores, exert a great influence on the organic life of the lake. There- 
fore the determination of the productive capacity of a natural lake is a more com- 
plicated problem than the estimation of the productivity of a small pond. 
There arises the question: To what degree may the amount of plankton in 
the lake be used as an indicator of its productive capacity ? This topic has been dis- 
cussed for a long time in the limnological literature. Some limnologists — as, for 
example, Schiemenz (1905) and Zander (1903) — are of the opinion that plankton 
