138 
BULLETIN OE THE BUREAU OE FISHERIES 
The question may arise as to how the addition of fertilizers increases fish produc- 
tion. The effect is, as already indicated, indirect. In general, the first step is through 
the plant life of the pond. The production of animal matter anywhere depends in the 
last analysis on photosynthesis or, in other words, on plant growth. The plant 
growth in turn depends on the energy derived from the sun, the carbon dioxide of 
either the atmosphere or of the water, and the presence in solution of suitable forms of 
nitrogen, phosphorus, potassium, etc. In other words, the amount of plant growth 
is limited by the amount of sunshine and the availability of those elements that go 
into the making of plant tissue, and also by the presence of those elements which, 
although they do not appear as constitutents of plants, are yet necessary to bring 
about proper growth. Whenever one or the other of these factors — that is, sunshine, 
carbon dioxide, nitrogen, phosphorus, etc. — becomes exhausted, plant growth ceases. 
The element that becomes exhausted and thus causes a cessation of plant growth 
is called a limiting factor, for it is those elements that either become exhausted 
completely or become reduced to concentrations too low to be effective that determine 
the amount of growth. Through the use of proper fertilizers all the elements, except 
sunshine, that enter into the process of plant growth may be intensified; and, provided 
enough sunshine is present, an increase in plant growth may be expected. This 
increase in plant growth should in the end mean an increase in fish production. The 
second step is through the organisms that feed on the plants and which, in turn, are 
consumed by the game fish. Among these intermediate organisms may be mentioned 
certain small Crustacea like copepods and cladocera, the immature stages of some 
aquatic insects, and herbivorous forage fish. 
In the case of fish ponds, one difficulty arises; namely, that certain plants may 
grow that are not available for fish food except in so far as they eventually die and 
decay and become fertilizer. In the fish ponds we are, therefore, more particularly 
interested in the algae, and the production of fish may be expected to be more closely 
related to the production of algae than to the total plant growth. 
In the practice of pond fertilization, the assumption has quite generally been made 
(see review of literature, p. 140) that phosphorus and nitrogen are the only elements 
that have to be supplied by the fertilizer, and that the other necessary elements are 
present in sufficient quantities. In a few instances the assertion has even been made 
that only phosphorus becomes exhausted and that this element is the limiting factor. 
One object of this paper is to present data to show that the addition of various 
fertilizers to the pond water increases the growth of the plankton algse (phytoplank- 
ton) and likewise increases the production of copepods, cladocera, and rotifers (zoo- 
plankton) — organisms that feed directly or indirectly on phytoplankton. Hence, 
there are presented in this paper the results of quantitative studies on the plankton of 
ponds that were fertilized and also of control ponds that were not fertilized. Along 
with the data on plankton counts and volumes (in the case of the net plankton) 
are presented data showing the amount of organic matter in the water — the organic 
matter in the bodies of the plankton organisms as well as the unorganized organic 
matter suspended in the water— that can be removed with an electric centrifuge 
(see section on methods). Both sets of data show that the addition of fertilizer 
has a beneficial effect. 
Another object of this paper is to present the results of a series of chemical 
determinations that have been made on the pond water. The reason for making 
these determinations is obviously as follows : If we are going to fertilize ponds intel- 
