4. MISC. PUB. 528, U. S. DEPT. OF AGRICULTURE 
concerned about the loss of fish through pond outlets and believe it 
necessary to screen the outlet. Such losses, however, are seldom 
serious, and obstructions to overflow water are likely to cause damage 
to the dam. Special spawning beds and shelters are unnecessary. 
Pond fish will spawn in adequate numbers without the aid of special 
facilities. The design of dams and spillways is best determined by 
an engineer or by reference to instructions to be found in such publi- 
cations as Low Dams (9), Stock-Water Developments (5), Fish for 
Food From Farm Ponds (4), and others (7, 7). 
Rooted aquatic plants, either emergent or submerged, must not be 
permitted to become established. These plants and any brush or 
debris should be removed soon after they appear. Submerged aquatic 
plants may be killed by applying either of the commercial fertilizers 
described later, directly over the plants, at monthly intervals during 
the winter. The addition of fertilizer at this time of the year, at 
least in the Southeast, results in a heavy growth of filamentous algae 
which shades the plants and causes them to die (/5, 16). The dead 
weeds will decompose and contribute to the fertility of the water. 
Rapid decomposition of a large quantity of aquatic plants may reduce 
the oxygen content of the water to the point where fish will suffocate. 
Therefore, the application of fertilizer should be discontinued when 
most of the plants are decaying. ‘This method has been successful 
in the control of various species of Wajas, Potamogeton, Nitella, Chara, 
and Myriophyllum. 
Ponds with only a small overflow, or none at all, will prove most 
economical to operate since from these only a small amount of fer- 
tilizer and plankton will be lost. Ponds with continuously muddy 
water have a low natural productivity since sufficient hght for the 
growth of phytoplankton (microscopic water plants) cannot pene- 
trate the water. The productivity of these ponds cannot be profitably 
improved by fertilization. 
FISH FOOD 
All animals derive their food, directly or indirectly, from plants 
because plants convert inorganic mineral matter into living tissue. 
The mouse eats the grass, and the fox eats the mouse. In water, the 
same conditions exist, but here it is the microscopic plants and animals 
known as plankton, instead of the large rooted plants, that are the 
source of food. In a pond the insect eats the plankton, the bluegill 
eats the insect, and the bass eats the bluegill. This relationship is 
sometimes called a food chain. 
Plankton is the assemblage of minute, often microscopic, plants 
and animals that live in water. Collectively, the plants are known 
as phytoplankton and the animals as zooplankton. The phytoplank- 
ton consists mainly of algae which include the diatoms, desmids, green 
aloae, and blue-green algae. It is these organisms that tint pond 
waters green or brown, make the green scum on the surface, and 
cause the effect known as water bloom. Bacteria are also part of 
the phytoplankton, although they usually are quantitatively unim- 
portant as food for other organisms. ‘They are responsible, however, 
for the decomposition and conversion of dead organic matter into 
simple chemical compounds available for plant growth (28). Since 
algae convert into organic matter the carbon dioxide, water, and min- 
