6 AQUATIC PLANTS IN POND CULTURE. 
methods outside the proper definition of pond culture.t In Europe 
the case is not wholly similar; although in a few instances American 
methods have been adopted, the term “pond culture” usually em- 
‘braces the rearing of trout by much the same methods as are in 
the United States pursued only with fishes that can not be artificially 
spawned—that is, the young trout may not be fed artificially, but 
often subsist in large part upon the natural food supply induced 
by culture of the ponds. 
IMPORTANCE OF AQUATIC PLANTS IN POND CTJLTURE. 
Since the young of the species of fishes to which pond culture is 
applied in the United States can not be successfully confined in 
the troughs or small ponds of the American trout breeder, and do 
not accept artificial food, they must depend for sustenance upon 
minute forms of animal life found in the waters and upon one 
another. At a very tender age they develop cannibalistic tendencies, 
and even where there is apparently an abundance of natural food 
they may reduce their own numbers 60 to 80 per cent within a month 
or six weeks from the time of hatching. It is therefore necessary 
in pond culture to provide not only sufficient natural food to satisfy 
the physiological requirements of the young fish, but, so far as pos- 
sible, an abundance which will divert them from the tendency to 
devour one another. 
Through the necessity for natural food, then, comes the primary 
importance of aquatic plants in pond culture. All animal life is 
dependent, directly or indirectly, upon plant life, the minute forms 
as well as many of the larger feeding directly upon plants, and the 
herbivorous species in turn serving as food for the carnivorous. The 
young fishes feed upon small crustaceans and other forms which are 
abundant only in an environment with abundant vegetation. Aquatic 
plants are therefore the food-producing agency in pond culture, and 
@It may not be amiss here to point out the distinction between trout culture by 
American methods and pond culture proper by reference to the procedure and the con- 
ditions at an American trout hatchery. 
Trout are not dependent upon natural food, and do not require a natural environment. 
It is customary to rear them in wooden troughs or in small rectangular ponds of earth, 
wood, or concrete, through which there is a constant flow of water containing no visible 
plant or animal life. The water supply may have come directly from a spring or from 
an artesian well; at many of the most successful commercial trout establishments in the 
United States the troughs and rearing ponds are supplied with water from artesian wells 
from 25 feet to 100 feet in depth. As the daily feeding of a large number of fish in a con- 
fined area necessitates frequent cleaning, any seeds or spores of vegetation introduced 
by the water supply have little or no opportunity to obtain a foothold. The trout fry 
will eat artificial food from the time the yolk sac has been absorbed, and by a judicious 
arrangements of troughs, tanks, or small ponds the trout raiser can maintain a very large 
number of fish within a comparatively small compass until they are of satisfactory size 
for distribution or for market. His dependence is artificial food or the artificial intro- 
duction of natural food, and without these means he would be powerless to conduct 
operations on an extensive scale. In American trout culture aquatic vegetation, So essen- 
tial in pond culture, is but a negative factor. 
