6 AQUATIC PLANTS IN POND CULTURE. 



methods outside the proper definition of pond culture.'^ In Europe 

 the case is not wholly similar; although in a few instances American 

 methods have been adopted, the term " pond culture "' usualW 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 CULTURE. 



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 manj^ 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 jDond culture, and 



« It may not be amiss here to point out tlie 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 opportimity 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. Ills 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. 



