312 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
is the depth, which should be as considerable as the nature of the ground and the 
absolute necessity of occasionally draining the ponds will allow. Even if there is a 
heavy flow of water into the raising-ponds, they should not have a depth of less than 
1 meter (3.28 feet) ; and if it were not desirable to be able at all times to survey the 
bottom of the pond, through the water, throughout its entire extent, I would rather 
have the depth 3 than 2 meters (10 rather that 6^ feet). 
The bottom of the pond should slope gently toward the aperture for letting the 
water off. The enormous pressure of the water, which is necessary in a pond of this 
depth, makes it desirable not to dam up the entire quantity of water, but to obtain 
at least half the depth by digging. It will be found advantageous to construct, if 
possible, several ponds (say four) at one and the same time, having a depth of 1.55 
meters (4.92 feet) and a surface area of about 9 by 20 meters (29.5 by 05.6 feet). 
It is necessary that every pond be arranged so that it can be absolutely drained; 
and it is an advantage if every pond has its inlet of fresh water independent of 
others. But with two pairs of ponds the lower ones should also receive the water of 
the upper ponds. The slopes on the banks and the bottom of the ponds should be 
well planed, and all hiding-places, sticks, stones, etc., should be removed, to provide 
for dragging nets through the ponds, which at a later period becomes necessary. 
It is advisable to sow watercresses (the seed can be obtained from any seedsman) 
on the bottom and slopes of the pond immediately after it has been finished, and let 
in the water only about a week later, uidess it is preferred to leave the second pair of 
ponds dry, to serve as reserve for emergencies. Wherever, in intensive feeding, it 
becomes necessary to deprive especially the shy brook trout of every hiding-place, 
the watercresses, which will soon grow luxuriantly, will later furnish shade for the 
fish and homes for the small aquatic animals which are as much needed for food in 
these ponds as in natural ponds. Long bean-poles (say, four to every poud) are laid 
in the surface of the water at regular intervals and bunches of watercresses are 
thrown among them; the cresses freely take root in the water and can easily be 
removed before the pond is dragged. 
So far, I have only had reference to ponds dug out of the ground and have ignored 
the existence of raising-ponds constructed of boards or laid in with cement, such as are 
found in many places. These ponds, whose construction is very expensive, can be 
managed very easily, and the perpendicular sides afford good shade and a clear view 
of the entire pond. The growth of the fish, however, is very unsatisfactory in them, 
compared with ponds dug out of the ground (not to mention the smaller expense of the 
latter), and the excrements are not put to as good a use as in the earth-ponds, where 
the aquatic animalcula, which serve the purpose of removing refuse excrements, etc., 
thrive much better. I also think that trout make use of earth in digesting food. 
Other fish thau trout should of course not be kept in the ponds, and if possible only 
one kind of trout should be kept in each pond (char in the coolest ponds, brook trout 
in the warmer, and raiubow trout in the warmest). It is urgently recommended to 
introduce into the raising-ponds, at the very outset, swamp snails, small crustaceans, 
etc., to furnish the fish as much good natural food as possible and provide a means of 
clearing the ponds of the refuse of food and the excrements of the fish. Wherever 
fish are crowded together in a small space, and dead food is introduced, the natural 
equilibrium is disturbed, and this should be restored as far as possible by the intro- 
duction of live animalcula. At Sandfort there are, above and between the raising- 
ponds, special smaller basins dug out of the ground, which are thickly covered with 
