48 U. S. BUREAU OF FISHERIES. 



tageously. It is advisable to give them as much room as is practica- 

 ble and to provide exercise for them several hours each day by lower- 

 ing the water level in the troughs to a depth of about 1 inch. The 

 extra overflow pipe previously mentioned is used for this purpose. 



REARING PONDS. 



Ponds for rearing trout from fry to fingerlings should be from 4 

 to 8 feet wide and of any desired length up to 60 feet, which, for 

 convenience in drawing off the water and feeding the fish, is about 

 the extreme limit. Between the advanced fry and No. 1| fingerling 

 stages it is of the utmost importance that the fish have several hours 

 of exercise each day. This can be provided for best by having the 

 bottom of the pond perfectly level across its width but with a slope 

 of about three-fourths of an inch to each 5 or 6 feet from the head 

 of the pond to the foot, with riffles 1 inch high at intervals (fig. 16, 

 p. 48). There should be a sufficient water supply t6 allow the riffles 

 to work effectively, and this will be evident from the formation of 

 water beads and air bubbles immediately beneath them. When the 

 water is drawn down to exercise the fish, it should be about 1| inches 

 deep above the riffle and one-half of an inch deep below the riffle. 

 It is very important that the pond have ample screen surface, other- 

 wise the water will dam and destroy the effectiveness of the riffles. 

 The screen may be placed vertically or inclined, and where it is possi- 

 ble a horizontal screen from 10 to 12 inches wide should be placed 

 level with the bottom of the pond in front of the vertical or inclined 

 screen, with a pit underneath for the reception of excrement. This 

 insures an even depth of water in the pond and affords an effective 

 means of ridding it of waste matter, which must be flushed out of 

 the pit every few days to guard against pollution. Under such an 

 arrangement the water supply enters the pond at the shallow end and 

 discharges at the opposite end at a point several inches below the 

 bottom of the pond. A successful means of preventing the fish 

 from entering the supply pipe is shown in Figure 16, page 48. 



STOCKING THE RIFFLE POND. 



A pond 60 feet long and 5 feet wide will safely carry from 30,000 

 to 40,000 fish of No. 1 to No. 3 fingerling size. Larger ponds may be 

 stocked with a proportionately larger number of fish. One-fourth 

 of the pond area should be shaded with a frame made of 1 by 21- 

 inch strips over which 2-inch poultry wire is stretched and then 

 covered with tarred roofing paper. This makes a cheap and satis- 

 factory cover, and with proper care it will last through several 

 seasons. 



Young fish in a riffle pond may be easily trained to take food. 

 Simply break up the material into small particles and allow it to 

 enter the pond with the water. As it passes over the riffles the fish 

 will seize it, and after having been fed a few times in this way the 

 food may be given to them by the usual method when the pond is 

 filled with water. 



