BULLETIN OF THE UNITED STATES FISH COMMISSION. 253 



of a pond so located as to receive its supply of water at one end and to 

 discharge it at the other are here indicated. 



Ponds should not be less than three feet in depth at their deepest 

 part — to insure the fish against being frozen in severely cold weather — 

 and should gradually lessen to a depth of one or two inches to provide 

 the shoals required for spawning. Small knolls and islands should be 

 removed, as they generally afford harbor for the enemies of carp. 



Foe Ponds in Tidal Regions. — The collector and drain ditches 

 t-hould be constructed and arranged as above, the collector being formed 

 at the lowest poiut within the embankment. The " overflow" may be 

 omitted, as the "flume" can be readily adapted to the purposes of car- 

 rying off any surplus water. As the flume requires constant attention 

 to insure its operation, the "overflow" should be retained, if practica- 

 ble, and be placed in firm ground. 



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Pig. 3. 



Fig. 3 represents a vertical section, and corresponding horizontal pro- 

 jection of the embankment B, showing the position of the flume F, and 

 its controlling valves, d d, together with that of the crib-work "0 C 

 0," which is constructed on the pond side, and is designed to support 

 the screens. 



The flume is placed, as before, on a level with the bottom of the col- 

 lector, and the valves are arranged for drawing the water from the pond, 

 at ebb tide. The inner valve has been slightly raised by sliding up- 

 ward its support g through the grooved trunnion a ; and the pressure 



