[7] UTILIZING WATER BY FI8H- CULTURE. 1107 



and the lower part of the outflow pipe should be ou a level with the bot- 

 tom of this pit. To make the pit more durable, its sides are frequently- 

 lined with boards, and from it a ditch, gradually growing shallower, 

 should extend the entire length of the pond. In very large ponds 

 smaller ditches should start from the main ditch on both sides, forming 

 a sharp angle with it (Plate II, Fig. 5). 



When the water is drained oft", the fish will gradually retire to these 

 ditches and thence to the fish pit, where they can be caught with very 

 little trouble. As a protection against fish thieves, it is well to drive 

 into the bottom of the pond small posts at intervals of a few meters, 

 which should protrude 20 to 30 centimeters above the bottom, thus pre- 

 venting the use of drag nets. The best posts for this purpose are young 

 pine trees, which are freed from their branches and driven into the 

 ground at the thin end, a few centimeters of the branches being al- 

 lowed to remain round the top (Plate II, Fig. 6). If no pine trees can 

 be obtained, simple posts are driven into the ground and a number of 

 nails fastened to the top, whose heads, however, should not be very 

 sharp, to prevent the fish from getting hurt. 



The arrangements for the outflow of the water serve two different pur- 

 poses : first to drain the pond either in part or completely, and second 

 to take off' superfluous water caused by sudden rains or brought into the 

 poad from the ditches which supply the water. 



The first purpose is served by sluices and taps, and the latter by weirs 

 and flood ditches. 



In many of the older ponds one still finds sluices such as are used in 

 mills (Plate III, Fig. 7). In new ponds it is not advisable to introduce 

 • them, as they require a good deal of carpenter's work, and easily get 

 out of repair. They, moreover, have this disadvantage,that, even when 

 they are only partly opened, the water rushes through the opening 

 with full pressure corresponding to the depth of the pond, carrying 

 with it fish and other objects, and if the opening is made larger carry- 

 ing away parts of the bottom and sides of the outflow ditch. 



The tap outflow (Plate III, Fig. 8) consists of a pipe of wood, clay, 

 or masonry led right through the base of the dike, open at the end 

 which is outside the pond, but closed at the one projecting into the 

 pond. The width of bore of this pipe, which generally consists of a sin- 

 gle trunk of an oak or pine tree, is as a rule 25 to 30 centimeters. In 

 large ponds several such pipes are sometimes led through the dike side 

 by side. At the end of the pipe projecting into the pond one or sev- 

 eral meters, a conical hole of about 20 centimeters is bored in the top, 

 which may be closed by a tap of the same size. This tap is connected 

 with a strong pole which rises perpendicularly from the water. At 

 equal distances from this pole (about 30 centimeters) four square posts 

 are firmly driven into the bottom of the pond, which rise to an even 

 height above the surface of the water, and to which horizontal beams 

 are fastened by means of which the pole may be raised or lowered, and 



