22 



FISH FARMING 



chapter. In arranging the depth of the falls at j and H and, 

 for that matter, any other falls in any other position in any 

 other fishery you must be acquainted with the fact that a. fish 

 cannot surmount them, and consequently cannot mount up- 

 stream, unless the depth of water at the foot of the fall is equal 

 to the height of the fall. This rule is not without exception, 

 but it may be applied generally, and especially in connection 

 with the construction of redds. If the stream is subject to 

 such heavy floods as would undoubtedly result in the redds 

 being covered entirely with flood water flowing back against 

 the less strong current emanating from the channels E E E, 

 then abandon any thought of constructing protected redds in 



Fig. 6. Method of Protecting Redds in Bed of River. 



that position. Similar advice applies to the breakwater M; 

 under normal conditions it is not required at all, but should 

 the flood be sufficiently strong to bring down, heavy debris it 

 should be erected in order to protect the inlets of the channels 

 E E E ; and if the little valley holds al roaring torrent, sweep 

 ing all before it at flood time, then the breakwater (or any 

 other contrivance) is useless, and the position is not suitable 

 for constructing redds. At spawning time you will probably 

 get your shaire of fish on the redds (of course, the screen F is not 

 in position until you have a sufficient number of spawning fish 

 on the redds), but, should they prefer the channel c c, you 

 must place some obstruction to> them mounting (preferably, a 

 screen at the lower end of the channel), which must be re- 

 moved at any sign of flood, or after spawning operations are 

 over, and you care little whether fish mount the channel 

 or not. 



