16 U. S. BUREAU OF FISHERIES. 



stave pipe is used, however, it should carry its full capacity of 

 water, as otherwise it deteriorates rapidly. 



In installing a long pipe line the expense involved in carrying 

 it at an even grade over depressions or through high places is fre- 

 quently excessive, and it is usually better to follow the general 

 contour of the land over which the pipe is laid. This method, 

 however, has the disadvantage of reducing the flow of water through 

 increasing the length of the pipe. There is also danger of " air 

 pockets " forming at the highest points in such a line, greatly reducing 

 or even entirely cutting off the flow of water. To guard against 

 such a contingency, vent cocks should be placed at all such high 

 points. 



AERATION. 



Fishes, like all animals, require an abundant amount of air — 

 oxygen — for their well-being. Unlike land animals, however, they 

 do not obtain their supply of oxygen directly from the atmosphere, 

 but it is absorbed from the water passing over their gills, organs per- 

 forming a function similar to that of the lungs of land animals. It is 

 obvious, therefore, that water suitable for the maintenance of fislies 

 must carry in solution a sufficient amount of gaseous oxygen to meet 

 the requirements of the fishes in this respect. Water from springs, 

 wells, or sometimes deep lakes is quite deficient in this vital require- 

 ment, or such water may contain an excess amount of air or gases 

 inimical to fish life. The remedy in either instance is the same — 

 intimate contact with the atmosphere. Water readily absorbs 

 oxygen whenever it comes in contact with the atmosphere, and by 

 the same process it readily gives off any surplus oxygen or other 

 gases that it may contain. The necessity for ample aeration of the 

 water to be used in a hatching house has already been mentioned 

 as a consideration of first importance, and some of the devices by 

 which it is accomplished have been alluded to. 



Water from a brook or stream that has been torn to from by 

 dashing down a steep stream bed will be saturated with the life- 

 giving oxygen, but such water, after supplying 16 to 48 feet of 

 hatching trough space, will have lost a part of its oxygen and will 

 need further aeration before it enters another series of troughs. 

 As mentioned above, water from other sources may be entirely 

 deficient in this respect, and it is therefore desirable to resort to all 

 practicable means for the correction of such possible faults in the 

 water supply. 



If the hatchery site commands a fall of 5 feet or more, suitable 

 aeration may be accomplished by a series of miniature riffles in the 

 conduit outside the building. The broader and thinner the sheet of 

 water provided the more thoroughly will it be exposed to the air. 

 If it must fall through the air instead of flowing down the face of 

 a perpendicular board, both surfaces of the sheet of water will be 

 exposed to the air, thus doubling the effect. When circumstances 

 permit, it is best to aerate in the conduit, which, as already sug- 

 gested, should be made wide and open for that purpose. 



If sufficient aeration can not be accomplished outside the building, 

 much may still be done as the water enters. "Wliile an open water 

 supply trough in the hatchery is somewhat unsightly perhaps, it has 



