370 KEPOKT OF THE COMMISSIOISTEE OF FISHERIES. 



from this cause occur in nature, for supersaturation does not arise 

 suddenly and aquatic animals would avoid the reoions of excess in the 

 rare cases where access to them is possible. The possibility of inju- 

 rious or fatal excesses of dissolved air, especially in natural waters, 

 seems not to have occurred either to fish oulturists or biologists. 



Tbe two faults, excess and deficiency of air, are so correlated that 

 the same process of correction applies to each. The same exposure 

 to the air which aerates water with a deficienc}' of air deaerates water 

 with an excess of air. In superaerated water, such as that of the Woods 

 Hole aquaria, there may be a deaeration in the more complete sense; 

 both nitrogen and oxj'^gen are to be removed. But in hardly any case 

 does the term aeration apply in its complete signification. Oxj-gena- 

 tion alone is usually the strict meaning. In natural waters the term 

 deaeration likewise does not in most cases completely apply. Denitro- 

 genation alone is the stricter meaning. Oxj^genation, however, may 

 accompany denitrogenation, and thus water is in the broad and looser 

 sense aerated and deaerated at the same time and by the same process. 



When an actual case of air-supersaturated water confronts the fish 

 culturist or the management of aquaria, the practical measures to be 

 taken w^ill suggest themselves according to the source of the excess of 

 air. If a gravity plant supplied by pumps is in operation the whole 

 suction system is open to suspicion of leaks. Such leaks, of course, 

 give out no water but suck air, and are therefore not always easily 

 recognized. By stopping the pumps and removing the proper valve 

 the hydrostatic pressure may bo allowed to rest back on the suction 

 pipe and will speedil}^ develop the leaks if the pipe is exposed. If it 

 is underground they may not show readily, or at all. Ilepair of all 

 the leaks will completely remedy the difficulty. The suction pipes, 

 especially if wooden, may be beyond repair, in which case nothing but 

 a complete renewal will entirely prevent trouble. Pending this, local 

 deaeration may be practiced at each aquarium, pond, or trough sup- 

 plied with the water. For an aquarium a large pan with many per- 

 forations may be suspended above, the higher the better, and the 

 water delivered into this. If the exposure of the slender streams and 

 the splashing at the surface are not suflicient correction, the scale of 

 the device has but to be increased, most conveniently b}^ adding more 

 perforated pans. The great desideratum is sufficient fall in which to 

 expose the water. 



When the supersaturated supply is from springs or wells the condi- 

 tion is more serious. A radical correction is impossible, for the air, 

 or modified air, which causes the excess is deep in the earth and can 

 not be controlled. If, as is usually the case, there is no great difi'er- 

 ence of level between the rising water and the ponds, troughs, or 

 tanks in which it is used upon fishes, it is the more difiicult or impos- 

 sible to completel.y deaerate. The natural remed}^ is to use the water 



