Internal Workings 147 



is one of the most difficult compounds in the world to break 

 down. To dissociate the one part of oxygen from the two 

 parts of hydrogen requires elaborate chemical processes. But 

 water exposed to air absorbs oxygen from it, and this oxy- 

 gen penetrates to the depths of the sea. It is this absorbed 

 oxygen which the blood-vessels in fish's gills take from the 

 water. 



Fishes vary in their oxygen requirements. The trout, for 

 instance, needs much more than the carp, which is why the 

 trout like swift streams while the carp can live in a mud- 

 puddle. The trout's fninimum requirements are about 3.5 

 parts per million by weight,^ but while it can exist under 

 such conditions, it shuns water with less than 5 parts per 

 million. The Federal hatchery at Leetown, West Virginia, 

 has 10 parts per million, the Klamath River the same. This 

 is pretty close to the saturation point, which is to say that 

 the water has absorbed just about as much oxygen as it can 

 possibly hold. Man, even if equipped with the gills of a fish, 

 could not live in water, for he requires fifty times as much 

 oxygen as a fish his own size. Air, with its 210 parts per 

 thousand, is none too good for him. 



Other things being equal, the colder the water is the 

 more oxygen it can hold — ^which is one of the reasons why 

 trout prefer cold water. The maximum temperature at which 

 the brook trout can live comfortably is 70° F., the brown 

 trout 76°, the rainbow slightly higher. Cold water being, 

 within limits, heavier than warm, it sinks, which is why 

 trout tend to stay on the bottom in hot weather. Exceptions 

 to this are certain inland lakes in which, on account of their 

 protected position, or their depth, there is very little circu- 

 lation in summer. In the hot weather the surface becomes 

 warm and light, the lower water cold and heavy. Two dis- 



^ 2.5 cubic centimeters per liter by volume. 



