18 NORTHERN FISHES 



with steep bottoms are relatively low in production, while shallow, or 

 only moderately deep, fertile lakes with gently sloping bottoms are 

 relatively high in production. The shape of the shore line also influences 

 the total production in a lake. Since the most food is produced in the 

 shallow waters near the shores, a lake with an irregular shore line and 

 many bays has much more productive area than a lake with a smooth 

 shore line and few bays. 



The fertility of a lake is not inexhaustible. It is conceivable that a 

 lake may become reduced in fertility just as soil may become drained 

 from overfarming. Though we have no evidence that such a reduction 

 in fertility has taken place to any serious extent in our lakes, it is only 

 good conservation practice to watch for and avoid any undue drain 

 on the fertility of our lakes. 



No matter how fertile a lake may be, the oxygen conditions some- 

 times prove a limiting factor in the production of fishes. Fishes, like all 

 living things, must have a constant supply of oxygen in order to live. 

 Oxygen depletion during a severe winter or as the result of excessive 

 vegetation may make it impossible for fish to survive in an otherwise 

 suitable lake. 



The oxygen fishes use for breathing is dissolved in the water, just as 

 salt or sugar may be dissolved. Water can hold only a small amount of 

 oxygen, and this dissolved oxygen is the only source available for the 

 respiration of fishes and most other water animals. The dissolved 

 oxygen in water comes from two main sources — aquatic plants and 

 the atmosphere. All green plants release oxygen as a by-product of 

 photosynthesis, the process in which they take in carbon dioxide or 

 carbonic acid gas and manufacture starch for plant food. This process 

 is the natural way of disposing of the carbonic acid gas produced by 

 decomposition and the respiration of animals and plants. Photosyn- 

 thesis takes place only in the presence of sunlight. 



Water plants, from the big weeds to the microscopic algae which 

 constitute the green scum, liberate oxygen on days when the sun shines. 

 On a bright day one can observe many streams of tiny bubbles ascend- 

 ing through the water from the leaves of a healthy water plant. These 

 bubbles are pure oxygen. They become smaller as they ascend, and 

 many disappear before reaching the surface because they dissolve com- 

 pletely in the water as they ascend. The water over a weed bed on a 

 bright day is usually supersaturated with oxygen, while on a cloudy 

 day or at night this same water may become very deficient in oxygen 

 because the plants cannot liberate this important gas in the absence of 

 sunshine. Even in summer a lake may occasionally suffer oxygen deple- 

 tion when there are long periods of cloudy weather without any wind 

 action. 



The only other source of oxygen in a lake is from the air above the 



