FEEDING FISH 221 



Sewage fishponds serve the additional function of purifying the water. 

 They accomplish this more effectively than any other method, especially 

 the irrigating system of sewage disposal, and require considerably less 

 area than the latter. One hectare of sewage pond area suffices for the 

 disposal of the sewage of a population of 2,000, while one hectare of 

 irrigation surface is required for every 250 inhabitants (Darmstadt 

 180, Berlin 240, Magdeburg 440). All other systems of sewage dis- 

 posal have the objectionable feature of being unproductive or unre- 

 munerative. 



The annual sewage water of the city of Munich, after preliminary 

 clarification, contains — 



3,200,000 kg. of nitrogen, 

 577,000 kg. of phosphoric acid, 

 637,000 kg. of potash. 



It has been planned to conserve this waste by means of fish culture. 

 The fresh suspended matter or sludge contains — 

 400,000 kg. of nitrogen, 

 224,000 kg. of phosphoric acid, 

 110,000 kg. of potash. 



It has been proposed to mix this sludge with the coarser rubbish or 

 "mull" and use it for fertilizing, and thus reclaiming, the worthless 

 heath regions and moors of the country surrounding Munich (Garchung, 

 Froettmaning, Dachau, Schleissheim, Erding, Aubing) and converting 

 them into valuable agricultural land. 



It is evident that the utilization of sewage in fishponds has a promis- 

 ing future. 



B. Artificial Feeding 



An increase in the yield of fish from ponds, by means of artificial 

 feeding, is possible or practicable under certain conditions only. Feed- 

 ing pays only in cultivated ponds with a deficient flora. Ponds overrun 

 with weeds cause too much loss of feed. Again, densely shaded ponds 

 with low water temperature may affect the results of artificial feeding 

 to the extent of 50 per cent. Warm water and an abundant supply of 

 oxygen favor the effects of artificial feeding. Artificial feeding should 

 not be practiced with the water temperature below 57° or above 77°F. 

 Hence the maxim, when feeding for high production, to "feed with 

 thermometer in hand." Carp practically stop feeding at a temperature 

 of 8°C. (46.4°F.). In winter, therefore, food should be given sparingly 

 and only sufficient to prevent starvation, or as much as they will eat. 

 Excess of food, beyond what is actually consumed, only serves to be- 

 foul the water. The warmer the water, the more active is metaboli- 

 zation in the body of the carp, more food is consumed and more di- 

 gested. At the same time the requirement for oxygen is raised. This 

 is an important point to consider. In order to avoid losses from lack 



