220 THE SCIENTIFIC FEEDING OF ANIMALS 



planted with aquatic plants (Ceratophyllum or hornwort, Myriophyl- 

 lum or water milfoil, Ranunculus or crowfoot). These are started in 

 pure water which is inoculated with the necessary lower forms of plant 

 and animal life. The clarified sewage water is not added for several 

 weeks. 



In the management of sewage fishponds putrefaction processes must 

 be kept in absolute check by suitable supplies of pure water and oxygen. 

 The oxygen content of the pond water must also be controlled. For 

 carp the water should contain 3 c.c. of oxygen per liter of water. The 

 presence of only 0.5 c.c. per liter causes asphyxiation of the carp. 



According to Hofer, the approximate oxygen content of water is easily determined 

 in the following simple manner : 



Take a flask with accurately fitted ground stopper and % liter capacity and fill to 

 overflowing with the water to be examined. Insert immediately (closed and down- 

 ward) a 2 c.c. glass tube filled with a 25 per cent solution of manganous chlorid. In 

 a similar manner insert a second tube filled with a 30 per cent solution of caustic 

 soda containing 10 per cent of potassium iodid. Keep the water flask stoppered at 

 all times except at the moment when each tube is inserted. Observe care that no 

 air bubbles are retained in the flask! Shake the flask for several minutes so that 

 the contents of the glass tubes become thoroughly mixed with the water in the 

 flask. A dirty white precipitate of manganous hydroxid is formed. The latter has 

 the property of combining with oxygen and forming a brown precipitate of pyro- 

 lusite (Mn02). According to the amount of oxygen in the water, this precipitate 

 assumes different shades of brown. If the water contains only 0.5 c.c. of oxygen 

 per liter (carp asphyxiate under this condition) the precipitate remains white. If 

 the water contains only 3 or 4 c.c. of oxygen per liter (this is just sufficient to 

 prevent asphyxiation of Salmonidae — trout and allied species) the precipitate be- 

 comes yellow. If the water contains 7 to 8 c.c. of oxygen to the liter (normal oxy- 

 gen content) the; precipitate becomes a pronounced brown. The Bavarian Bio- 

 logical Experiment Station for Pisciculture, in Munich, has issued a color scale to 

 aid in recognizing and valuing the different shades of color in this test. This simple 

 method of oxygen determination can, of course, also be applied successfully in the 

 problem of live fish transportation, wintering fish under ice, etc. 



Two-year-old carp are best adapted for stocking sewage ponds where 

 table fish are desired. In Strassburg, Koenigsbrunn and Bruenn the 

 gain per hectare of pond surface is estimated at 1,200 pounds. DemoU 

 also recommends stocking with younger carp and artificial breeding. 

 Rainbow trout and tench (Tinea vulgaris — European species allied to 

 carp) may be planted as secondary crops. These fish are exceedingly 

 palatable, but tench should be placed in fresh water ponds or traps for 

 some time before marketing, which should also be done under other 

 conditions. 



The extensive establishments at Strassburg and Amberg, in addition 

 to the sewage ponds above described, are provided with fresh-water 

 ponds, spawning ponds, reservoirs, etc., with all facilities for convenient 

 access in winter. An extensive duck-breeding establishment is also 

 maintained and considered of great importance in connection with fish 

 culture in sewage ponds. These ponds become infested with duckweeds 

 (species of Lemna) which mukiply rapidly and often cover the entire 

 water surface with their growth, shutting out light and air, and keeping 

 the water too cold. These plants are a favorite food of ducks. 



