Sewage Pollution and Fertility 43 



is for all animals, a progressive decrease in the favorability of the en- 

 vironment associated with a progressive increase in population density, 

 until growth and reproduction arc inhibited.-"' Hey ^^ noted that when the 

 two indigenous species of alga-eating tilapias (T. mossambica and T. 

 sparrmani) were released in equal numbers in South African sewer ponds, 

 T. sparrmani eventually disappeared. If a few T. mossambica were placed 

 in a population of T. sparrmani, the former disappeared. Neither of these 

 species can be considered as primarily predatory, but both will eat small 

 fishes when they are available. 



The biological domination man exerts over most artificial lakes not only 

 upsets interrelationships of aquatic organisms, but enters the picture in 

 other ways, most commonly, perhaps, in water pollution from silt, from 

 organic waste, and from chemicals. These pollutants are damaging to 

 fishes in relation to the capacity of the recipient environment to absorb 

 tlieir eJBFects without itself becoming greatly changed to the detriment of 

 fish populations. Of the three types of pollutants, silt and chemicals are 

 almost uniformly undesirable, while sewage pollution from organic waste 

 may represent a mixed benefit: Organic sewage increases production once 

 certain demands that it makes upon water are met. 



SEWAGE POLLUTION AND FERTILITY 



Some ponds and lakes receive sewage runoff from septic tanks, over- 

 loaded tile disposal fields, and domestic-stock feed lots, or effluent from 

 primary or secondary sewage works. In Europe, the use of municipal 

 sewage as fertilizer for fish ponds -^ is widespread, and cities as large as 

 Munich dispose of most of their effluent in this manner. Rainbow trout 

 and carp from sewage-fed ponds are very acceptable as food in Germany 

 and other countries. Detailed descriptions of the methods used in the 

 propagation of fish in sewage-fed oxidation ponds are given by Kisskalt 

 and Ilzhofer ^^ and by Wundsch.*^ In this country some interest has been 

 shown in the development of oxidation ponds for disposing of the sewage 

 of small communities. Also, many stock farmers are building ponds close 

 to cattle barns and hog houses, so that animal waste can be piped directly 

 into them. Although fish cannot be raised in most of these ponds receiving 

 undigested sewage, they can be produced in supplemental ponds con- 

 nected with the former. 



Those interested in unpolluted streams, ponds, and lakes should be 

 made aware of the dangers associated with the predicted increase in 

 human population, even if current practices are followed in sewage dis- 

 posal. Some of these dangers will soon be apparent. 



A part of the fertilizer applied to crop lands is leached from the soil. 

 In some locations as much as 10 per cent of tlie inorganic phosphorus 



