found that 1.0 p. p.m. would kill goldfish. Field tests in fish 

 ponds and bio-assays on pond fishes with this chemical apparently 

 have not yet been carried out. 



Biological Control of Aquatic Vegetation 



Species of Tilapia have been experimented with (Swingle 1957, 

 1958, 1960) in this country to determine their potential for algae 

 and weed control, as well as food and sport-fish production. In 

 Alabama ponds, effective filamentous algae and weed control, as 

 well as outstanding fish production, has been obtained by stocking 

 eighty, 4-7 inch Tilapia per acre in April, or two thousand 1-2 

 inch finger lings per acre in June, for the production of this fish. 

 Smaller numbers of stocked fish would be required in ponds already 

 stocked with bluegills and bass. 



These fish are sensitive to cold weather, and Tilapia mossambica 

 is killed when the temperature descends as low as 48°F. They may 

 be wintered-over in troughs in heated rooms, as has been done at 

 the School of Fisheries at Auburn University, Alabama. 



CONTROL OF WATER- BLOOM ALGAE 



The water-bloom algae are microscopic, free-floating plants 

 which create turbidity in the water, making it appear green, brown, 

 or blue-green in color. Fish ponds are fertilized to stimulate 

 their growth. These algae are usually desirable in shallow lakes, 

 because the turbidity they create by their growth and multiplica- 

 tion tends to shade out the coarser weeds, thereby maintaining more 

 space for the growth of fish. Many species of midgeflies, which 

 are a staple food of fish, cannot become abundant on the bottom 

 where muskgrass and coarse weeds have become very dense. 



Water-bloom algae, usually blue-greens, sometimes become a 

 problem in the larger lakes of fertile agricultural areas or where 

 sewage effluents, or other wastes, have abnormally increased the 

 natural productivity of the water. In Iowa, for example, most of 

 the kills of livestock, waterfowl, and fish brought about by 

 excessive growths of blue-green algae have been traced to very 

 abundant growths of Anabaena f los - aquae or Aphanizomenon f los - aquae 

 (Rose, 1953). However, a number of species of algae can create 

 hazards to domestic stock, fish, and wildlife under certain con- 

 ditions (Ingram, 1954) . 



In fish ponds, water-bloom algae sometimes become so abundant 

 the fish are killed during the night due to exhaustion of the 

 oxygen supply. During the daylight hours, plants give off oxygen 

 as a by-product of their photosynthetic activity, but at night 



