82 limnology, water supply and waste disposal 



Algal Control and Production of Fish Food 



It has long been recognized by fish culturists that maximum fish pro- 

 duction is usually obtained with the aid of fertilization. Good results have 

 been obtained with inorganic fertilizers and with such organic fertilizers as 

 cottonseed and soybean meal. The aim of fertilizing with inorganic fertil- 

 izers is to produce a plant plankton crop. These minute plants are of 

 little direct importance as fish food, but by their growth and decay, organic 

 material is released into the water. This promotes the growth of water 

 bacteria and the smaller animals which are the food of fish. The role of 

 organic fertilizers is more direct. Such material is available for immediate 

 bacterial decay and may even serve directly as food for some aquatic 

 animals. The application of either type of fertilizer usually results in an 

 increase in both plant plankton and fish crops (Smith and Swingle 1940). 

 Because plankton and fish production are known to be related in ponds, 

 the question arises as to whether or not the elimination of part of the 

 plankton crop with copper sulphate will seriously affect fish production. 

 To answer this question, it is necessary to consider a more basic one. Is 

 there a direct and proportionate relationship between plant and animal 

 production in waters? 



Huff (1923) found that the growth of the plant plankton crop or its 

 reduction with copper sulphate had no effect on the population trends of 

 microscopic animals in Lake Vadnais near St. Paul. Pennak (1946) in 

 reviewing his work on the plankton of Colorado lakes concludes that "a 

 relatively low plankton crop . . . may support populations of grazers which 

 range from very low to very high and that dense phytoplankton popula- 

 tions are not necessarily associated with dense populations of grazers." 

 Similarly, in natural Minnesota ponds used for the rearing of pike-perch 

 from fry to fingerling size, a concentration of nutrients was found above 

 which additional natural fertility, and presumably the amount of organic 

 matter produced by that fertility, did not increase the yield. In a series 

 of such ponds, having an excess of calcium and nitrogen, those in which 

 total phosphorus concentrations were between .02 and .05 ppm yielded an 

 average 11.4 pounds of fingerlings per acre; those within a total phosphorus 

 range of .051 -.1 ppm yielded 71.2 pounds per acre; those within a range 

 of .11-.2 yielded 65.5 pounds, and those with a total phosphorus concen- 

 tration greater than .21 yielded at the average rate of 67.8 pounds per acre 

 (Moyle 1949). It appears, therefore, that above a certain optimum level, 

 added fertility and the production of organic material by algal growth 

 does not greatly increase the production of aquatic animals. Above an 

 optimum level, it is likely that other biological and spatial factors become 

 more important than basic food supply. 



The main purpose of pond fertilization is to make certain that enough 

 organic material is present for maximum fish production. To be sure of 

 this, ponds are fertilized heavily enough to raise an excess of algae. In 

 algal control with copper sulphate, the reverse is accomplished. Excess 

 organic producing power as represented by algae is removed by chemical 



