Fish Population Adjustment 167 



There has been a tendency on the part of some aquatic biologists to 

 over-simpHfy the problem of pond fertilization and to consider results 

 obtained under some conditions to be universally meaningful. ^^^ Actually 

 die problem of fertilization of waters is so complex that it is difficult to 

 duplicate results from one pond to another, to say nothing of duplicating 

 results from one research station to another. 



There are dangers inherent in the fertilization of any eutrophic lake by 

 any means.^^- ^^^ Hasler and Einsele ^^ cite the changes that have taken 

 place in Lake Snake, Vilas County, Wisconsin; Pontiac Lake, Michigan; 

 Lake Okoboji, Iowa; Sylvan Lake, Noble County, Indiana; and Stadlsee 

 near Waldsee in Wiierttemberg, Germany. They also describe the pos- 

 sibility that fertilization may upset an efficient natural food chain for 

 one that is much less efficient. "For example, in a natural lake, a rich 

 growth of Cyclotella, a small diatom, fulfills the ideal food requirement 

 of Daphnia, but fertilization might encourage previously rare or non- 

 existent algae which are not adapted at all well as food for Daphnia, 

 while the desirable form, Cyclotella, is suppressed." 



Swingle and Smith ^^^ demonstrated that by applying inorganic ferti- 

 lizer to ponds in the proper amount they could increase the standing crop 

 of bluegills from 130 pounds per acre to between 300 and 500 pounds 

 per acre. These results have not been demonstrated in Michigan,^^ in 

 Indiana,^^ in Illinois ^- or in any other part of the United States outside 

 of the southeastern states. 



In ponds in some of the least productive soil types in Illinois the addi- 

 tion of recommended amounts of inorganic fertilizer increased the average 

 standing crop of fish by about 1.22 times.^- The improvement in fishing 

 was such that uninformed fishermen could not tell which ponds were 

 fertilized and which were not; yet in terms of total yield, rate of catch, 

 and average size, the fertilized ponds produced considerably better blue- 

 gill fishing than did unfertilized control ponds. In contrast, the controls 

 usually produced a higher yield of bass 10 inches or larger than did the 

 fertilized ponds. 



The fertihzation of ponds and lakes cannot be recommended as a 

 general fish management technique outside of the southeastern United 

 States, because the results are too variable and uncertain. Once the 

 fertility of small impoundments in productive soils has been built up, this 

 fertility may manifest itself in luxuriant annual crops of filamentous algae, 

 bluegreen algae, or rooted aquatic vegetation. There are already numer- 

 ous examples of such ponds, most of which are quite productive of fish; 

 but they are problem waters because a treatment to kill rooted vegetation 

 will be followed by obnoxious blooms of algae which in turn may require 

 chemical treatment. These lakes have reduced aesthetic values, and fishing 

 and swimming are hmited by plant growths of one type or another. 



