Carrying Capacittj and Standing Crop 63 



Fertilization. Fisheries literature contains listings of many censuses of 

 fish populations made through the draining of ponds and lakes or through 

 the use of rotenone. Many of these censuses have been republished by 

 Carlander ^^ in his compilations of growth and population statistics. A 

 great deal has been published on the increase in the standing crops of 

 fishes resulting from the use of various fertilizers.^' ^^' -^' ^^' ^^' ^" These 

 studies furnish evidence that various inorganic and organic fertilizers 

 introduced into ponds will temporarily increase the standing crop of most 

 fishes, although there is evidence that one species may be benefited 

 through the use of fertilizer to a much greater extent than another 

 inhabiting the same water.-^ 



A pond fertilized for a period and then left without the addition of 

 fertilizer will show a reduced standing crop of fishes 3 or more years after 

 fertilization is stopped."^ This indicates that some of the fertilizer is no 

 longer available, either because it has been washed out of the pond or 

 has become bound up in insoluble compounds in the pond bottom. How- 

 ever, if the standing crop of fishes, even though reduced, is still not as 

 low as it was prior to the beginning of fertilization, this indicates that 

 fertilizer in an available form has accumulated in the pond bottom. This 

 accumulation of fertilizing materials may go on as a natural process 

 (even if a pond or lake owner adds no organic or inorganic fertilizers) 

 through the death of plants and animals in the pond, through the ac- 

 cumulation of dust and leaves blown in from outside sources, and through 

 the addition of nutrients leached from the soils of the pond watershed. 



Chemical Basis for Fertility. Moyle ^- demonstrated a positive rela- 

 tionship between the presence of varying amounts of certain chemicals 

 (total phosphorus, total nitrogen, and total alkalinity) in the surface 

 waters of Minnesota lakes and the poundage of fishes supported by those 

 lakes, although this could be expressed as a direct relationship only in the 

 case of total phosphorus. 



Kinds of Fishes. Standing crops of fishes vary greatly on the basis of 

 the kinds of fishes making up a population and the relative abundance 

 of each of several kinds ( Figure 4.2 ) . Standing crops of fishes in Illinois 

 ponds varied from 75 pounds per acre in soft-water-Ozark-hills ponds of 

 southern Illinois where the population was largemouth bass and green 

 sunfish, to 1100 pounds per acre in a black-soil-flood-plain pond in central 

 Illinois where the population was composed of crappies and bigmouth 

 buflFalo. In Iowa the standing crops of fishes ranged from 28 to 1235 

 pounds per acre.^^ Where the standing crop exceeded 300 pounds, usually 

 bullheads or buffalo were present. Populations of bluegills usually ex- 

 ceeded 100 pounds per acre. In Kentucky poundages ranged from 200 to 

 1000 pounds per acre in unfertilized ponds. ^^ 



A number of ponds have been censused one or more times, and these 



