366 Ittinois NATURAL History SURVEY BULLETIN 
by-passing of barnyard drainage, so that 
it cannot flush directly into the pond, and 
adequate planting and protection of steep 
hillsides adjacent to the ponds, fig. 1. In 
two instances, however, adequate by-passes 
were not built, and hillsides were not prop- 
erly protected by the lake owners; as a 
V ol 2 2y Aes 
size before they live out their life span. 
Therefore, it is obvious that the popula- 
tion level should be held at a figure below 
the maximum carrying capacity of the 
lake. In natural waters, especially in those 
away from human habitation, a continuous 
reduction of the fish population may come 
Fig. 1—A farm pond in Adams County, IIl., built by the U. S. Soil Conservation Service. 
Although located below a barn lot, this pond is protected from yard drainage by a levee. No 
loss of fish from pollution has ever occurred here. 
consequence, fish were unable to survive 
in the lakes, fig. 2. 
The observations of Swingle & Smith 
(1939) in Alabama ponds, and the fish 
yields for several years from Fork Lake 
(see section following), indicate that a 
lake once stocked with fish approaches its 
maximum carrying capacity within a single 
year. However, lakes in regions with a 
short growing season, and large reservoirs 
stocked with small numbers of fish, may 
require two seasons. 
‘The carrying capacity of an individual 
lake may vary with the kinds of fish intro- 
duced ; a lake reaches its maximum capaci- 
ty when the most efficient use is made of 
its food resources. 
As a fish population multiplies and its 
total weight approaches the maximum 
carrying capacity, the growth rate of the 
fishes gradually decreases until actual 
stunting begins. Unless measures are 
taken to reduce the population, conditions 
unfavorable to growth will continue, and 
the lake will produce few fish of desirable 
sizes. A rapid growth rate must be main- 
tained in order to produce fish of large 
about through predators. ‘These predators 
are largely absent in artificial waters. 
From age analyses of the complete fish — 
populations of several lakes, it has been 
found that the broods of fish of the several 
kinds present vary in numbers from year 
to year. Creel census records over a period 
of years likewise show fluctuations in the 
abundance of the various kinds of fish 
caught. ‘These fluctuations in brood sizes 
are related not only to the success of 
spawning but, more especially, to the sur- 
vival of young fish during their first sea- 
son. Fluctuations in the numbers of young 
fish surviving after their first season are 
a source of trouble to the fish manager. 
If large broods escape severe losses during 
the early months of life, their members 
may grow rapidly until their food require- 
ments begin to tax the food resources of 
the lake. “Then they grow very slowly or 
become stunted. On the other hand, very 
small broods may have little food competi- 
tion and make rapid growth to desirable 
size, but they may not be numerous enough 
to bear the fishing load when their mem- 
bers should make up the bulk of the hook- 
