368 ILtino1s NATURAL History SurRvVEY BULLETIN 
maining in the lake are growing rapidly 
and gradually moving up in their relative 
positions in the pyramid of numbers. The 
effect of controlled cropping is to hold the 
total weight of fish present at a level some- 
what lower than the maximum carrying 
capacity of the lake. This cropping insures 
an abundance of food at all times, which 
in turn supplies the nourishment necessary 
for rapid growth. Meanwhile, the popu- 
lation, composed of individuals covering 
a wide range of sizes, is making efficient 
use of the food resources. The food re- 
sources of a lake are utilized most efficient- 
ly when the largest possible percentage 
goes to produce new flesh and the smallest 
percentage to maintain metabolism in the 
fish population. If the lake is overpopu- 
lated it is conceivable that all food will be 
used to maintain metabolism. If it is un- 
derpopulated the fish may grow very rapid- 
ly but use only a small portion of the 
available food. Somewhere between these 
extremes is an optimum population in 
which the greatest amount of new flesh 
may be produced from the food resources 
of an area unit of water. 
Several experiments now in progress in- 
dicate that this type of cropping reduces 
Oe 
BROOD 
BROOD 
V ol. 22; 2072.9 
the numbers in all broods so that no single 
brood of any species may dominate the 
lake. From the standpoint of the angler, 
the fish grow rapidly, reach large sizes 
and are numerous enough to maintain good 
fishing. 
The illegality of taking small fish in 
many states makes the application of this 
cropping plan difficult. “Technicians should 
make arrangements within their states to 
demonstrate the effects of this method. 
Equal cropping of all broods in a lake 
assumes that all kinds of fish present are 
of equal value in fishing. Frequently this 
is not the case. Often fishermen are aware 
that a more valuable species is being re- 
placed by one of less value, but are at a loss 
as to what should be done. In these cases 
special effort must be made to crop the 
less desirable species much more heavily 
than the others, especially among the 
younger broods. 
In the artificial lakes of Illinois, under- 
fishing is a serious problem. As may be 
seen in fig. 3, underfishing takes a very 
small slice from the top of the pyramid of 
numbers. In a lake that has been under- 
fished for several seasons, only the oldest 
fish are of desirable size. In a lake that is 
PLANNED 
CROPPING 
Fig. 3——Pyramid of numbers representing a theoretical population of fishes in an artificial 
lake. The population is divided into six annual broods (including all species). Successively 
smaller sections of the diagram from the base to the apex indicate reduction in the numbers of 
fish in each brood from natural or accidental causes (exclusive of angling). In an underfished 
population, anglers remove only a small slice of the pyramid, composed of a fraction of the 
older broods. Overfishing removes nearly all of the older and larger fish, leaving principally 
young fish too small to be taken. A theoretical plan of cropping proposes the removal of all 
of the oldest brood, and a fraction of the other broods equal to approximately one-fourth of each. 
“4 
4) 
