February, 1943 
and-line catch. Abnormally large or small 
broods tend, therefore, to make fishing un- 
certain. A successful method of cropping 
must be adjusted to cover these variations. 
The average length of life of important 
pond fishes is variable but, in Illinois, 6 
years 1s near the maximum for most spe- 
cies. Hansen (ms. unpublished), working 
on the fish of Lake Decatur, reports that 
dominant broods of crappies and gizzard 
shad practically disappear in 4 years. 
Largemouth bass sometimes reach the age 
of more than 10 years, but few attain this 
age in the warm waters of artificial lakes. 
Cropping for high yields, year after year, 
involves a consideration of the number of 
broods making up a population. Theoreti- 
cally these broods (for convenience we 
will use six) can be arranged to form a 
pyramid of numbers, such as is illustrated 
in fig. 3. The brood of the year, forming 
the base of the pyramid, is the largest. 
The smallest and oldest brood forms the 
apex of the pyramid. 
It has been estimated by Dr. Thompson, 
BENNETT: MANAGEMENT OF ARTIFICIAL LAKES 
367 
in his work on the bottomland lakes of the 
Illinois River, and by Dr. D. F. Hansen, 
Assistant Zoologist of the Illinois Natural 
History Survey, in his study of Lake De- 
catur, that the normal or accidental death 
rate (exclusive of angling) of fishes re- 
duces the numbers of any given brood 
during the earlier years about one-half 
within a year’s time. If this is true, and 
if spawning is equally successful in all 
years, at any given time the number of 
individuals belonging to a selected brood 
is twice that of the brood spawned in the 
preceding year, and one-half as numerous 
as the one of the following year. As the 
natural death rate of fish 6 or more years 
old is very high, cropping should remove 
all of these older fish, and a fraction of 
each of the other broods, amounting to at 
least one-fourth of their total numbers. 
Some means should be devised for remov- 
ing a portion of the fingerlings less than 
a year old and yearlings still too small to 
be of value for food or angling, fig. 3. 
As the crop is being taken, the fish re- 
Fig. 2—A farm pond built by the U. S. Soil Conservation Service in Schuyler County, III. 
Measures outlined by the S.C. S. for by-passing barnyard sewage around the lake were not 
followed and the first fish stocked died from lack of dissolved oxygen. Later, 
fencing and 
planting of the slopes and by-passing most of the runoff from the barn lot so improved water 
conditions that fish could live. 
