J 
358 IttiNno1s NAaturAL History SurvEY BULLETIN 
This represents approximately the mini- 
mum size for table use. For example, in 
the bluegill this minimum has been set at 
6 inches total length or about 0.25 pound; 
in the crappie, 8 inches or about 0.30 
pound. A suggested definition of a good 
fishing lake is one which produces an aver- 
age catch of one fish of desirable size per 
man-hour of effort during the fishing sea- 
son, and one which will maintain this rate 
year after year. Swingle & Smith (1938) 
in their Management of Farm Fish Ponds 
and in other publications (1939; 1941, 
1942) say little about maintaining sus- 
tained yields, except through periodic 
draining and restocking. 
The fish manager should have some 
knowledge of the carrying capacities of his 
lakes, and the kinds of fish that are present, 
as well as the intensity of fishing pressure. 
He can assume through experience some 
reasonable poundage figure (based on nor- 
mal increment of the fish and carrying 
capacity of the lake) for cropping each 
lake, to serve as a guide in the yearly har- 
vest of fish. Only by controlling the fish- 
ing pressure within a minimum range 
around the actual optimum yield can he 
have any assurance of sustained good fish- 
ing. Moreover, he should have the ability 
not only to recognize, but to control, cer- 
tain outside influences which may limit or 
upset his planned yield. For example, the 
introduction of rough fish, such as carp 
and buffalo, through use of bait minnows 
may in time so restrict the size and abun- 
dance of hook-and-line species as greatly 
to reduce the yearly catch. 
The importance of rough fish in limiting 
good fishing in artificial lakes has often 
been underestimated or overlooked. When 
carp or buffalo gain entrance into these 
lakes, they greatly influence the success of 
more desirable kinds. A change in the 
status of the game-fish population may 
come about through disturbance during the 
spawning period of the nest-building fishes, 
or rough fish may cause (through greatly 
increased turbidity) such a modification of 
the environment that it becomes unsuitable 
to the game and pan fish. Roily artificial 
lakes in Illinois invariably contain many 
rough fish or bullheads and small numbers 
of stunted crappies and sunfish. In lakes 
containing large populations of rough fish, 
additional stocking of centrarchids will 
not improve fishing. When the rough fish 
V ol. 22, Art. 3 
are entirely removed, the lakes become 
clear, and there is a marked improvement 
in size, condition and numbers of the 
game-fish population. ; 
In some waters poor fishing may be due 
to an abundance of so-called forage fish, - 
those species that are popularly supposed 
to serve as food for the desirable game fish. 
How these limit the numbers of more de- 
sirable fish is unknown, but crowding and 
food competition may be important factors. 
Popular misconceptions have led Illinois 
fishermen or their organizations to insist 
upon the introduction into artificial lakes 
of several species of valuable fishes that are 
not suited to this environment. The most 
common fishes of this classification are the 
smallmouth bass and white bass. Other 
important fishes somewhat less commonly 
stocked are the walleyed-pike, northern 
pike, lake trout and muskellunge—the lake 
fishes of the north. The warm water and 
low oxygen concentrations seasonally char- 
acteristic of artificial lakes insure the early 
death of such fish. In a few instances in 
which they managed to survive for a time, 
they were unable to reproduce successfully. 
There are, however, a few species of 
warm water fish apparently well suited to 
artificial lakes; these include largemouth 
bass, crappies, bluegills and bullheads. Up- 
on these the Illinois lake fisherman must 
depend for angling. Yet problems arise 
when these fish are used, because of their 
great reproductive potential and the lack of 
natural predators. Stunting is the most 
common cause for poor fishing in artificial 
lakes stocked with these species. Although 
the largemouth bass is the most predatory 
of these fish, it seems unable to control 
populations of bluegills and crappies with- 
out the assistance of heavy hook-and-line 
fishing. 
Evaluation of Species 
More than 40 species of fish were found 
in 22 artificial lakes of Illinois in the 
course of censusing their fish populations. 
While most of these lakes furnished little 
or no fishing for a time previous to renova- 
tion, several represented average fishing 
waters, and a few were producing excel- 
lent catches. Past yield and stocking 
records are vague for many of these lakes. 
Such information may be gathered from 
lake owners or fishermen, but at best it is 
