250 
In 1949, 1,027 bass were returned to 
the lake following the March census, 
table 5, about 25/ of which were between 
9 and 10 inches in length. The lake area 
by June 1 was only 11 acres. An attempt 
had been made to remove all bluegills in 
March. After the census, the bass popu- 
lation, which had developed in 18 acres 
ot water with a large available food supply 
in the form of crayfish, bluegills, and 
aquatic insect nymphs, was concentrated 
into 11 acres of water containing a much 
reduced food supply. Bass caught through- 
out the summer were thin, and many of 
those less than 10 inches long in March 
were still below that length when caught 
in July and August. The bass spawn was 
relatively small; the estimate was 24,000 
try (2,182 per acre, table 10). 
In 1950, small bluegills were again 
abundant, most of them having originated 
from the 1949 spawn of those fish that 
had remained in the lake basin during the 
1949 draining operation. The estimated 
population of small bluegills was 2,878 
per acre; this was comparable to the 
number present in 1946 and more than 
twice the number present in 1948, table 
10. However, in 1948 the large bluegills 
numbered 299 per acre as compared with 
58 in 1950. In 1950, as in 1948, the 
survival of bass to the fry stage apparently 
was very low. 
The number of bass replaced in the 
lake after the 1951 census amounted to 
29 large fish per acre, plus an additional 
16 per acre that ranged between 8 and 10 
inches long, a total of 45 per acre, table 5. 
The bluegills replaced averaged 36 large 
fish per acre. The large warmouths re- 
placed averaged about 3 per acre and the 
small warmouths 31 per acre. Small war- 
mouths were replaced in an attempt to 
build up the population of these fish. In 
this year the estimated number of bass fry 
produced per acre amounted to more than 
1,700, table 10. 
Several factors related to density of 
population appear to have influenced the 
production of largemouth bass spawn. 
During the years 1941 to 1944, inclusive, 
when the fish population was essentially 
one of largemouth bass alone, the spawn 
estimate decreased each year without any 
apparent relationship to the number of 
bass of spawning age present in the lake. 
Ituinois NATURAL History Survey BULLETIN 
Vol. 26, Art. 2 
Later, when bluegills were present, the 
years of good spawn production were those 
in which the bluegill population (particu- 
larly of small fish) was greatly reduced at 
the time of the spring draining census 
(1945, 1947, 1949, 1951). ‘There is 
strong evidence here that a large bluegill 
population may control the survival of 
the spawn of bass (1946, 1948, 1950). 
If this relationship of bass with a crowded 
bluegill population were allowed to con- 
tinue over a period of years, the bass 
population might become gradually smaller 
because of the lack of adequate replace- — 
ment for the adults lost. Interference — 
with spawning and attacks on bass eggs 
and fry by small fish (of other species as 
well as the bluegill) may be the most im- 
portant single factor responsible for poor — 
bass populations and poor bass fishing in — 
Illinois lakes. 
The level of survival of young bass 
throughout the first year of life may not 
be correlated very closely with the level 
of spawn production. In 1942 and 1943, 
moderate numbers of young were observed, 
but age determinations made from scales 
removed from bass taken by fishermen and 
from bass handled during the draining 
censuses in later years indicated that very 
few fish of the 1942 and 1943 broods sur- 
vived the postfry stage. In 1946, the num- 
ber of bass fry was relatively small, but 
quite a number of fish from the brood of 
this year appeared in the catches of later 
years, in spite of the fact that this brood 
could have been eliminated entirely 
through draining operations of March, 
1947. Bass of the 1946 brood that ap- 
peared in the catches of 1947 and 1948 
and as unmarked fish in the 1949 drain- 
ing census must have remained in the 
lake basin or in the feeder stream above — 
the lake during the 1947 census. 
How often must a successful spawn be 
produced to maintain a largemouth bass 
population that approaches the maximum 
a body of water will support? At Ridge 
Lake the bass population was maintained 
at a high level throughout most of the 
10-year study period by relatively good 
spawns in most odd-numbered years 
(1941, 1943, 1945, 1947, 1949, and 
1951) alternating with relatively poor 
spawns in most even-numbered years 
(1944, 1946, 1948, 1950), table 9. In 
