262 
influence on the total exploitation for a 
given season was the relative abundance ot 
natural food. In no year in which natural 
food was “normally” abundant (had not 
been reduced through artificial means) 
did the catch of bass exceed 40 per cent 
of the available crop, either in numbers or 
pounds, even under a fishing pressure that 
in one season averaged more than 300 
man-hours per acre. It is improbable that 
greater fishing pressure would have had 
much effect upon yield under “normal” 
Ittinois NarurAL History Survey BULLETIN 
Vol. 26, Art. 2 
Unaccountable Mortality and Length 
of Life 
One of the more difficult problems in 
studying a population of fish is to de- 
termine the final disposition of individuals 
in that population. Marked fish that are 
caught by co-operating anglers and most 
of those that are accidentally killed in 
draining operations are accountable; so 
are fish that die from accidents or from 
hooking injuries and are found floating 
Table 18—Number and weight of available largemouth bass per acre, catch of bass per — 
acre, fishing pressure, and abundance of bass forage in the form of small fish, Ridge Lake, 
1942-1950. Calculations involving acreage were based on a water area of 18 acres except — 
where stated otherwise. 
| 
| : 
AVAILABLE AVAILABLE Bass ; 
aires a Roce Carcu or Bass PER ACRE : 
FIsHING 
FISHING | PRESSURE, Per Per 
SEASON | : Man-Hours rae A wae oe Cent 
‘ | Weight, | Per Acre ma ma Jeight, Co) o 
N umber! Pounds Bass Bluegills Number Pounds | Avail- | Avail- 
! able able 
Number| Weight 
1942.... 290! 6323- 212 Low None 27 15.1 9 24 
19443... . 90 | 54.62 | 90 Very low |Moderate 26 15.0 29 27 
1945... .| 36 28.74 105 High ow 23 19.8 64 69 
1946... .| 87 46.1? 168 Very low |Very high 20 14.6 23 3a 
19472223) 22 27.04 155 Moderate |Low 10 10.9 45 40 
1948....| 149 | 76.0? | 320 None Very high 36 25.6 24 34 
1949. ...| 57 44.74 | 220 Low Very low 33 30.0 58 67 
19495... 93 13.28 361 Low Very low 54 49.2 58 67 
1950... 73) | 62.6 224 Verylow |Veryhigh| 29 18.3 40 29 
1 Only about 24 legal-sized bass per acre. Fishermen 
included fish of less than legal size. 
2 Weight estimate based on actual catch plus weight of 
of spring following. The estimate is too high, because no 
result of cropping. 
® Ridge Lake was not opened to public fishing in 1943; 
* Weight estimate based on weight of bass actually put 
(except possibly that for 1949) because no allowance was made for weight gains between the time bass were returned 
the n In 1949, bass may have decreased in weight because they were crowded. 
5 Figured on the basis of a lake area of 11 acres, the actual area for the fishing season. 
to the lake and the time they were caught. 
food conditions. But in 1945, for example, 
when food conditions were probably con- 
siderably below “normal,” a fishing pres- 
sure of 105 man-hours per acre resulted 
in a total exploitation of 64 per cent of 
the number and 69 per cent of the weight 
of available bass. Thus, the culling tech- 
nique that was associated with the bi- 
ennial draining censuses at Ridge Lake 
not only allowed the fish replaced in the 
lake to grow rapidly to sizes satisfactory 
to anglers, but it also increased the rate 
of catch and the yield in numbers and 
pounds. 
took only legal-sized fish; Natural History Survey catches 
catchable bass (about 6 inches or more in length) in census 
adjustment was made for compensatory weight increase as a 
public fishing in 1944 was begun in August. 
back into lake following census. Estimate is too low 
on the water. However, in the Ridge 
Lake study, many marked fish simply dis- 
appeared from the lake. The disappear- 
ance of the fish may be attributed to one 
of the following causes: (1) natural mor- 
tality of fish that were unobserved after 
they had risen to the surface of the water 
or were torn apart before decay had pro- 
gressed sufficiently to cause them to float, 
(2) escape of fish from the lake by way 
of the surface spillway in time of high 
water, (3) injury to fish during draining 
and censusing operations, followed some 
time later by death and failure of th 
