266 
The program of marking and censusing 
has made it possible to trace the reduction 
in numbers of each marked group through- 
out the period of the experiment. ‘The 
435 fish of the original stock had been re- 
duced to 17 by the end of the first 2-year 
period, table 20. As 2 of these fish were 
killed in the draining operation, only 15 
were available for restocking the lake 
after the census. 
All 15 could be accounted for at the 
end of the next 2-year period (1943- 
1945): + had been caught by fishermen 
and 1 was found after it had died, evi- 
dently as a result of a hooking injury; 
10 were recovered in the 1945 census and, 
of these, 1 was injured in the census and 
9 were replaced in the lake after the 
census. These 9 were fish that had been 
brought from Lake Chautauqua. At the 
beginning of the 1945-1947 period they 
averaged 3.9 pounds each and were prob- 
ably 7 to 10 years old; during the period 
3 were caught, and only 1 was recovered 
at the time of the 1947 census. Thus, 
the unaccountable loss in this period was 
5 (55.6 per cent) in contrast to no un- 
accountable loss in the preceding 2-year 
period. The fish recovered in the 1947 
census weighed 6.75 pounds when re- 
turned to the lake in 1947. It was re- 
captured (7.0 pounds) at the time of the 
1949 census and was caught by a fisher- 
man (6.5 pounds) in July of 1949. Scales 
collected at the time of capture indicated 
that this bass was 10 years of age. 
Of the 1,500 marked bass of the 1941 
and 1942 broods or year-classes that were 
returned to the lake following the 1943 
census, +6+ were taken in the 1944 creel 
and 543 were retaken in the 1945 drain- 
ing census, leaving +93 to be accounted 
for, or a loss of 32.9 per cent. In the next 
2-year period (1945-1947), 496 were re- 
turned to the lake, and of these 337 were 
caught in 1945 and 69 in 1946. Sixty- 
three were taken in the 1947 census, leav- 
ing 27 unaccounted for, or a loss of only 
5.4 per cent. 
During the 1947-1949 period, the num- 
ber of bass of the 1941 and 1942 broods 
was reduced by all causes from 61 to 23; 
the unaccountable loss was 6 fish, or 9.8 
per cent of the number returned to the 
lake in 1947. Only 4 of the 23 bass re- 
turned to the lake following the 1949 
Intinois NatuRAL History SurvEY BULLETIN 
Vol. 26, Art. 2 
census were recaptured in the 1951 census. 
Ten of these 23 fish were caught by fisher- 
men in 1949, none in 1950; 9 were un- 
accountable losses. 
The 1941-1942 group is of particular 
interest because of its changes in unac- 
countable losses from 1943 to 1951. In 
1945, the losses in this group for the 
1943-1945 period were figured as 32.9 
per cent. In March, 1943, these bass 
averaged 0.17 pound each; by 1945, they 
averaged 0.77 pound; and, by 1947, 2.44 
pounds. In the 1945-1947 and the 1947- 
1949 periods the unaccountable losses 
were 5.4 and 9.8 per cent, respectively. 
In the 1949-1951 period, when the bass 
of this group averaged between about 4 
and 5 pounds each, the unaccountable 
losses were 39.1 per cent. These fish were 
approaching an age of 10 years and it 
seems reasonable to believe that natural 
mortality may have been responsible for 
much of the relatively high unaccountable 
loss in the 1949-1951 period. 
When the reduction in numbers of bass 
in the 1941-1942 year-classes that resulted 
from fishing and all other causes over the 
period 1943-1951 is compared with a 
hypothetical 50 per cent annual reduction 
in numbers, the regressions are found to 
be quite similar, table 21. 
In the Ridge Lake study, the 1941- 
1942 year-classes constituted the only 
group of fish large enough and old enough 
to give satisfactory figures covering the 
approximate life span of bass. The less 
comprehensive data from 1945-1946 and 
1947-1948 groups failed to follow the 
reduction pattern of the 1941-1942 group, 
and it seems likely that the similarity of 
the regression pattern of the last-named 
group to a 50 per cent annual reduction 
pattern may be coincidental. 
Exploitation Rates 
The rates at which the populations of 
marked bass were exploited by anglers are 
shown in table 19. As it was impossible 
to assign instantaneous times of occur- 
rence to the unaccountable losses that were 
brought to light when the lake was 
drained at the end of each 2-year period, 
the exploitation rate shown in table 19 — 
for any one fishing season is a simple per- 
centage derived from the number of bass — 
