AGE AND GROWTH OF THE CISCO 
253 
While the Clear Lake cisco undoubtedly has an advantage over the other three 
populations in the length of the growing season, this advantage may not be as great 
as might at first be suspected. The autumn of 1931, in which year the length of 
the growing season of the Clear Lake cisco was estimated, was abnormally warm. 
The records 16 of the United States Weather Bureau station at Minocqua, about 6 
kilometers west of Clear Lake, show that the air temperature in September 1931, 
was 4.5° F. above the normal, while in October and November it was 3.0° F. and 
6.5° F. above normal, respectively. This warm autumn may have extended the 
growing season of the Clear Lake cisco beyond its normal limit. If this is true the 
growth beyond early September in normal years may not be as much as 20 percent 
or more of the season’s growth as tables 37 and 38 would indicate. 
If the length of the growing season in the different lakes is considered in relation 
to the growth rates of the various populations, it may be seen that poorest growth 
occurs in the lake with the shortest growing season (Trout Lake) while the best 
growth occurs in the lake with the longest growing season (Clear Lake). The two 
lakes (Muskellunge and Silver Lakes) in which the growth rate was intermediate 
also have growing seasons of intermediate length. Thus it appears that differences 
in the length of the growing season are important contributing factors toward differ- 
ences in growth rates. However, these differences in the length of the growing 
season can by no means explain entirely the observed differences in the amount of 
growth in the four populations. The growth of the Clear Lake cisco up to July, for 
example (tables 37 and 38), is well in excess of that found in Trout Lake cisco (table 
35) or the Silver Lake cisco (table 36) of corresponding ages at any time in the 
growing season or for the entire growing season. 
RELATIONSHIP BETWEEN DENSITY OF POPULATION AND RATE OF 
GROWTH 
It was stated in the introduction that these studies on the growth of the cisco 
have been made on populations from “type” lakes that were selected on the basis 
of their biologically productive capacities as estimated chiefly from the amounts of 
fixed carbon dioxide in their waters. It was hoped that the comparison of the growth 
of the various populations might yield information as to the extent and nature of 
the effect of the natural richness of the waters upon the growth rates of the fish popu- 
lations that live in them. The investigators who collaborated in planning and ini- 
tiating the studies of which the present one is a part harbored no delusions to the 
effect that the relationship between the growth of fishes and the environment is in 
any sense simple. It was nevertheless surprising to find early in the investigation 
of the growth of the cisco that the growth rates of the populations did not fall at all 
in the order of the estimated productive capacities of the lakes in which they occur- 
red, but rather that the reverse was true. Trout Lake with 18.7 parts per million of 
fixed C0 2 contains the slowest growing cisco of the four populations, while the cisco 
of Clear Lake, in which body there occur only 2.2 parts per million of fixed C0 2 , 
stands out by far as the fastest growing of the four stocks. In Silver Lake and 
Muskellunge Lake where the amounts of fixed C0 2 are intermediate in value (Sil- 
ver, 15.5 parts per million; Muskellunge, 10.0 parts per million) the growths of the 
cisco populations are also intermediate. 
It can hardly be concluded that an inverse causal relationship exists between 
the growth rate of the cisco and the fixed C0 2 content of the lake it inhabits. Rather 
18 Climatological Data of the United States by Sections ,vol. 18, 1931. 
