AGE AND GROWTH OF THE CISCO 
287 
Before entering into the discussion of the relationship between growth rate and 
the environmental factors that may affect it, attention should be called to the fact 
that the order of the four lakes with respect to growth in length is not the same as 
their order with respect to growth in weight. Although the Muskellunge Lake 
cisco shows better growth in length than the Silver Lake cisco, its growth in weight 
is inferior to that of the Silver Lake population. The reason for this reversal of 
order lies in the very poor condition of the Muskellunge Lake fish. Weight un- 
questionably furnishes a better measure of increase in living matter than does length, 
but for the purposes of the present discussion it will probably be sufficient to consider 
both populations merely as intermediate between the extreme conditions represented 
by the Trout Lake cisco and the Clear Lake cisco. 
PHYSICAL-CHEMICAL FACTORS 
Certain factors may be ruled out immediately as inadequate for the observed 
differences in the growth rates of the four populations. 
Temperature fails to account for the observed differences in growth rate. Since 
all four lakes are located within a short distance of each other the climatic conditions 
that affect each of them are essentially the same. Differences in the size and form 
of the various basins may lead to differences in average water temperature during 
the summer, but here it should be pointed out that while the cisco finds its coldest 
summer habitat in Trout Lake and Clear Lake, the populations of these two lakes 
represent the extremes in growth rate. 
Oxygen conditions also fail to explain the differences in the growth rates of the 
four populations. Here, as was the case with temperature, Trout Lake and Clear 
Lake resemble each other most closely. Each possesses large masses of well oxy- 
genated water in the hypolimnion, the favorite habitat of the cisco. 
The abundance of bound C0 2 and the closely related hydrogen-ion concentration 
and conductivity are the only physical-chemical characteristics known to show any 
correlation with growth rate. Growth rate in length and the abundance of bound 
carbon dioxide stand in an inverse relationship to each other. It is hardly reasonable 
to assume that an abundance of bound C0 2 impedes growth directly or that a scarcity 
of bound C0 2 accelerates it. The effect of the abundance of C0 2 on growth rate is 
probably indirect and operative through its modification of the biological nature of 
the cisco’s environment. 
In general, the amount of bound C0 2 in a lake’s waters is roughly indicative of 
the biological productive capacity of that lake. In view of this fact it would hardly 
be expected that the poorest growth of the cisco would occur in the lake with the 
greatest concentration of bound C0 2 . This apparently paradoxical situation is 
explained, however, if it is assumed that an abundance of bound C0 2 makes not only 
for a greater production of food organisms, but also makes for a much greater abund- 
ance of the ciscoes themselves, and that the abundance of the ciscoes in turn de- 
termines their growth rates. Such an assumption has the support of the observed 
fact that the cisco population is most dense in Trout Lake with the greatest amount 
of bound C0 2 and sparsest in Clear Lake with the least amount of bound C0 2 , while 
Muskellunge Lake and Silver Lake with intermediate concentration of bound C0 3 
have populations of intermediate densities. 
