288 
BULLETIN OF THE BUREAU OF FISHERIES 
DENSITY OF POPULATION 
The section on the relationship between density of population and growth rate 
inclined toward the opinion that differences in the severity of competition for food 
related to differences in the densities of the various populations are largely responsible 
for differences in the growth rates of the four cisco stocks. This opinion was pre- 
sented as the most logical, even in the face of the facts, first that the actual existence 
of any competition for food in any one of the four populations remains to be demon- 
strated, and second that there exists a strong possibility that crowding may impede 
growth independently of competition for food (Willer’s “Raumfaktor”). 
As to the causes of the observed differences in the densities of the four populations 
it may be seen, as was mentioned previously, that the greatest density of the cisco 
population occurs in the lake with the heaviest concentration of bound C0 2 (Trout 
Lake) while the least density occurs in the lake with the lightest concentration of 
bound C0 2 (Clear Lake). The positions of Silver Lake and Muskellunge Lake are 
reversed with respect to the concentration of bound C0 2 and the density of population, 
but both are intermediate to the conditions found in Trout Lake and Clear Lake. 
Since it is generally held that the success or failure of a hatching of fish depends 
primarily on conditions that determine the survival of the young at a very early stage, 
it does not appear unreasonable to hold that differences, traceable to the concentration 
of bound C0 2 , in the amount and land of plankton available to newly hatched ciscoes 
may account for the observed differences in the densities of the four populations. A 
study of the plankton cycle in each lake should throw light on the subject. 
There does not appear to be any complete correlation between the amount of 
organic matter in surface samples of plankton taken during the summer and the den- 
sity of the cisco populations. Although Clear Lake with the least amount of organic 
matter in the surface plankton has the sparsest cisco population, Trout Lake, with the 
densest cisco population does not have the greatest amount of organic matter in the 
surface plankton. 
It is possible further that these differences in the densities of the cisco populations 
may depend on yet other factors such as the availability of suitable spawning areas 
or the destruction of eggs and young by predators. 
Regardless of whether it is held that differences in growth rate depend directly 
on differences in population density or that both are dependent on yet other factors, 
it must he admitted that growth rate and density of population show a very close 
correlation. 
LENGTH OF THE GROWING SEASON 
In the general section under this title it was pointed out that differences in the 
growth rates of the four cisco populations can be explained in part by actual differences 
in the length of the cisco’s growing season in the various lakes. The Trout Lake 
cisco has definitely the shortest growing season, followed by the Muskellunge Lake 
cisco, the Silver Lake cisco, and the Clear Lake cisco in the order named. Thus it 
may be seen that in the four populations the length of the growing season follows the 
same order as their growth rates (in weight) and the inverse order of their densities of 
population. 
These differences in the length of the growing season cannot be accounted for on 
the basis of temperature and oxygen conditions for the same reasons that tempera- 
ture and oxygen conditions fail to account for the differences in growth rate. (See 
p. 287.) 
