AGE AND GROWTH OF THE CISCO 
257 
the growth of the kleine Marane and stated that he had found the same situation 
in the smelt (Wilier, 1926). 
In view of the numerous demonstrations by other investigators of a relationship 
between density of population and growth rate in fishes it was considered advisable 
in this study to investigate the relative densities of the four cisco populations with 
particular reference to a possible relationship between these densities and the observed 
differences in the growth rates of the various stocks. 20 
An important source of difficulty in the study of the relationship between popu- 
lation density and growth rate lies in the fact that the estimation of the density of a 
fish population, either absolute or relative, is at its best difficult and in a large measure 
uncertain. Ordinarily the chief sources of information are the results of experimental 
fishing and the statistics of the commercial yield. In the present investigation 
dependence must be placed entirely upon the size of the catch per unit of effort 
as based on the action of experimental gear (gill nets). In this connection some of 
the chief difficulties and sources of error in the gathering and the interpretation of 
data on the gill-net catches should be mentioned. 
Gill nets fish from a fixed position , and their effectiveness is dependent upon the 
movements of the fish. If the amount of movement varies according to time, locality, 
or population, such variations will be reflected in the catch of the net. The effect 
of the movements of the cisco is complicated by a tendency for the fish to shoal or 
move in groups. Group movements cause a great variation in the catch from day 
to day from nets set in exactly the same location. 
Further difficulty lies in the fact that the catches of the same net or the same 
group of nets in different lakes are not always directly comparable. 21 For example, 
the seven mesh sizes of the “new" gear employed in collecting the samples for the 
later part of this investigation failed to take equally adequate samples of all the age 
groups in the different populations. The Clear Lake catches probably represent 
reliable samples of the I group and all older groups as well. In Silver Lake and 
Muskellunge Lake, however, the I groups cannot be considered adequately repre- 
sented, while in Trout Lake the representation of the II group as well as the I group 
are to be considered too small. The comparison of the Clear Lake catches with 
those in which the I and II groups are absent or poorly represented would of course 
lead to a too high estimate of the relative density of the Clear Lake population. 
It must be recognized also that the fishing effort represented by a series of nets 
varies from one lake to another, for obviously no effort can be accredited to a net 
that is wholly ineffective for the capture of the members of a particular population. 
On the other hand, in a population with a wide length range two or three sizes of 
mesh may be necessary to capture a sample directly comparable to the catch of a 
single mesh size in a population with a more compact size range. Because of these 
difficulties it is not possible on the basis of gill-net catches to derive “index numbers” 
that can be taken directly as reliable measures of relative densities in populations of 
greatly dissimilar size ranges. In general, however, the examination of the catch of 
each size of mesh in each lake will, in spite of these aforementioned difficulties, give 
a fairly good idea of relative population densities in different lakes. 
!0 It is realized that competition for food is not at all confined to competition among individuals of the same species. With the 
possible exception of the Muskellunge Lake cisco population, however, the competition of other species is probably of insignificant 
importance in this study. At present there are available few data on the degree of competition between the Muskellunge Lake 
cisco and the perch that occur abundantly in the same regions of the lake. (See the section, Fishes Associated with the Cisco.) 
!l For a more detailed discussion of the action of gill nets see the section, “The selective action of gill nets.” 
