AGE AND GROWTH OF THE CISCO 
219 
the method is equally valid for the populations considered in this investigation. As 
will appear later, the results of the analysis of the growth material presented here 
seem to justify the assumption. Throughout this paper the individual ages are 
designated as the number of years of life that have been completed. Although the 
cisco spawns in late autumn or early winter, the life of the individual is assumed to 
begin the following spring when hatching takes place. During the first year of its 
life the individual is a member of the 0 group, during the second year it is a member 
of the I group, etc. Fish hatched in the same year belong to the same year class, 
regardless of their age at the time of capture. 
Two examinations for age determination were made of the scales of each individ- 
ual specimen. At the second examination either a definite assignment of age was 
made for the troublesome scales or the slides were marked as unreadable. At the 
time of the final examination one of the three scales on the slide was measured. 
This scale was selected on the basis of clearness of markings and symmetry of form. 
The diameters of the different growth areas were measured along the anteroposterior 
axis of the scale. For this measurement the ruler was placed in such a position that 
its edge passed through the focus of the scale and approximately bisected the posterior 
field (that portion of the scale which is exposed in its natural position on the fish). 
Scale measurements were made with a tested millimeter ruler and recorded to the 
nearest millimeter (occasionally the nearest half millimeter). 
The assessment of age was based on the determination of the number of annuli 
or lines of discontinuity between growth areas of succeeding years. Two difficulties 
were encountered in the determination of individual ages. The later annuli of the 
older individuals (particularly those of the slower-growing populations) were some- 
times so crowded together as to make their recognition difficult or even impossible. 
In the latter case the scales were discarded. Accessory annuli (summer checks) 
were not infrequent, but their characteristically indefinite appearance and their 
position with respect to the true annuli were such that they were ordinarily easily 
recognizable. Somewhat less than 5 percent of the total number of scales examined 
were discarded as unsatisfactory for the determination of age. 
Accessory checks were found to occur regularly in all the samples of the scales 
of the Muskellunge Lake cisco. They could be separated from the true annuli quite 
easily in the growth fields of the earlier years, and consequently they presented no 
great difficulty in the age and growth analysis of the earlier (1928-31) collections which 
were composed almost entirely of I-, II-, and Ill-group fish. In 1932, however, IV- 
group fish were present in abundance for the first time. In this year’s collection the 
presence of accessory annuli, along with the IV-group’s excessively slow growth 
during the fourth year of life, made the separation of the III group and the IV group 
of that year’s collection both difficult and uncertain. Because of these difficulties 
and because of the abundance of data from the preceding years, the 1932 collections 
were not used in the computation of a general growth curve for the Muskellunge 
Lake population. However, certain of the data from the 1932 Muskellunge Lake 
samples will be used from time to time for other purposes. 
Van Oosten (1929) demonstrated that after the formation of the first annulus 
the ratio of the total scale length to body length is so nearly constant in the lake 
herring that the assumption of the absolute constancy of the ratio affords the most 
