170 
BULLETIN OF THE BUBEAU OF FISHERIES 
of growth of the first year. Again, the marginal circuli of specimens taken in the 
spring or in the fall show the spacing characteristic of the season in which the fish 
were taken. 
While the number of circuli per zone or the total number on the scale is an 
unreliable basis for age determination in many species and also in old specimens 
of the sucker, it is a striking fact that in suckers 4 or 5 years old the number of 
circuli per zone is quite constant — namely, about 10. 
White suckers of the same age, in terms of years, vary in length within fairly 
narrow limits, but they have the same number of zones of circuli. This number is 
a more accurate index of the age of the fish than is the length. Such are the con- 
clusions to which the following observations appear to lead. 
A series of specimens was selected, ranging from 60 to 300 millimeters in length, 
with an interval of 10 millimeters between specimens. Ten scales of each specimen 
were examined, and the annual zones and number of circuli in each zone were 
Fig. 51. — Scale of second year, taken in November from 
a specimen 60 millimeters in length 
Fig. 52.— Sector of scale of the third year, from specimen 
90 millimeters in length 
counted. It at once became evident that the fish belonged to various groups deter- 
mined by the number of annual zones, and that their age could be thus fixed. 
Specimens measuring 60, 70, and 80 millimeters showed two zones (annuli) and 
were therefore 2-year fish. (Fig. 51). It is interesting to note that the number of 
circuli averaged 19 (30 scales). This is in accord with the finding that about 10 are 
added in each new zone. Fish of 90, 100, 110, and 120 millimeters showed three 
zones and belonged to the third year. (Fig. 52.) The number of circuli varied within 
the narrow limits of 28 to 32, or approximately three times 10. In the larger speci- 
mens similar results obtained, except that as the fish becomes older the zones tend 
to become narrower and contain a smaller number of circuli. (Figs. 53 and 54.) 
SEASONAL IRREGULARITIES IN THE CIRCULI 
Not all the circuli are even and continuous lines. Those of early spring (February 
and March) are quite irregular. This irregularity may consist of sinuous curvings, 
interruptions of the ridge, or branching. More rarely a chainlike condition is pro- 
duced by junctions between two parallel ridges. These early spring circuli usually 
