TIME OF ANNULUS FORMATION 



The fish of Fork Lake offered unusual opportunities to study the time of annulus for- 

 mation. All bass were known to have been spawned in Jime, 1938, and the bluegills, imtil 

 midsummer, could be separated with certainty into two distinct groups, namely, original 

 breeders and the 1938 brood spawned in Fork Lake. Toward the end of the season a few 

 small, 1939 bluegills appeared in the nets. In the 1938 broods of bass and bluegills, the 

 actual ages were known, and in March, 1939, the scales of the fish collected were free 

 from any marks that might be interpreted as annuli. Collections were taken at monthly 

 intervals, March through November, making it possible to measure accurately when annuli 

 were formed. Table 6 shows the percentages of the collected fish having new annuli. These 

 percentages are shown also in fig. 3. The midpoints of annulus formation as read from the 

 graphs were the following dates: May 4 for yearling bluegills. May 16 for breeder blue- 

 gills, Jime 9 for yearling bass. 



TABLE 6. —PERCENTAGE OF FISH WITH NEW ANNUAL RINGS. 



Among the yearling bluegills, growth was both rapid and uniform throughout the grow- 

 ing season of 1939, fig. 1. In a few Individuals, annulus formation took place before 

 April 18, fig. 3, and in all of the fish collected annuli were visible at the end of May. 

 All yearling bluegills collected had formed annuli by the time nest building and spawning 

 began. 



_ 100- 



_l 

 D 

 Z 

 Z 

 < 



X 



80- 



60- 



40- 



I- 

 Z 

 UI 

 O 20-1 



a. 

 iij 



Q. 



OH 



O O^ lO— O--^ 



MAR APR MAY JUN JUL AUG 



19 3 9 



SEP 



OCT 



T r 



NOV 



FIG. 3. — The formation of the 1939 annuli in bass and bluegills taken from Fork Lake, 

 March-November, 1939. 



