Figure 5. — An accessory check on a gizzard shad scale. 



Time of Annulus Formation 



No new annuli were observed on the scales of any 

 age group of gizzard shad collected in the lake prior 

 to the second quarter of May (table 3). Data for 

 individual days in this second quarter indicated 

 that two of the five males and eight of the nine 

 females of age-group I, taken in the open lake on 

 May 15, had already formed their first annulus. 

 Only one other of the I-group fish, a female cap- 

 tured on May 12, 1955, collected in this quarter 

 had the new annulus. No fish of this age group 

 exhibited annuli in the small collections of May 

 16-23. (All of the 48 male and 56 female I-group 

 fish caught in Sandusky Bay on May 22, 1953, had 

 their new annulus — data not given in table.) All 

 I-group shad collected after May, however, had 

 completed the new annulus. The new annulus of 

 the Il-group fish began to appear in the first 

 quarter of June and the year-mark was present in 

 all of these fish by the first quarter of July. Shad 

 older than the Il-group seem to have formed the 



new annulus about a week later; data for these 

 higher ages, however, are meager. The time of 

 annulus formation, as judged from examination of 

 scales, is in general agreement with the time of 

 resumption of growth in length as indicated by 

 changes in the length of fish (see fig. 8). 



Once annulus formation had started, the per- 

 centage of females having the new annulus was 

 greater than that of the males until all fish of both 

 sexes had new annuli. On the whole, the female 

 gizzard shad seems to form the annulus as much as 

 a week earlier than the male. 



Among the Il-group shad, neither length nor 

 weight of either sex was a determining factor in 

 time of annulus formation (a similar study was 

 not made of other age groups). A comparison 

 (made only for June) of the Il-group female 

 gizzard shad which had not spawned with those 

 which were spent demonstrated that the percent- 

 age of fish having the new annulus was about the 

 same in both groups. Hence, we may assume 

 that the act of spawning neither hastened nor 

 retarded the formation of the new annulus. It 

 may not be concluded, however, that the gonad 

 development which ultimately results in spawning 

 has no effect on the structural appearance of the 

 developing annulus or upon the initiation of a 

 check. Probable evidence of the effect of spawn- 

 ing is seen in the accessory check found between 

 the first and second annuli in the scales of some 

 shad (see previous section on description of the 

 scale) . 



Validity of the Annulus as a Year-M?rk 



Lagler and Applegate (1943) and Lagler and 

 Van Meter (1950) demonstrated that the annulus 

 is a true year-mark on scales of gizzard shad in 

 Indiana and Illinois. The following data on shad 

 in Lake Erie add support to the belief that annuli 



Table 3.- — Percentage of gizzard shad having a new annulus in May, June, and July 

 [Combined collections of 1953-55; lake specimens only. Number of fish in parentheses] 



GIZZARD SHAD IN WESTERN LAKE ERIE 



397 



