fields where the termination of the circuli of the 

 first zone is "cut across" by the circuli of the 

 second zone, it may escape notice under low 

 magnification. 



A second type of demarcation line, characterized 

 by a narrow band devoid of clear-cut circuli, 

 normally is found between all growth zones be- 

 yond the first. Here, too, the earlier circuli of the 

 more recent zone cut across the paths of the later 

 circuli of the preceding zone in the lateral fields. 

 This type of annulus is clearly evident under low 

 magnification (fig. 3). 



Further study is required, but on the basis of my 

 observations I suggest that this second type of 

 annulus is a combination spawning mark and 

 annulus. It is found only on the scales of fish of 



Figure 4. — Second annulus of a gizzard shad scale 

 resembling typical first annulus. 



Figure 2. — The first annulus of a gizzard shad scale. 



Figure 3. — The second annulus of a gizzard shad scale, 

 with clear line between second and third growth zones. 



spawning age. The prominence of this annulus 

 and its possession of a few fragmentary circuli 

 indicate not only a cessation and resumption of 

 growth, but also a period of either poor circulus 

 formation or even of limited scale resorption. 

 Such a period is not indicated by the first annulus. 

 Annulus formation and spawning take place at 

 about the same time (see next section on time of 

 annulus formation). Although no direct causa- 

 tive relation between spawning and annulus 

 formation has been demonstrated, the period of 

 disturbed circulus formation may well be the 

 result of rapid prespawning gonad growth. A few 

 gizzard shad do not spawn during their third year 

 of life (near the start of which the second annulus 

 is formed) . These fish may provide the scales on 

 which the second annulus resembles the normal 

 first annulus closely. This type of annulus (fig. 4) 

 lacks the narrow band of fragmentary circuli 

 which makes the usual second and succeeding 

 annuli so conspicuous. 



Further evidence on the.possible effect of spawn- 

 ing on scale structure comes from gizzard shad that 

 show an accessory check between the first and 

 second annuli of their scales (fig. 5) ; in fact, all 

 accessory checks found were in this position. Fish 

 with this check may be the few shad which spawn 

 during their second year of life. Because the first 

 annulus is already present by the first of June, 

 whereas any precocious second-year spawning 

 occurs in late July — or later in the year (see 

 section on spawning season) — the effect of this 

 late spawning, if it is to be shown by the scale, 

 would have to appear beyond the first annulus. 



396 



U.S. FISH AND WILDLIFE SERVICE 



