FISHERY BULLETIN: VOL. 76, NO. 2 



Table 6. — Mean distance from last ring to scale edge expressed as the percent of the 

 distance from focus to scale edge for Gulf menhaden. 



Fish with three rings were not clearly differen- 

 tiated as a distinct age-group. Because of small 

 numbers neither modes nor general shapes of the 

 distributions could be clearly determined. Means 

 tended to increase slightly as the season pro- 

 gressed. There was considerable overlap of the 

 lower end of the length range with the upper end of 

 the length range of two-ring fish. 



If the rings we observed were true annuli, the 

 distance from the last ring to the scale edge should 

 have decreased as the number of rings increased, 

 and should have increased throughout the season 

 for fish having the same number of rings. Both of 

 these trends were apparent (Table 6). 



Mean lengths back-calculated to the age of an- 

 nulus formation from fish with one, two, or three 

 rings were similar, and the mean lengths at the 

 time of first ring formation calculated from two- 

 ring fish were slightly smaller than those calcu- 

 lated from one-ring fish, as would be expected 

 (Table 7). This tendency for mean lengths back- 

 calculated from successively older ages of the 

 same year class to become progressively smaller is 

 commonly known as Lee's phenomenon and may 

 be caused by a variety of factors. Mean lengths 

 calculated from three-ring fish, however, were 

 slightly larger, rather than smaller, than mean 

 lengths calculated from either one- or two-ring 

 fish. 



Frequency distributions of lengths back-calcu- 

 lated to the time of the first, second, and third ring 



Table 7.— Mean lengths (millimeters) of Gulf menhaden at the 

 time of each ring formation calculated from one-, two-, and 

 three-ring fish, 1968-72 year classes. 



formation were well separated from each other, 

 with only a small overlap between one- and two- 

 ring and two- and three-ring fish (Figure 2). Those 

 at the time of the first ring formation were similar 

 in shape, whether calculated from one-ring, two- 

 ring, or three-ring fish. Those calculated from 

 two-ring fish were shifted slightly farther to the 

 left than those calculated from one-ring fish, as 



Year Class 



1972 



1970 



>- 

 u 



z 



o 



1970 



1970 



1969 



1969 



100 



120 140 160 180 



FORK LENGTH IN MM 



200 



220 



Figure 2. — Length-frequency distributions at time of first, sec- 

 ond, and third ring formation, back-calculated from one-ring, 

 two- ring, and three- ring Gulf menhaden, 1968-72 year classes. 



320 



