NICHOLSON and SCHAAF; AGING OF GULF MENHADEN 



frequency distributions of sampled fish, returns of 

 tagged juvenile menhaden, and spacing of rings on 

 the scales. 



Length-Frequency Distributions 



From the general shape and the number of 

 modes of a length-frequency distribution curve, it 

 is often possible to infer the number of age-groups 

 represented. Length-frequency curves of Gulf 

 menhaden sampled during 1964-73 fishing sea- 

 sons had two distinct modes. Since distributions in 

 all years were similar, we have shown only those 

 for 1967-70 (Figure 1). A prominent mode, usually 



evident in May at around 135 to 150 mm, shifted 

 progressively to the right during the season and by 

 September varied from about 155 to 170 mm. A 

 smaller mode at about 170 to 180 mm in May 

 tended to shift farther right during the season and 

 disappear by midsummer, so that the curve be- 

 came unimodal and greatly skewed. This small 

 mode apparent in May appeared to be a continua- 

 tion of the mode that was prominant during the 

 preceding September. 



From the general shape of the length-frequency 

 curves there appears to be only two dominant 

 age-groups in the fishery. The younger and more 

 numerous tends to dominate the fishery as the 



u 



Z 



o 



>- 

 z 



UJ 



o 



1970 



N = 118 



N = 1018 



N = 2523 



N = 828 



N = 456 



N = 2152 



N = 23 



100 



120 140 160 180 200 220 100 120 



FORK LENGTH IN MM 



N = 36 



160 180 



200 220 



FIGURE 1.— Length-frequency distributions of Gulf menhaden in percent, by month, 1967-70. 



317 



