BECKMAN ET AL.: AGE AND GROWTH OF RED DRUM 



RESULTS 



Length-weight regressions for males and females 

 were not significantly different (P = 0.842 for inter- 

 cepts, P = 0.605 for slopes). The combined length- 

 weight regression was 



Weight = 2.9 x 10-« FL^-^z 

 N = 1,626. 



r2 = 0.91 



The length-frequency distributions of red drum 

 collected by purse seine (Fig. 2) were significantly 

 different from those obtained by gill net (P < 0.01) 

 and hook and line (P < 0.01). Therefore, to avoid 

 gear selectivity bias, only purse seine samples were 

 assumed to represent the age-frequency distribution 

 of the offshore spawning population. 



Because the sagittae were extremely thick and 

 opaque, they needed to be sectioned before they 

 could be aged. Distinct opaque and translucent 

 growth zones were observed in transverse sections. 

 Annuli were most distinct and the most consistent 

 growth patterns were observed in the region from 



the core to the proximal surface of the sagitta along 

 the ventral margin of the sulcus acousticus. All 

 counts were made in this region (Fig. 3). 



The percentage of sagittae with opaque margins 

 was plotted by month to determine the timing of an- 

 nulus formation. Opaque zones were deposited in the 

 sagittae during winter and spring months in three 

 successive years of sampling (Fig. 4A). As a consis- 

 tent pattern of annulus formation was exhibited 

 each year, data were combined for all years in order 

 to compare annulus formation between size groups 

 (Fig. 4B). Data were grouped according to matur- 

 ity (Overstreet 1983) and growth patterns. Group- 

 ings were chosen to include an adequate sample size 

 within each group for analyses as follows: 0-4 an- 

 nuli — immature and early maturity, rapid growth; 

 5-9 annuli — mature, rapid growth; 10-19 annuli 

 — mature, reduced growth; and 20-36 annuli — 

 maximum ages, reduced growth. A single peak per 

 year in all plots indicates that one annulus was 

 formed each year in all groups. Age in years for red 

 drum was equal to the number of annuli observed 

 in sections of sagittae. Age estimates were obtained 



FORK LENGTH (cm) 



UNKNOWN Kzxa MALE 



FEMALE 



Figure 2.— Length-frequency distribution for red drum captured by purse seine from offshore northern Gulf of 

 Mexico waters. "Unknowns" are individuals for which sex identifications were not available. 



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