Franks et a\ Age and growth of Rachycentron canadum 



461 



were collected from April through July 

 (n=787); peak samples were taken in May 

 (n-349). Fewer fish were collected in August 

 (n=49) and September through November 

 (n=157). No samples were collected in Decem- 

 ber, and only 12 samples were collected from 

 January through March. 



For most fish, the date and location of catch 

 were recorded along with fork length (FL, 

 mm), total length (TL, mm), and total weight 

 (TW, nearest 0.1 pound converted to kilo- 

 grams), although some fish had been gutted. 

 All lengths reported are FL. The sex of most 

 fish was also recorded, including that of sev- 

 eral young-of-the-year (YOY). Sex-specific 

 length-weight regressions were calculated by 

 linear regression of logj^-transformed data, 

 and the slopes and elevations of the regres- 

 sions were compared by using analysis of 

 covariance (Snedecor and Cochran, 1967). 

 Relationships of fork length to total length 

 were calculated by using the generalized lin- 

 ear regression model: FL=a+bTL. 



Sagittal otoliths were removed from most 

 specimens, then cleaned with distilled wa- 

 ter, air dried, and stored dry in labeled vials. 

 Cobia sagittae are small and fragile. They 

 are elongate, laterally compressed struc- 

 tures, with a rounded posterior, a pointed 

 rostrum, and a smaller, pointed antirostrum 

 (Fig. 2). The distal surface is concave, and a 

 wide, curved sulcus traverses the proximal 

 surface longitudinally. Initially, we randomly 

 selected ten sagittal otolith pairs (fish 

 FL=700-1613 mm) to determine the number 

 of opaque bands in each. Paired counts of 

 opaque bands agreed in all cases. Therefore, 

 the left sagittal otolith was used for age esti- 

 mation unless missing, broken, or illegible, 

 in which case the right sagitta, if available, 

 was substituted for age analysis. Whole left 

 sagittae were weighed on a microbalance to 

 the nearest milligram to evaluate otolith 

 weight as a predictor of age. Sex-specific lin- 

 ear regressions were fitted to otolith weight 

 and age data and were compared by using 

 analysis of covariance ( Snedecor and Cochran, 1967 ); 

 degree of significance set at a = 0.05. Sagittae were 

 embedded in Spurr (Secor et al., 1992) and sectioned 

 through the core along a transverse, dorsoventral 

 plane with a Buehler Isomet low-speed saw contain- 

 ing a diamond wafering blade. Two or three thin-sec- 

 tions (0.3 mm) were mounted on a microscope slide 

 with CrystalBond 509 adhesive, sanded with wet 600- 

 and 1500-grade sandpaper, polished on a felt wheel 



Figure 2 



Whole I A and B) and sectioned iCi sagittal otolith from an age 9 

 (1621 mm FLl female cobia, Rachycentron canadum. The otolith's 

 distal ( A ) and proximal ( B ) surfaces were viewed with reflected light, 

 and C was viewed with transmitted light. Labels for A and B: a = 

 anterior; p = posterior: d = dorsal; v = ventral; c = core; r = rostrum; 

 ar = antirostrum; sa = sulcus. Labels for C: d = dorsal; v = ventral; 

 di = distal; pr = proximal; c = core; ds = dorsal sulcal ridge; vs = 

 ventral sulcal ridge; vm = ventral margin. Numbers indicate selected 

 annuli. Scale bars = 1.00 mm for A and B; 0.50 mm for C. 



with 0.3 |im alumina micropolish, then examined un- 

 der a binocular dissecting microscope at 20-40x mag- 

 nification with transmitted light. 



Three experienced readers independently counted 

 opaque bands from the core to the outer otolith mar- 

 gin. Opaque bands were most distinct and easily 

 counted in the midportion of the ventral lobe of a 

 section, and our analyses were made in that region 

 (Fig. 2). Opaque bands were often obscured at the 



