Franks et al.: Age and growth of Rachycentron canadum 



463 



were significantly larger than males 

 (Mann-VVhitney f7-test, P<0.001 ), and 8591 

 of fish >1000 mm were female. The sex 

 ratio of 1:2.7 was significantly different 

 from 1:1 (x^=205.Q, df=l, P<0.6o01). 



Neither slopes (ANCOVA, df=914; 

 F=2 . 156, P=0. 142 ) nor elevations ( ANCOVA, 

 df=914. F=2.334, P=0.127) of the length- 

 weight regressions by sex were found to 

 be significantly different; therefore, data 

 were pooled and one relationship estab- 

 lished (Table 1; Fig. 4). Weight was ap- 

 proximately a cubic function of length, 

 implying nearly isometric growth. The re- 

 lationships between FL and TL are pre- 

 sented in Table 1. 



When viewed with transmitted light, 

 thin-sectioned sagittae revealed a pattern 

 of distinct, alternating narrow opaque and 

 wide translucent bands (Fig 2). The dis- 

 tance between the first two opaque bands 

 distally from the core typically was wider 

 than the distance between subsequent 

 opaque bands. Mean marginal increment 

 analysis (Fig. 5) demonstrated that April 

 through August was the time of annulus 

 formation and suggested that opaque 

 bands form once each year. All otoliths 

 exhibited a zone of translucent material 

 beyond the last annulus from September 

 through February. Mean increment was 

 minimal during June and increased to a 

 maximum in February (no samples were 

 collected during December). The sample 

 size was too small to plot marginal increments for 

 each year and age-group separately; however, a vi- 

 sual examination of the data indicated that marginal 

 increments for individual years 1987-90 and age- 

 classes 2-5 were similar, with a consistent seasonal 

 minimum during summer. Timing of annulus forma- 

 tion was similar for each sex. 



Of the 645 left sagittae processed for age estimates, 

 187 (29%) were judged illegible. Right sagittae from 

 168 of the latter group were available and processed, 

 and 76'^?^ (128/168) were readable. Readers agreed 

 on ages for 96% (565/586) of usable otoliths, 170 

 males (range 345-1330 mm FL) and 395 females 

 (range 335-1651 mm FL). Only 21 (4%) of the us- 

 able otoliths were rejected because of disagreements 

 among readings, owing primarily to disparities over 

 the presence of an annulus adjacent to the core or at 

 the otolith's margin. Of the sagittae found accept- 

 able for age estimations, 33 were from YOY (335- 

 510 mm) and 42 were from age 1 fish (493-910 mm). 

 Ten age 1 fish were 838 mm (minimum legal size) or 



larger. Most (n=463, 82%) of the 565 fish that we 

 aged were estimated to be ages 2-5 (27% age 2; 29%- 

 age 3; 17% age 4; and 9% age 5). Age 6 fish and older 

 were conspicuously uncommon. There was a signifi- 

 cant difference between the age-frequency distribu- 

 tions of males and females (Kolmogorov-Smirnov 

 two-sample test, dn=0.308, P<0. 05). An age 11 female 

 ( 1568 mm) and age 9 males (n=2, 1240 and 1260 mm) 

 were the oldest cobia sampled (Table 2). Twenty five 

 females (1170-1651 mm) were age 6 or older, but only 

 six males (1035-1330 mm) were older than age 5 

 (Table 2). 



Growth in length for both sexes was relatively fast 

 through age 2, after which growth slowed gradually 

 (Fig. 6). We found a wide range of lengths within most 

 age groups for both sexes (Tables 3 and 4). For ex- 

 ample, age 4 males and females ranged from 850 to 

 1250 mm and from 900 to 1250 mm, respectively. We 

 also found a wide range of ages within some of the 

 length groups. For example, the 1000 mm and 1200 

 mm groups of males ranged fi-om ages 2 to 7 and ft-om 



