Franks et a\: Age and growth of Rachycentron canadum 



465 



ages 4 to 9, respectively (Table 3), whereas 

 the 1350 mm group of females ranged fi'om 

 ages 5 to 9 (Table 4). 



The results of likelihood-ratio tests 

 showed a significant difference in the over- 

 all von Bertalanffy growth models for 

 males and females ()(-=175.06, df=l, 

 P<0.0001) (Table 5), a finding substanti- 

 ated by approximate randomization test- 

 ing of the growth models {P<0.0001 ). Like- 

 lihood-ratio tests showed that estimates 

 of LJx^=24.60, df=l, P<0.0001) and K 

 (X-=7.02, df=l, P=0.008) were signifantly 

 different between sexes, however, t^ was 

 not significantly different ( X"=-0 . 1 1 , df= 1 , 

 P=0.752). Growth param.eters indicated 

 that females achieved a greater theoreti- 

 cal asymptotic length and grew at a faster 

 rate than males. Predicted lengths-at-age 

 derived by the von Bertalanffy equations 

 agreed with observed lengths, except for 

 age 9 males (n=2) and age 8 and 10 females 

 (n-VZ) (Table 2), where observed lengths 

 were considerably larger than those pre- 

 dicted. Average observed lengths-at-age for 

 females were greater than those of males 

 for age 1 and older (Table 2), and predicted 

 lengths of females were greater than those 

 of males for all ages. 



Otolith weight was significantly related 

 to age (Fig. 7), and the slopes of the otolith 

 weight-age regressions for males and fe- 

 males (Table 1) were significantly different 

 (ANCOVA , df=385, F=34.13, P<0.0001). 



Age-length keys were constructed to estimate the 

 age structure of legal-sized cobia (>838 mm FL) 

 caught from 1987 to 92 (Fig. 8) which we believe was 

 representative of the northeastern Gulf recreational 

 fishery. Most (84^^) of those fish were age 2-4, 

 whereas age 3 represented 37*^ of the catch. Age at 

 full recruitment to the fishery was age 4 (modal age 

 plus one). Ages 1-3 represented 66% of the fishery, 

 age 4 represented 19%, and ages 5-11 only 15% . The 

 instantaneous rate of total mortality (Z) estimated 

 by our catch curve analysis for ages 4-8 was 0.75 

 (Fig. 9). 



Discussion 



Despite acquiring many of our cobia samples at fish- 

 ing tournaments, we believe our overall collections 

 reflect the recreational hook-and-line fishery for co- 

 bia in the northeastern Gulf during the late 1980s 

 and early 1990s. Although anglers typically enter 



large fish in tournaments, substantial numbers of 

 small fish were also entered during the competitions, 

 particularly if aggregate weight awards were pre- 

 sented during multiday competitions. We frequently 

 sampled anglers' entire catch which included small 

 fish not entered in competition. Nontournament fish 

 were also examined at docks and marinas, and these 

 specimens ranged from less than minimum legal size 

 to some of the largest fish that we encountered. 



Although the length-weight relationships between 

 the sexes did not differ significantly, females were 

 typically larger than males. Thompson et al.'^ re- 

 ported similar results for cobia taken off western 

 Louisiana. In our study, females predominated (2.7:1 

 overall sex ratio) during all study years. Females 

 were dominant in all age groups, and the magnitude 

 of that dominance varied with increasing age. Dur- 

 ing a five-year study (1987-91) of cobia from west- 

 ern Louisiana waters (west of the Mississippi River 

 delta), Thompson et al.'^ reported an overall sex ra- 

 tio of 2.1:1 that was skewed towards males (464, 



