468 



Fishery Bulletin 97(3), 1999 



migration has been suggested for swordfish (Berke- 

 ley and Houde, 1983;Tserpes and Tsimenides, 1995) 

 and Atlantic bluefin tuna (Compean-Jimenez and 

 Bard, 1983). Other authors (Nelson and Manooch, 

 1982; Sturm et al., 1989; Beckman et al., 1990; 

 Ferreira and Russ, 1994) also suggested that repro- 

 duction may not be the sole determining factor and 

 commented on the physiological nature of annulus for- 

 mation and the importance of environmental factors. 

 Longevity of male and female cobia differed con- 

 siderably. Males older than age 7 were rare, and 

 maximum age was 9. Females older than age 8 were 

 rare, and maximum age was 11. Maximum ages of 

 cobia from Louisiana (age 10, Thompson et al.'^) and 

 Virginia (age 10, Richards, 1967) were similar to our 

 observations. However, Smith (1995) reported a 

 maximum age of 14 for males and age 13 for females 

 for cobia from North Carolina. We also found, as did 

 Richards (1967) and Smith (1995), that mean ob- 

 served lengths at age for females were larger than 

 those for males for all age classes, except age fish. 



400 



300 



CD 



n 



E 



200 



100 



n 



Males 



Females 



mm. 



\r/y7i 



10 



Age (years) 



Figure 8 



Age structure of cobia, Rachycentron canadum, >838 mm FL (regula- 

 tion minimum size) in the northeastern Gulf of Mexico recreational 

 hook-and-line fishery, 1987-92. n=992 



Considerable variation in size was observed within 

 most age gi-oups, including YOY, for both males and 

 females, which, according to Goodwin and Johnson 

 (1986), is not unusual for warm-water fishes. The 

 variation in size makes it difficult to estimate pre- 

 cisely the age of cobia from length alone. For example, 

 our largest cobia weighed 62.2 kg, which was slightly 

 greater than the all-tackle world record weight for 

 cobia (61.5 kg) reported by the International Game 

 Fish Association ( 1997 ). At a fork length of 1610 mm 

 and at age 8. this specimen was neither the longest 

 fish in our sample nor the oldest. A prolonged spawn- 

 ing season and multiple spawnings characteristic of 

 cobia (Lotz et al., 1996) probably account for the wide 

 variation in size of YOY cobia and other age groups 

 as well. Annual growth was most rapid through age 

 2 for both sexes, then gradually decreased thereaf- 

 ter, particularly for females. 



Otolith weight was a good predictor of age, ac- 

 counted for 78*^ and 84*^ of the variability in age of 

 male and female cobia, respectively, and explained 

 as much variation in age as fork length in 

 the von Bertalanffy model for each sex. 



Our estimates of growth parameters are 

 the only estimates available for R. cana- 

 dum in the northeastern Gulf We found 

 that the von Bertalanffy theoretical growth 

 models for males and females were signifi- 

 cantly different, as did Thompson et al.'^ 

 Lengths predicted from the theoretical 

 growth curves agreed with the average 

 observed lengths. Theoretical asymptotic 

 lengths seemed realistic, even though few 

 fish >1200 mm were sampled. Theoretical 

 growth coefficients (L and t^) reported by 

 Thompson et al.*^ for cobia from Louisiana 

 were smaller than our estimates ( Table 5 ), 

 although their estimates ofK were larger, 

 particularly for females. Asymptotic 

 lengths for males and females taken off 

 Virginia (Richards, 1977) were consider- 

 ably larger than L^ values reported by 

 Smith (1995) for cobia from North Caro- 

 lina, values reported by Thompson et al.^ 

 for cobia from Louisiana and our study 

 (Table 5), although our asymptotic length 

 for males was similar to that in Richards' 

 ( 1967) study. The differences in estimates 

 of growth coefficients for cobia throughout 

 their range in U.S. waters may be due to 

 methodological differences, e.g. sectioned 

 otoliths ( this study) versus scales (Richards, 

 1967), or differences in geographical cov- 

 erage. Nevertheless, we believe our growth 

 parameter estimates are appropriate for 



