Life History of Cobia 
19 
of opaque zones as annuli. First, mean size of cobia increased with 
opaque zone count. Second, young-of-the-year cobia (based on length 
frequency distributions) had no opaque zone distal to the sagittal core 
or focus, whereas age 1 fish had one opaque zone distal to the sagittal 
core. Moreover, recent research in the northern Gulf of Mexico (Franks 
et al. 1991, Thompson et al. 1991) confirmed the validity of the formation 
of one translucent and one opaque zone on cobia sagittae each year. 
Assuming that opaque zones on cobia sagittae were valid annuli, 
my results indicated that cobia grew rapidly during the first few years 
of life, and by age 3 mean mass ranged from 6 to 8 kg. Results 
from public tagging programs report equally dramatic growth for recaptured 
specimens (Anonymous 1986, Richard 1989, Franks 1995). My study 
agreed closely with Richards (1967) on mean length for both sexes 
at age 1 and 2 (Table 2). For age 3 and older, Richards (1967) reported 
that mean sizes were larger. Eleven specimens were estimated as age 
11 to 14, while Richards’ (1967) maximum age for cobia was age 
10. Perhaps, erosion on scale edges caused him to underestimate cobia 
ages, as has been shown in other fishes (Chilton and Stocker 1987). 
Male cobia have a higher growth coefficient, k, than females, 
and the difference between sexes was greater for my study (0.37 to 
0.24) than previous work (0.28 to 0.23; Richards 1977). Mean asymptotic 
FLs (Table 3) for both sexes were lower than Richards (1977) reported, 
possibly reflecting a greater availability of larger cobia in Chesapeake 
Bay during the 1960s. Age 3 females ( n = 50) predominated in the 
present study, whereas Richards (1967) found age 5 females ( n = 34) 
were most numerous. No doubt, estimates of mean asymptotic size 
in the present study were underestimates as the current North Carolina 
state record cobia (1988) weighed 46.7 kg. 
Cobia were primarily demersal feeders along the North Carolina 
coast, and they preyed on portunid crabs, penaeid shrimps, stomatopods, 
numerous teleosts, and small elasmobranchs. Overall, the blue crab 
was the most important food item in the cobia diet, which reinforces 
the colloquial name of “crab-eater” used along the southeastern coast 
of the United States (Knapp 1951, Manooch 1984). Most portunids 
were ingested whole, except for Ovalipes which was usually macerated. 
Similar to the results of the present study, Knapp (1951) found demersal 
prey, such as portunids, stomatopods, penaeids, and eels in cobia stomachs 
from the northern Gulf of Mexico. Cobia from the western Indian 
Ocean consumed mostly portunids, cephalopods, and eels (Darracott 
1977). In the sounds of North Carolina, cobia greater than 9 kg showed 
a predilection for smooth dogfish pups and small dasyatid sting rays, 
and these were among the largest prey items ingested. Cobia may 
