666 
Fishery Bulletin 99(4) 
700 
600 
♦ C. sapidus 
* O. ocellatus 
♦ 
*♦ 
500 
400 
300 
200 
100 
♦ 
50 
60 
70 
80 
90 
100 110 120 130 140 150 
Cobia fork length (cm) 
Figure 1 
Volume (mL) of portunid crab consumed in lower Chesapeake Bay as a function of cobia 
size. 
(1996), and Franks et al. (1996). Frequency (%F) was the 
percent of all stomachs that contained food. 
Results and discussion 
Stomach contents from 114 adult cobia (37-141 cm FL) 
were examined. Seventy-eight stomachs (68%) contained 
at least one identifiable, nonbait prey item. One species of 
bivalve, one species of hydroid, six species of crustacean, 
one elasmobranch, and 16 species of teleost were observed 
(Table 1). Mean volume of prey per stomach was 150.6 mL 
(range: 2-680 mL). Mean number of different prey per 
stomach was 1.9 species (range, 1-5 species). Blue crab 
( Callinectes sapidus) and lady crab ( Ovalipes ocellatus ) 
dominated the diet of cobia. 
Index of relative importance (IRI) values for blue crab 
(5257) and lady crab (3665) were two orders of magnitude 
higher than IRI values for other prey items. Larger cobia 
consumed greater volumes of crab (Fig. 1). Additionally, 
larger cobia consumed larger (sublegal, <13 cm) blue crabs, 
although similar-size lady crabs were consumed by all siz- 
es of cobia (Table 2). Atlantic croaker ( Micropogonias un- 
dulatus), hogchoker ( Trinectus maculatus ), and fish and 
crab remains constituted the top food items volumetrical- 
ly after portunid crabs. Although relatively high volumes 
were observed, IRI values for Atlantic croaker and hog- 
choker were low owing to infrequent occurrence of these 
items. High volume of crab remains was consistent with 
high volumes, counts, and frequency of occurrence of por- 
tunid crabs in cobia stomachs. Fish remains likely result- 
ed from finfish bait (predominantly menhaden, Brevoortia 
tyrannus, and Atlantic croaker, Micropogonias undulatus). 
Crab and fish remains not identifiable to family and fin- 
fish bait were excluded from IRI calculations. 
Consumption of blue crab was greatest along the west- 
ern shore of the Bay and least at the mouth of the Bay. 
Conversely, consumption of lady crab was greatest at the 
mouth of the Bay and least along the eastern shore of 
the Bay (Fig. 2A). A greater percentage of lady crabs were 
consumed by male cobia than by female cobia, whereas 
a greater percentage of blue crabs were consumed by fe- 
male cobia (Fig. 2B). IRIs for blue crab and lady crab were 
similar in June, but dramatically different in July. In July, 
IRI for blue crab was twice as high as that for lady crab. 
Prey availability and habitat utilization were likely re- 
sponsible for determining location, sex, and within-season 
differences in composition of portunid crabs consumed by 
cobia. June samples were predominantly male cobia col- 
lected from the Bay mouth and eastern shore of the Bay, 
whereas July samples were predominantly female cobia 
collected from the western Bay shore. 
Cobia feeding habits in lower Chesapeake Bay were 
more similar to feeding habits reported for cobia from 
North Carolina (Smith, 1995) than to feeding habits of 
cobia from the northern Gulf of Mexico (Franks et al., 
1996; Meyer and Franks, 1996). Portunid crabs dominated 
the diet of cobia in Chesapeake Bay and North Carolina 
(Smith, 1995). Elasmobranch fishes were consumed exclu- 
sively by large cobia in Chesapeake Bay and North Caro- 
lina (Smith, 1995). In North Carolina (Smith, 1995), cobia 
fed on stingrays ( Dasyatis sp.) and smooth dogfish ( Muste - 
lus canis). Cownose ray (Rhinoptera bonasus ), previously 
unreported as a food item of cobia, was the only elasmo- 
branch consumed by cobia in Chesapeake Bay. Cownose 
ray was observed from eight cobia stomachs; however, on- 
