665 
Stomach content analysis of cobia, 
Rachycentron canadum, 
from lower Chesapeake Bay* 
Michael D. Arendt 
School of Marine Science 
College of William and Mary 
Virginia Institute of Marine Science 
Gloucester Point, Virginia 23062 
Present address: Marine Resources Research Institute 
South Carolina Department of Natural Resources Division 
217 Fort Johnson Road 
Charleston, South Carolina 29422-2559 
E-mail address: arendtm@mrd.dnr.state.scus 
John E. Olney 
Department of Fisheries Science 
School of Marine Science 
College of William and Mary 
Virginia Institute of Marine science 
Gloucester Point, Virginia 23062 
Jon A. Lucy 
Sea Grant Marine Advisory Program 
Virginia Institute of Marine Science 
Glooucester Point, Virginia 23062 
Cobia ( Rachycentron canadum) is a 
migratory pelagic species that is found 
in tropical and subtropical seas of the 
world, except in the central and eastern 
Pacific Ocean. The United States ranks 
third in total commercial production 
of cobia, however, recreational land- 
ings generally exceed commercial land- 
ings by an order of magnitude (Shaffer 
and Nakamura, 1989). In the western 
Atlantic Ocean, cobia migrate to Ches- 
apeake Bay in spring and summer 
to spawn, and the productive waters 
of the Bay are believed to constitute 
important foraging grounds (Joseph 
et ah, 1964; Richards, 1967). Cobia 
are known to move to areas of high 
food abundance, particularly crusta- 
cean abundance (Darracott, 1977). 
Recent feeding studies in the north- 
ern Gulf of Mexico and off North Car- 
olina have reported geographic differ- 
ences in cobia diet and have indicated 
that the relative importance of fishes 
versus crustaceans is variable and that 
cephalopods constitute the least signif- 
icant prey items. In the northern Gulf 
of Mexico, Franks et al. ( 1996) reported 
that fish (primarily anchovies, Anchoa 
sp. ) dominated (% index of relative im- 
portance [IRI | , Pinkas et ah, 1971) the 
diet of juvenile cobia (236-440 mm 
FL). Meyer and Franks (1996) reported 
crustaceans (primarily portunid crabs) 
occurred in 79.1% of stomachs and rep- 
resented 77.6 % of total prey items con- 
sumed by cobia (373-1,530 mm FL) 
in the northern Gulf of Mexico. Fish 
(primarily hardhead catfish, A/'ius fe- 
lis, and American eel, Anguilla rostra- 
ta) increased in importance with in- 
creasing cobia size. Fish were found 
in 58.5% of all cobia stomachs (20.3% 
of total prey) but occurred in 84.4% 
of stomachs of cobia 1150-1530 mm 
FL (Meyer and Franks, 1996). In con- 
trast, Smith ( 1995) observed decreased 
importance of teleosts in the diet of 
cobia (39-142 cm FL) in North Caroli- 
na. Elasmobranch fishes and portunid 
crabs dominated the diet of cobia >9 kg 
(Smith, 1995). 
Tag-recapture data collected between 
1995 and 1999 document localized 
movement of cobia within lower Chesa- 
peake Bay during summer, as well as 
the return of individual cobia to spe- 
cific locations or general regions of the 
lower Bay in subsequent summers. * 1 Al- 
though Chesapeake Bay is an impor- 
tant destination for migrating cobia, 
feeding habits of cobia in the Bay have 
never been thoroughly examined. Our 
study documents cobia feeding habits 
in Chesapeake Bay and compares find- 
ings with similar cobia studies from 
North Carolina and the northern Gulf 
of Mexico. 
Methods 
Cobia were sampled opportunistically 
at marinas and fishing tournaments 
in lower Chesapeake Bay between 
June and July 1997. Intact stomachs 
were removed by cutting above the car- 
diac sphincter (esophagus) and below 
the pyloric sphincter (large intestine). 
Stomachs were labeled, bagged, trans- 
ported on ice to the VA Institute of 
Marine Science, and examined in rela- 
tively fresh condition. An incision was 
made along the longitudinal axis and 
the contents of stomachs were emp- 
tied onto a 500-pm mesh sieve for rins- 
ing and sorting. Contents were blotted 
dry on paper towels before counts, dis- 
placed volumes ( 1-L graduated cylin- 
der), and identifications to the lowest 
possible taxon were made. When pos- 
sible, carapace widths (mm) of crabs 
were measured with calipers. An index 
of relative importance (IRI) for all prey 
items combined was calculated with 
the formula (% Number + % Volume) x 
(% Frequency ), as described by Pinkas 
et al. (1971) and subsequently used 
by Smith (1995), Meyer and Franks 
* Contribution 2390 of the Virginia Insti- 
tute of Marine Science, Gloucester Point, 
VA 23062. 
1 Annual reports. Virginia Game Fish Tag- 
ging Program, Marine Resources Com- 
mission, 968 Oriole Dr. South, Suite 102, 
Virginia Beach, VA 23451. 
Manuscript accepted 13 April 2001. 
Fish. Bull. 99:665-670 (2001). 
