Butler et al.: Feeding ecology of Thunnus thynnus in North Carolina 
57 
(Rooker et al., 2007; Teo et al., 2007). Beginning in 
late November, bluefin tuna migrate into North Caro- 
lina coastal waters and feed upon local prey resources 
(Kade, 2000; Boustany, 2006). Atlantic menhaden ag- 
gregate off North Carolina to spawn during winter 
(Checkley et al., 1999) and may be the primary prey 
of bluefin tuna. 
When compared to other teleosts, bluefin tuna have 
standard metabolic rates that are among the highest of 
any fish species (Dickson and Graham, 2004; Blank et 
al., 2007). These high metabolic demands require them 
to consume large amounts of prey. Thus, bluefin tuna 
have the potential to influence the abundance of other 
species within an ecosystem. Overholtz (2006) modeled 
predation demand of bluefin tuna on Atlantic herring 
( Clupea harengus) in the Northwest Atlantic during 
summer months. Consumption of Atlantic herring by 
bluefin tuna was highest in 1970, declined to a low in 
1982, and increased through 2002. To our knowledge, 
the predatory impact of bluefin tuna on Atlantic men- 
haden during winter has not been examined. 
Here, we describe the diets of bluefin tuna (>185 cm 
curved fork length) off North Carolina during winter. 
We also estimate field-derived gastric evacuation rates 
and daily ration; daily ration is used to estimate the 
population-level consumption of bluefin tuna on Atlantic 
menhaden. Lastly, we compare the population-level con- 
sumption of Atlantic menhaden by bluefin tuna with the 
predatory demand from other known Atlantic menhaden 
predators. The latter question was addressed at both 
current and rebuilt bluefin tuna populations to investi- 
gate predator and prey management implications. 
Materials and methods 
Study area 
From 2003 to 2006, we sampled stomachs from commer- 
cially caught bluefin tuna landed in Beaufort and More- 
head City, North Carolina. The fishery operated from 
November through January (length of season varied 
by year) within a 28-km radius (centered at approxi- 
mately 34°26'N lat., 76°28'W long.) south of Cape Look- 
out shoals. Bluefin tuna were predominantly captured 
by trolling, where a dead-baited hook (with or without 
a lure) is pulled behind a moving vessel to imitate live 
prey. Generally, ballyhoo ( Hemiramphus brasiliensis) 
was used as bait. 
Collection of samples 
Bluefin tuna stomachs were collected during the winters 
of 2003-04 (71 = 42), 2004-05 (tz = 219), and 2005-06 
(71 = 187) off the coast of North Carolina; during the 
first winter a pilot collection was undertaken and the 
results were included only in the overall diet analysis. 
For most bluefin tuna, stomachs and other viscera were 
removed at sea by the fishermen. Upon excision, all 
stomachs were stored on ice until they could be collected 
by researchers. In addition to the stomach, the fisher- 
man was responsible for providing information on time 
and location of capture, curved fork length ( CFL , cm), 
and dressed weight {DW, kg). Curved fork length was 
measured from the tip of the snout to the fork of the tail 
over the contour of the body. DW was obtained after the 
head, tail, and viscera had been removed. In instances 
where a DW was not recorded, one was estimated by 
using the allometric relationship of CFL to DW defined 
from the current study (n = 379) as 
DW = 7.625 x HU 6 • CFL 3 088 , r 2 = 0.871. 
Dressed weights were converted to round weights (i.e., 
the total weight of a live fish; RW, kg) by using the regres- 
sion equation (77 = 685) developed by Baglin (1980): 
RW = -7.922 + 1.296 • DW, r 2 = 0.874. 
Diet analysis 
Individual stomach samples were opened and the con- 
tents placed in labeled plastic bags. Contents that could 
not be analyzed immediately were frozen for later analy- 
sis. All stomach contents were identified to the lowest 
possible taxon. Identifiable prey items were grouped 
by taxa and wet weight (g) was recorded. Teleosts or 
invertebrates that could not be identified were measured 
and recorded as unidentified species (e.g., “unidentified 
fish remains”). 
Diets were expressed by indices of percent frequency 
of occurrence (%0) and percent weight (%W ) (Hyslop, 
1980). Percent frequency of occurrence was calculated 
as the number of bluefin tuna that had ingested a spe- 
cific prey item divided by the total number of bluefin 
tuna that contained prey. Percent weight was estimated 
as the total wet weight of a specific prey type divided by 
the total wet weight of all prey across the total number 
of stomachs samples. 
Cumulative prey curves were constructed a posteriori 
by within-winter period (early December, late Decem- 
ber, and January), year (2004-05 and 2005-06), and 
size class (large medium [individuals between 185.4 
and 205.7 cm CFL ] and giant [individuals >205.7 cm 
CFL]) to determine if the sample sizes were sufficient 
to describe bluefin tuna diets (Ferry and Cailliet, 1996). 
Prey species were grouped by family and the mean and 
standard deviation of the cumulative number of unique 
prey was calculated by randomly resampling the num- 
ber of stomachs that contained prey 500 times (Bizzarro 
et al., 2007). The cumulative mean number of unique 
prey taxa calculated from randomized stomach samples 
was then plotted against the number of stomachs exam- 
ined (Ferry and Cailliet, 1996). Sample size sufficiency 
for each prey curve was tested by the linear regression 
method (Bizzarro et al., 2007), where the slope from a 
regression of the mean number of unique prey items 
from the last four stomach samples was compared to 
a slope of zero (Student’s Utest of the equality of two 
population regression coefficients; Zar, 1999). Sample 
