Butler et al.: Feeding ecology of Thunnus thynnus in North Carolina 
67 
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Figure 5 
Estimated population-level consumption (metric tons, t) of Atlan- 
tic menhaden ( Brevoortia tyrannus) by other important preda- 
tors (see references provided on figure for further details from 
other studies); bottom and top of bars represent minimum and 
maximum estimated consumption (harvest) levels. Bluefin tuna 
(Thunnus thynnus ) consumption estimates from this study are 
represented by the current (2005) abundance estimate for age- 
6+ western Atlantic bluefin tuna and a restored (to 1975 levels) 
age-6+ western Atlantic bluefin tuna population. 
cial bluefin tuna fishery which is limited to 
only those fish greater than 185 cm (CFL). 
Although infrequent, smaller bluefin are cap- 
tured locally during the winter. If smaller size 
classes of bluefin tuna are consuming Atlantic 
menhaden during winter or at other times of 
the year, then they too should be considered 
in future consumption estimates. Currently, 
there is no evidence for this during non-winter 
months (Eggleston and Bochenek, 1990). Sec- 
ond, the stock assessment of western Atlantic 
bluefin tuna is uncertain given the debate re- 
garding the influence of trans-Atlantic mixing. 
The most recent assessment (ICCAT, 2007) 
assumed a mixing rate of 1-2% between the 
east and west bluefin tuna stocks. Rooker et al. 
(2008) found that as many as 40% of the age 
5+ bluefin tuna captured in the Mid-Atlantic 
bight were from the eastern stock. Bluefin tuna 
mixing rates for the U.S. South-Atlantic bight 
during the winter could be used to assign ad- 
ditional biomass from the eastern stock to the 
B bft variable described above. 
In summary, this study has filled a gap in 
the knowledge of the natural history of bluefin 
tuna and determined their potential impact 
on Atlantic menhaden. During the winter, the 
continental shelf of NC serves as a spawning 
ground for several estuarine-dependent spe- 
cies, including Atlantic menhaden. The diets of 
large medium and giant bluefin tuna have been 
dominated by Atlantic menhaden during recent 
winters (Kade, 2000; this study) and likely in earlier 
years. Therefore, the migrations to and residence times 
of bluefin tuna in NC will likely be dependent on the 
abundance of Atlantic menhaden. Within-winter shifts 
in bluefin tuna diets do occur. However, it is uncertain 
whether dietary changes are a result of the relative 
abundances of Atlantic menhaden compared to other 
prey. Quantitative data of prey type availability are 
lacking and should be considered in future studies. 
Acknowledgments 
This study was funded by a NC Sea Grant Fishery 
Resource Grant 04-EP-04. We would like to thank the 
commercial fishermen and tuna buyers who participated 
in the study, especially G. Leone, A. Ng, and J. Cox. 
We would like to give special thanks to J. Miller and 
J. Osborne for their assistance with data analyses and 
manuscript editing, D. Bethea and J. Bizzarro for their 
help with cumulative prey curve analysis, J. Morley for 
his assistance with prey identification, and J. Merrell for 
his help with data collection. We also thank J. Smith, 
N. McNeill, B. Block, A. Boustany, A. Walli, B. Chase, 
A. Bianchi, and E. Loew for providing either data or 
knowledge pertinent to the current research. The com- 
ments from three anonymous reviewers substantially 
improved this paper. 
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