Atlantic Bluefin Tuna (Thunnus thynnus) Feeding Ecology and Potential Ecosystem Effects During Winter in North Carolina.

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Atlantic bluefin tuna (Thunnus thynnus) occupy North Carolina waters during winter months. Their potential impact on prey populations during this time has largely been unexplored. Diet, prey-size selectivity, predator-prey size relationships, gastric evacuation rates, daily ration, and population-level predatory demand were estimated for Atlantic bluefin tuna in North Carolina during winter. Quantitative analyses of bluefin tuna stomachs collected from commercial fishers during two winters (2004-2005) were examined. Bluefin tuna diet was dominated by Atlantic menhaden with other teleosts, portunid crabs, and squid being of mostly minor importance. By weight, I found no major inter-annual differences in diet; however, intra-seasonal diet variability was evident with more diverse diets in late-fall and less diverse diets (predominantly Atlantic menhaden) during winter for one out of two years examined. Lengths of Atlantic menhaden collected from bluefin tuna stomachs were compared with lengths of Atlantic menhaden captured from the Atlantic menhaden purse seine fishery; no significant differences were observed suggesting no prey size selection. Minimum and median-sized Atlantic menhaden prey increased with increased bluefin tuna size, while maximum-sized Atlantic menhaden did not change. Diel patterns in mean gut fullness values were used to estimate the first known field derived gastric evacuation rate for this species. Daily ration from mean gut fullness values and gastric evacuation rates were used along with a range of bluefin tuna population sizes and residency to estimate population-level consumption by bluefin tuna on Atlantic menhaden. I found that, at current population levels, bluefin tuna predation on Atlantic menhaden is minimal relative to consumption of Atlantic menhaden by other known predators and commercial harvest. This was corroborated with an independent estimate of Atlantic menhaden consumption using an Ecopath model. Bluefin tuna appear to occupy coastal waters in North Carolina to prey upon Atlantic menhaden; thus, changes in the Atlantic menhaden stock status or distribution could alter winter foraging locations of bluefin tuna. This study has helped fill a gap in the knowledge of bluefin tuna natural history and provided data necessary for implementing multispecies fisheries management.



quantile regression, gastric evacuation, daily ration, consumption, Ecopath, prey size-selectivity, correspondence analysis, cumulative prey curves, ecosystem modeling, feeding, North Carolina, Atlantic menhaden, Bluefin tuna