Prey-muscle carbon and nitrogen stable-isotope discrimination factors in Atlantic bluefin tuna (Thunnus thynnus)

Analyses of carbon and nitrogen stable isotopes naturally occurring in animal tissues prove useful in trophic ecology investigations. The correct interpretation of field stable-isotope data should be supported by previous experimental work, but the development of isotopic models is frequently based on untested assumptions. This study was conducted to provide reference isotopic data that may be of application in further studies designed to reconstruct diets and elucidate trophic relationships of Atlantic bluefin tuna, Thunnus thynnus. The influence of the fat content on carbon and nitrogen isotope ratios (δ13C and δ15N), as well as prey-predator discrimination factors (Δ13C and Δ15N), were assessed in white-muscle samples of wild-caught bluefin tuna reared on chub mackerel for approximately five months. Lipid extraction resulted in significantly lower values of C:N ratio, and higher values of δ13C and δ15N. Four significantly different Δ13C and Δ15N estimates were obtained using all possible combinations of bulk and lipid-extracted muscle data from predator and prey. Removal of lipids from bluefin tuna muscle resulted in significantly different Δ13C and Δ15N estimates, whereas lipid extraction from prey samples did not significantly change the results. We suggest that, where specific experimental data are not available, the values proposed here for Δ13C (− 0.16‰) and Δ15N (1.64‰) be used in diet reconstruction and food-web investigations on bluefin tuna and other closely related species.

Research highlights
► 13C and 15N analyses are useful in studies on bluefin tuna trophic relationships.
► Isotopic models require accurate estimates of prey-tissue discrimination factors.
► Lipid extraction resulted in depleted C:N values, and enriched δ13C and δ15N values.
► Removal of lipids from tuna samples resulted in significantly different Δ13C and Δ15N.
► The best estimates of the discrimination factors were Δ13C = − 0.16‰ and Δ13N = 1.64‰.