S2. Detailed methods for stable isotope mixing model used to calculate dietary prey proportions.
The mixing model, Stable Isotope Analysis in R (SIAR), was used to estimate dietary proportions (Parnell et al. 2010). Like earlier models (e.g., Phillips and Gregg 2003), SIAR can calculate absolute estimates of dietary proportions for n + 1 sources using n isotopes. For cases where sources are > n + 1, SIAR generates a range of possible combinations of source proportions that could produce observed consumer isotope values and reports mean proportion estimates as well as 95 % confidence intervals.
While diet-tissue discrimination is often assumed to be ~ 0.4 ‰ (Peterson and Fry 1987) for δ 13
C and 3.4 ‰ for δ
15
N (DeNiro and Epstein 1981; Minagawa and Wada 1984), discrimination factors vary among individuals (Gaye-Siessegger et al. 2004; Barnes et al. 2008), species (Macko et al. 1982), tissue types (Pinnegar and Polunin 1999), and diet types (Adams and Sterner 2000). For mixing model calculations, diet-tissue discrimination was assumed to be
1.4 ‰ for δ
13 C and 1.3 ‰ for δ 15
N based on available liver data for other fish species (Sweeting et al. 2007a; Sweeting et al. 2007b). To account for uncertainty and inherent variability in trophic discrimination values, standard deviation values of 1.8 ‰ and 1.4 ‰ were applied for
δ 13 C and δ 15
N, respectively, to approximate inter and intra-species variability (Barnes et al.
2008). Prey values were entered into SIAR models as mean values ± SD. When only a single prey sample was analyzed for a given prey group, standard deviation estimates of 0.40 ‰ were applied for each isotope to account for inter-individual variability. For samples from the Mid
Atlantic Bight, all liver samples were included in a single analysis within SIAR. For the Bay of
Biscay, liver samples were analyzed separately for summer and fall.

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