Age for herring and sprat was determined using length-at-age regression models that are derived during routine pelagic trawl surveys for stock assessment (Saunders et al. 2010). Carbon and nitrogen isotope composition of whale skin was determined using continuous flow elemental analysis isotope ratio mass spectrometry (CF-EA-IRMS) at the University of Southampton using a EuroVector EA 3000 (EA) combined

with a PDZ Europa Scientific 20-20 (IRMS). Isotope ratios are presented in delta notation as parts per thousand differences from an internal standard (ACROS L-Glutamic Acid) according to the following equation: δYX = [(Rsample/Rstandard) − 1] Carfilzomib in vitro × 10−3, where R denotes the heavier:lighter isotope ratio and Y is the atomic mass of the stable isotope X (δ13C or δ15N). Internal standards calibrated with International Atomic Energy Agency IAEA (Vienna, Austria), i.e., Vienna Pee Dee Belemnite (for C), atmospheric N2 (for N), were routinely analyzed between samples in order to determine instrument precision. Based on the two standard deviations of these standards, the analytical precision of two runs at separate laboratories was similar 0.4‰ and 0.2‰ for nitrogen, and 0.2‰ and 0.1‰ for carbon for Southampton and University click here of California

Davis respectively. Prey items (fish muscle and homogenized krill) were analyzed at UC Davis by CF-EA-IRMS using a PDZ Europa ANCA-GSL (EA) combined with a PDZ Europa 20-20 (IRMS). The analytical precision at Southampton, calculated as the standard deviation of routinely measured bovine liver and glutamic acid standards, was 0.40‰ for nitrogen, and 0.20‰ for carbon. At the UC Davis laboratory, this was 0.15‰ and 0.06‰ for nitrogen and carbon, respectively. In exoskeletons of crustaceans such as krill, carbonates (CaCO3) are derived from isotopically heavy HCO3− ions from the environment, Rho and

are thus a nondietary fraction and must also be removed as their enriched 13C affects whole-body δ13C values (Søreide et al. 2006). Lipids are depleted in 13C, thus altering the δ13C values of tissues. The elemental carbon to nitrogen ratio (C:N) is a useful proxy for lipid content (McConnaughey and McRoy 1979) and was used to assess lipid effects on isotopic values in light of those previously published species- and tissue-specific values for lean tissue. Lipid-free C:N values for whole zooplankton (range) are 3.30–4.03 for marine zooplankton (Kiljunen et al. 2006, Søreide et al. 2006), (± SD) 3.6 ± 0.1 for M. norvegica (Bentaleb et al. 2011) and 3.3 ± 0.1 for white muscle in sprat and herring of (Kiljunen et al. 2006, Caut et al. 2011). These were used as a threshold lipid-free and/or carbonate-free values for each species, which if exceeded indicated that all δ13C values for that species should be corrected arithmetically (i.e., lipid-normalized) to correct for the presence of isotopically lighter lipid (Table 1).