Tips were scrubbed in soapy water, sterilized, solvent-rinsed, and foil-wrapped prior to use. Samples were removed from the tips using solvent-rinsed forceps, wrapped in aluminum foil, and transported at −20°C to the laboratory whereupon

they were stored in glass vials at −80°C until analysis. Photo identification of individual whales was used to avoid duplicate sampling in the field (see Whooley et al. 2011). Briefly, lipids were extracted only from whale skin samples, but carbon SB203580 purchase and nitrogen isotope ratios were determined separately to circumvent undesired effects on δ15N. Zooplankton were acidified without further treatment but were corrected for the presence of lipids when deemed appropriate, based on C:N ratios. Finally, fish muscle samples were analyzed without treatment, but were also lipid-corrected when appropriate based on C:N ratios. As such,

all known sources of bias arising from tissue treatment and the presence of both lipids and carbonates were accounted for, with the exception perhaps of δ15N in zooplankton which may decrease by up to 0.32‰ following acidification (Jacob et al. 2005). Although several studies have shown that acidification has a negligible effect on δ15N as long as lipids are not extracted (Sotiropoulos et al. 2004, Søreide et al. 2006, Sweeting et al. 2006) and provided that the acidified samples are not washed with water (Jacob et al. 2005). Epacadostat purchase From above the lateral line, white muscle samples from each fish were excised, homogenized and 0.5 g aliquots were freeze-dried for 24 h. Whole individual zooplankton specimens were also freeze-dried for 24 h followed by removal of carbonates by soaking in a 2 M HCl for 5 min, or until effervescence had ceased (Søreide et al. 2006). After drying in a fume hood, individual zooplankton samples were homogenized and ground to a fine powder using a pestle and mortar. Whale skin samples were duplicated and diced finely using solvent-washed scalpels on clean glass slides. Entire longitudinal profiles of the skin biopsies to a depth

of ca. 10 mm were analyzed. Lipids were extracted for 24 h (6 h refluxing, 18 h soaking) in Soxhlet washed glass microfibre thimbles with 150 mL of 1:1 n-hexane and acetone Protein kinase N1 (Ryan et al. 2013). Both lipid-extracted and bulk skin samples were homogenized using a mortar and pestle. Ca. 1.5 g of prepared tissues were weighed accurately into tin cups. The age of sprat and herring was considered an important factor given that stable isotopic composition is likely to change with size and thus age in pelagic fishes (e.g., Overman and Parrish 2001, Jennings et al. 2002). Furthermore, selective foraging for certain prey age-classes by predators such as baleen whales is possible (Griffiths 1980).