Sampling Methodology: Zooplankton, small mesopelagic fishes, and squids were collected from July 28 to December 8, 2006 during the National Oceanic and Atmospheric Administration’s (NOAA's) Stenella Abundance Research (STAR) surveys (Gerrodette et al. 2008). We defined our study area to include a subset of sample locations from the STAR surveys based on the presence of both east-west and north-south productivity gradients across the region, with greater surface chlorophyll a concentrations at the eastern end of the study area and along the equator, according to published oceanographic data. Zooplankton samples were collected with a cylindrical-conical bongo net (333 um mesh), fished to 200 m approximately two hours after sunset, and the samples were frozen within one hour of collection. Specimens of euphausiid crustaceans, Euphausia distinguenda (Ed) and E. tenera (Et) were sorted from the thawed zooplankton samples in the laboratory. Specimens of mesopelagic myctophid fishes Myctophum nitidulum (Mn) and Symbolophorus reversus (Sr) were collected by dipnet at night. Specimens of the squids Dosidicus gigas (Dg) and Sthenoteuthis oualaniensis (So) also were collected at night, using handlines and jigs. (See Olson et al. 2010, Philbrick et al. 2001 for detailed methods).

Analytical Methodology: Methods are described in Hetherington et al. (2016). Briefly: Isotopic analysis of bulk muscle tissue or whole animals was performed at the University of Hawaii’s Isotope Biogeochemistry Laboratory. Stable isotope values of nitrogen were determined using an on-line carbon-nitrogen analyzer coupled with an isotope ratio mass spectrometer (FinniganConFlo II/Delta-Plus). Isotope values are reported in conventional delta-notation relative to the international standards atmospheric N2 and V-PDB, for N and C, respectively. Mean accuracy of all stable isotopic analyses was < +/- 0.1 ‰ (1 sd) based on triplicate analysis of in-house reference materials (glycine standard and tuna muscle) with known δ15N values.