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).

Three species of tuna, yellowfin (Ta.; Thunnus albacares), skipjack (Kp.; Katsuwonus pelamis), and bigeye (To.; Thunnus obesus) tunas, were sampled year-round during 2003-2005 by observers of the Inter-American Tropical Tuna Commission onboard purse-seine fishing vessels. Samples of dorsal white muscle were taken from each fish adjacent to the second dorsal fin. Fish of uniform size were used for analysis: skipjack tuna 450-550 mm, yellowfin tuna 500-700 mm, and bigeye tuna 450-550 mm. All samples were stored frozen until further processing in the laboratory.

Analytical Methodology: Methods are described in Hetherington et al. (2016). Briefly: Amino acid (AA) compound-specific isotope analysis (AA-CSIA) was conducted on a subset of 48 of the samples used for isotopic analysis of bulk muscle tissue or whole animals. The basis for sample selection was to represent the range of variability in bulk δ15N values and the range of sample locations along the transect. The d15N values of individual AAs were measured using an isotope ratio mass spectrometer (IRMS) (Delta PlusXP, Delta V Plus or MAT 253) interfaced with a gas chromatograph (Trace GC) through a GC-C III combustion furnace (980 degrees C), reduction furnace (650 degrees C), and liquid-N cold trap. All samples were analyzed in triplicate and the measured AA-d15N values were normalized to known d15N values of two coinjected internal reference compounds (norleucine and aminoadipic acid with d15N reference values of 19.06 ‰ and -5.8 ‰, respectively).