The majority of micronekton fish samples were collected in August 2011 using R/V Kilo Moana (KM1123) at two sites in offshore waters to the north and west of the island of O'ahu in the NPSG. Station ALOHA is the Hawaii Ocean Time-series site located north of O'ahu (22.45 N, 158 W) and Station KAHE is an offshore site located west of O'ahu at (~21.3 N, 158.3 W).

Large pelagic fish tissue samples were collected at sea by trained longline fishery observers of the National Oceanic and Atmospheric Administration’s (NOAA) Pacific Islands Regional Observer Program during the years 2009-2011. Approximate catch locations were reported in accordance with NOAA’s Fisheries Operations Data Confidentiality Policy (see Figure 1 of Choy et al. 2015).

Methods are described in Choy et al. (2015). Briefly:
Outer layers of large pelagic fish tissue samples were removed with clean tools to avoid any potential sampling contamination. Only dorsal white muscle tissue of micronekton fish specimens was used for analysis. Bulk sample nitrogen (N) and carbon (C) isotope compositions of all fishes were determined using an isotope ratio mass spectrometer (IRMS; Delta PlusXP) coupled to an elemental analyzer (Conflo IV/Costech ECS 4010). Isotope values are reported in conventional delta-notation relative to the international standards atmospheric N2 and V-PDB, for N and C, respectively. A subset of large pelagic and micronekton fishes (60 of 222) was selected for amino acid (AA) compound-specific isotope analysis (AA-CSIA). The d15N values of individual AAs were measured using an 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 [Nor] and aminoadipic acid [AAA] with d15N reference values of 19.06 o/oo and -5.8 o/oo, respectively).

Related Publications:
Choy C. A., Popp B. N., Hannides C. C. S. and Drazen J. C. (2015) Trophic structure and food resources of epipelagic and mesopelagic fishes in the North Pacific Subtropical Gyre ecosystem inferred from nitrogen isotopic compositions. Limnology and Oceanography doi:10.1002/lno.10085. (PDF)