Dataset: Fish larvae abundances across the Eastern Tropical North Pacific Oxygen Minimum Zone determined from MOCNESS sampling conducted on R/V Sally Ride cruise SR2114 from December 2021 to January 2022

Data were collected on R/V Sally Ride cruise SR2114 in the Eastern Tropical North Pacific from December 2021 to January 2022.

The MOCNESS was equipped with 10 nets of 1 square meter (m^2) mouth opening and 333 micrometer (µm) mesh size, a SeaBird SBE9+ CTD, a SeaBird SBE 43 dissolved oxygen (DO) sensor, and flow meter and angle sensors. The MOCNESS tows were performed at 1.5-2 knots net speed and at a 40-50° net angle. A horizontal net tow strategy was followed to sample selected DO concentrations (oxypleths) in five sampling levels: 1) the oxic level (~200 micromoles per kilogram (μmol/kg)­­­­, near surface), 2) the hypoxic (~100 μmol/kg­­­­) and 3) suboxic (~10 μmol/kg­­­­) levels in the upper core boundary, 4) the anoxic core level (<1 μmol/kg­­­­, at the center of the anoxic core), and 5) the deep level (~10 μmol/kg in the lower boundary below the anoxic core). The remaining 5 MONCESS nets were opened during transitions between target depths and were not used for this study. This horizontal sampling protocol allows for discrete, punctual sampling events, but cannot provide continuous, vertically-integrated abundances as oblique tows would (Wiebe et al. 2015).

Fish larvae and juveniles were separated and counted under a stereoscope using metal tweezers. Fish larvae were identified to the most specific taxonomic level possible using a specialized bibliography. The larval stages (preflexion, flexion, postflexion, and transformation) were defined according to Moser (1996). Preflexion and flexion larvae were considered "early larval stages" as they both lack fully-developed fins. Larval abundances were standardized to larvae per 1000 cubic meters and were considered absolute abundances. Juveniles and adults were separated, counted, and identified to family level, except for the Gonostomatidae, that were represented entirely by the genus Cyclothone. Adult abundances were standardized to fish per 1000 cubic meters. Because the increased swimming ability of adult fish could affect fishing efficiency, abundances were considered relative, and should not be compared with the absolute abundances of fish larvae.