Dataset: MOCNESS net data from R/V Sally Ride cruise SR2114 in the Eastern Tropical Pacific 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 locations of particular biological interest (see Figure 1 of Gutiérrez-Bravo, et al. (2024)), such as the CRTD (M1), San Jose submarine canyon (M2), the center (M3, M5) and borders (M4, M6) of two anticyclonic eddies (abbreviated ACE-1 and ACE-2), and across the northern portion of the OMZ (M7 and M8).

Using manufacturer software (SBE Data Processing), CTD data were filtered and aligned. CTD-rosette data were binned to 1-meter depth and MOCNESS data were binned to 10 seconds. Conservative Temperature and Absolute Salinity were calculated. CTD-rosette data were used to construct hydrographic sections, while MOCNESS sensor data were used to describe the environmental conditions of zooplankton samples.

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 dissolved oxygen (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). An example of the configuration of horizontal sampling levels along the water column is shown in Figure 2 of Gutiérrez-Bravo, et al. (2024). The oxic, hypoxic, and suboxic levels were sampled for ~10 minutes (~500 cubic meters (m^3) filtered volume). The deep and anoxic levels were sampled for ~20 minutes (~1000 m^3 filtered volume) to obtain sufficient sample material.

Due to time constraints and to eliminate any differences between stations caused by the phase of diel vertical migration, all the net deployments were performed during nighttime. Hence, the sampling results reflect nighttime distributions only.

Sample handling:
Gelatinous organisms, non-planktonic groups, and excess water were removed from the samples after they were retrieved from the cod ends. The samples were preserved with Ethanol 96%. At least two ethanol changes were performed during the cruise and a third was performed on land.

Zooplankton biovolume was measured by the displacement method (Steedman 1976) and standardized to milliliters per 1000 cubic meters (mL/1000 m^3) by dividing the zooplankton displacement volume (mL) by the volume of water filtered by the net (m^3).

Fish larvae and juveniles were separated and counted under a stereoscope. Fish larvae were identified to the most specific taxonomic level possible, using a specialized bibliography (Evseenko 1990; Moser 1996; Aceves-Medina et al. 1999, 2003; Evseenko and Shtaut 2000; Richards 2005; Jiménez‐Rosenberg et al. 2006; González-Navarro et al. 2013; Silva-Segundo et al. 2021). 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 (larvae/1000 m^3) 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 (fish/1000 m^3). 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.