Dataset: Larval fish counts and larval tuna counts with corresponding plankton net type and geographic coordinates from R/V Roger Revelle cruise RR2201 (BLOOFINZ-IO, January-March 2022) in the Argo Basin region off NW Australia

Larval tuna sampling was done by three net systems fished in the upper 25-30 meters (m) of the water column. The main gear utilized was the Bongo90. Standard larval sampling was done with 10-minute (~2 knots) oblique hauls in the upper 25 m of the water column using a dual 90-centimeter (cm) diameter bongo frame with 505-micrometer (μm) mesh nets (Laiz-Carrion et al., 2015). Filtered volumes were calculated with mechanical flowmeters (2030R, General Oceanics Inc.) centered in each net mouth. Bongo90 tows were generally done with a smaller "mini-bongo" net (20-cm diameter mouth; 200 and 50-μm mesh nets with flowmeters) attached above the larger nets to sample zooplankton prey (Shiroza et al., 2021). A second net, the 'Black Widow' was the name given to a modified Bongo90 frame with black-dyed nets (1000 µm mesh), was towed at higher speed at night with a depressor (and without the mini-bongo) to collect larger, more evasive larvae. The Black Widow net was lost due to a break in the Kevlar line on tow 54 (Cycle 2, 9 February).The third net utilized was the neuston net. The neuston was a square single-frame coarse-mesh net (1 square meter, with 1000-µm mesh) that was towed at the sea surface. Although operated at lower speed (≤2 kts) than the Black Widow, it was also successful in collecting larger larvae after the widow was lost.

Freshly collected samples (one side of the bongo net collections or the Neuston) were put on ice and live sorted and sized to species and developmental stage (preflexion, flexion, postflexion) at-sea following Richards et al., 2005. Sorted specimens were numbered and either frozen or EtOH preserved in individual vials for later analyses of gut contents (feeding), otolith microstructure (growth), and isotopic composition (trophic position). The second sides of bongo net collections were preserved whole by a multi-step process that replaced residual water with (ethyl ethanol 95%, EtOH) after 24 hours. Shipboard sorting yielded over 3700 larval tuna specimens, dominated (~86%) by southern bluefin tuna (SBT, Thunnus maccoyii) which were most abundant along the southern margin of the Argo Basin compared to the central basin. Yellowfin tuna (YFT, T. albacares) was second in relative abundance (~7%), and the remainder were relatively equally divided among T. albacore tuna (ALB, T. alalunga ), big eye tuna (BET, T. obesus), and skipjack tuna (SKJ, Katsuwonus pelamis). A mass of freshly spawned eggs sampled on one occasion and subsequently confirmed to be SBT by genetic analysis will be important in isotopic interpretation of larval trophic position by providing a correction for the maternal contribution.

Laboratory analysis at the University of Miami was conducted by first measuring plankton displacement volume and wet weight of plankton samples. Next, the preservative (EtOH) was buffered by adding 125 milliliters (mL) of Tris buffer to a 5-gallon carboy. All plankton jars were transferred into buffered EtOH, and subsequent vials contained buffered EtOH. Next, a thorough examination of the right side (R) Bongo90 plankton samples was conducted under 0.63 - 4x magnification using a stereomicroscope to make sure that shipboard sorting did not miss any scombrid larvae. All larval fish were removed from a subset (n=67) of tows and from a 1/4 aliquot split (n=21) of Bongo90 samples. Scombrid larvae were identified to the lowest taxonomic level possible, and were labelled as 'Thunnus Sp.' when species-level identification was not possible. Scombrid larvae were removed from an additional 38 sets of tows without removing other taxa to expedite sample processing of scombrid larvae for collaborative projects.

Over 2400 Thunnus larvae were individually examined, assigned a developmental stage, and given a unique identification number; a subset (n=480) was measured and photographed. Larval scombrids were shipped to collaborators for stomach content identification, prey preference analysis, otolith removal for ageing, genetics (PCR multiplex-identification, genome mapping), isotope analysis, and compound-specific isotope analysis.