Southern Bluefin Tuna (thunnus Maccoyii) Dominated the Larval Tuna Assemblage During the 2022 Spawning Season in the Eastern Indian Ocean

Southern bluefin tuna (SBT), Thunnus maccoyii, is a long-lived, migratory pelagic species that spawns exclusively south of the Java Sea, off the northwestern coast of Australia. Among the bluefin tunas, the larval ecology of SBT remains the least studied, despite major technological advances in early life history dynamics to inform management strategies for larval and juvenile stages. During the peak spawning season in the austral summer of 2022, plankton tows were conducted in this region for the first time in 35 years, targeting SBT and other co-occurring scombrids from surface waters (similar to 30 m depth). This study reassessed the presence, spatial distribution, and relative abundance of larval scombrids as part of the BLOOFINZ (Bluefin Larvae in Oligotrophic Ocean Foodwebs, Investigations of Nutrients to Zooplankton - Indian Ocean) cruise. Herein we describe collection methods, abundance, and diel patterns of larval scombrids in the study area whose growth, diet, and trophic ecology are examined in companion studies in the BLOOFINZ-INDITUN special issue of this journal. Larval identifications were conducted using taxonomic keys, with 11% confirmed by multiple genetic tools (multiplex PCR, COI sequencing, and high resolution melting). SBT dominated the larval scombrid assemblage and most frequently co-occurred with Thunnus alalunga and Thunnus albacares. SBT abundance averaged 35.8 +/- 47.7 (mean +/- Standard Deviation, with median 14.25) individuals per 1000 m(-3), with a peak occurring approximately one week after the full moon. A high-density patch of fish eggs (>82,000 1000(-1) m(-3)) was also recorded. Developing robust, spatiotemporally relevant abundance estimates is critical for continued effective management of tuna fisheries.