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The region off northwestern Australia in the eastern Indian Ocean is an oligotrophic open-ocean ecosystem characterized by low nutrients and high stratification, and is the only known spawning ground for Southern Bluefin Tuna (SBT). We conducted four multi-day Lagrangian experiments (cycles) and transect sampling to investigate plankton trophic flows during the peak SBT spawning season (Jan–Feb 2022). These cycles were carried out after recent storm mixing and subsequent warming and stratification of the water column. We quantified mesozooplankton size-fractionated biomass and grazing, targeting processes that coincide with SBT larval habitat (upper 30 m) and the full euphotic zone (150 m). Higher grazing occurred in the upper 30 m at the start of our study, coinciding with the deeper mixed layer. Euphotic-zone integrated grazing peaked later, coinciding with higher stratification and increased abundance of diatoms at depth. Mesozooplankton grazing impact averaged 0.11 ± 0.01 d-1 for the region, equivalent to 21.2 % of phytoplankton growth. Combined with microzooplankton grazing removals, phytoplankton growth and total zooplankton grazing losses were approximately in balance for the euphotic zone. While direct consumption of phytoplankton only supported respiratory needs of a portion of the assemblage in two of the four experimental cycles, respiratory and production needs were largely met by including the consumption of microzooplankton production. Thus, the production of mesozooplankton prey required to sustain SBT larvae in this nutrient-poor environment was obtained by both high grazing and efficient trophic coupling. This study highlights the role of metazoan zooplankton in warm oligotrophic waters of the eastern tropical Indian Ocean, and indicates that efficient ecosystem transfer can be achieved even under conditions assumed to severely limit production and biomass accumulation. 

Maternal effects play a fundamental role in shaping early larval growth and survival in marine fishes. This study explores the relationship between maternal trophic ecology and larval growth in bluefin tunas, with a focus on Southern Bluefin Tuna (SBT) and an expanded dataset from multiple Atlantic Bluefin Tuna (ABT) populations incorporated into the General Bluefin Model (GBM). Daily growth and stable isotopes (δ15N and δ13C) were obtained from 80 pre-flexion SBT larvae and 355 pre-flexion larvae from the GBM dataset. Results revealed a significant negative linear trend between larval age and δ15N values, consistent with the gradual attenuation of maternally inherited isotopic signatures during development. Faster growing larvae showed significantly lower δ15N and δ13C values, indicating that maternal trophic behaviour and prey sources critically influence larval growth potential. Maternal isotopic niche breadth, inferred from larval isotope data, was markedly narrower in groups with optimal larval growth, suggesting that specialized (stenophagous) maternal feeding strategies promote enhanced offspring performance. These patterns were observed consistently in two bluefin species across seven different populations, despite geographic and temporal variability, highlighting a robust ecological link between maternal foraging behaviour and larval development. From these findings, we introduce the hypothesis of an Optimal Maternal Feeding Isotopic Niche, representing a constrained isotopic range associated with increased larval growth and survival. This framework advances our understanding of the influence of maternal trophic ecology on offspring fitness and offers valuable insights for the conservation and management of highly migratory pelagic species with complex reproductive strategies. 

This study analyzed the growth patterns and survival of Southern bluefin tuna (SBT, Thunnus maccoyii) larvae collected during January–February 2022 in their only known spawning area in the eastern Indian Ocean (IO). Otolith microstructure was examined to characterize both population-level and intra-population growth (OPToptimal and DEF-deficient group), with special emphasis on the flexion process, as well as to provide insights into larval survival. SBT larvae began flexion at sizes and ages comparable to those reported in other bluefin tuna species. At the intra-population level, OPT larvae reached flexion earlier in a better physical condition, with greater length, weight, and body depth, likely increasing their chances of survival at later stages. The observed larval growth rates (0.38 mm d−1) exceeded that from a historical study in 1987 (0.33 mm d−1), likely due to a ~2 ◦C increase in sea surface temperature and shifts in prey availability. Larval survival appears to depend on a selective process based on growth, in which only a small proportion of individuals (<2 %) exhibited width increment in otoliths similar to those of surviving larvae, allowing for faster development and earlier access to larger prey. These findings highlight the need for expanded research on the early life stages of SBT, particularly in the context of ongoing ocean warming and climate change. 

We investigated ontogenetic variability in feeding and prey selection by Southern Bluefin Tuna larvae (SBT; Thunnus maccoyii) in their only global spawning region, located in the eastern Indian Ocean between northwestern Australia and Indonesia. Zooplankton prey and SBT larvae were sampled during four multi-day Lagrangian experiments conducted in the southern Argo Basin during the peak midsummer spawning season in January–February 2022. The zooplankton were identified, enumerated, sizes measured, and their developmental stages ascertained both in situ and from the stomach contents of the larvae. Larval stomach contents revealed high feeding incidence of 95 % with at least one prey item ingested with an average of 3.9 zooplankton prey per larva. Diet and prey selection transitioned during larval ontogeny from copepod nauplii to calanoid and corycaeid copepodites, and ultimately to cladocerans, as well as fish larvae when these were available. However, for all developmental stages and experiments, appendicularians stood out as the most significant prey taxon, constituting an average of 57 % of prey carbon biomass consumed and up to 79 % for postflexion stages. We observed some indication of increasing selection for appendicularians and fish larvae where they were most abundant, even when other suitable prey items were more readily available. Our study documents unprecedented high feeding incidence and positive selection for appendicularians compared to previous investigations of bluefin species, highlighting a pathway that enhances food web transfer efficiency. Appendicularians are uniquely able to thrive in oligotrophic environments and could be an optimal food source supporting SBT larvae in the future ocean. 

Whereas recruitment success for many fisheries depends on coincident timing of larvae with abundance peaks of their prey, less can be more in the tropical/subtropical spawning areas of bluefin tunas if lower but steady food resources are offset by reduced larval vulnerability to pelagic predators. To understand larval habitat characteristics for Southern Bluefin Tuna (SBT), we quantified microbial community carbon flows based on growth and grazing rates from depth profiles of dilution incubations and carbon biomass assessments from microscopy and flow cytometry (FCM) during their peak spawning off NW Australia (Indian Ocean) in February 2022. Two Chla-based estimates of phytoplankton production gave differing offsets due to cycling or mixotrophy, exceeding 14C net community production on average (677 ± 98 versus 447 ± 43 mg C m−2 d−1). Productivity was higher than in the Gulf of Mexico spawning area for Atlantic Bluefin Tuna but less than similar studies of oceanic upwelling regions. Microzooplankton grazing averaged 482 ± 63 mg C m−2 d−1 (71 ± 13 % of production). Two measurement variables for Prochlorococcus gave average production and grazing rates of 282 ± 36 and 248 ± 32 mg C m−2 d−1 (86 ± 6 % grazed). Prochlorococcus comprised almost half of production and grazing fluxes in the upper (0–25 m) euphotic zone where SBT larvae reside. Prochlorococcus declined and eukaryotic phytoplankton and heterotrophic bacteria increased in relative importance in the lower euphotic zone. These results describe relatively classic open-ocean oligotrophic conditions as the food web base for nutritional flows to SBT larvae. 

We examined trophic ecology and its influence on larval growth variability for three scombrids, southern bluefin (Thunnus maccoyii, SBT), albacore (T. alalunga, ALB), and skipjack tunas (Katsuwonus pelamis, SKJ), that share a common spawning ground in the eastern Indian Ocean. We combined otolith-based ageing with bulk nitrogen and carbon stable isotope analysis (SIA) of individual larva. Significant interspecific differences in δ15N and δ13C indicate adaptive resource partitioning that allows these tunas to coexist during early ontogeny. Trophic position and isotopic niche were estimated with both frequentist and Bayesian approaches, enabling the evaluation of ontogenetic isotope shifts, niche overlap, and resource use in relation to growth. ALB grew fastest, had the highest trophic position, and the broadest isotopic niche. Optimally growing tuna larvae occupied the narrowest trophic niche and had lower trophic positions for all three species, supporting the hypothesis that strong trophic specialization supports better growth performance and that feeding on more efficient shorter food chains (e.g., microbial loop via appendicularians) can enhance larval fitness. Using lower C:N ratio as a proxy of larval condition found in optimal growing groups supports the broader hypothesis that growth potential is closely tied to energy allocation strategies during early ontogeny. A detailed understanding of how larval trophodynamics, niche breadth, and resource partitioning interact with growth and survival during these vulnerable stages is essential for ecosystem-based management, particularly in systems where growth rate modulates predation risk and competitive fitness. 

Not AvailabSouthern Bluefin Tuna (SBT, Thunnus maccoyii) range broadly in rich feeding grounds of the Southern Hemisphere but spawn only in a small tropical region off northwestern Australia directly downstream of the Indonesian Throughflow. Here, we describe goals, physical context, design and major findings of an end-to-end process study conducted during the peak SBT spawning season (January–March 2022) to understand nutrient sources, productivity, pelagic food web structure and their relationships to larval SBT feeding, growth and survival. Mesoscale variability was investigated by continuous underway measurements of surface waters and station sampling along the cruise track. Biogeochemical and community relationships, process rates, and trophic interactions were determined in four multi-day Lagrangian experiments in the southern Argo Basin. The study revealed strong system balances among nitrogen fluxes, phytoplankton production, grazing processes, and export. Highly selective feeding on appendicularians allows efficient trophic transfer from picophytoplankton-dominated production to SBT larvae. Plankton productivity, phytoplankton carbon and zooplankton biomass were proportionately elevated compared to similar measurements from the Atlantic bluefin larval habitat in the Gulf of Mexico, but with less advective input from the coastal margins. Individual-based otolith and stable isotope analyses identify larvae of lower trophic position, narrow diet, and narrow maternal diet as the fastest growers most likely to contribute to stock recruitment. Our study highlights the importance of system-level studies to document and understand the subtleties of how food webs of oligotrophic regions respond to climate change, which may not be predictable from the acquired knowledge of historical studies. 

Two cohorts of Atlantic bluefin tuna (Thunnus thynnus) larvae were sampled in 2017 and 2018 during the peak of spawning in the Gulf of Mexico (GOM). We examined environmental variables, daily growth, otolith biometry and stable isotopes and found that the GOM18 cohort grew at faster rates, with larger and wider otoliths. Inter and intra-population analyses (deficient vs. optimal growth groups) were carried out for pre- and post-flexion developmental stages to determine maternal and trophodynamic influences on larval growth variability based on larval isotopic signatures, trophic niche sizes and their overlaps. For the pre-flexion stages in both years, the optimal growth groups had significantly lower δ15N, implying a direct relationship between growth potential and maternal inheritance. Optimal growth groups and stages for both years showed lower C:N ratios, reflecting a greater energy investment in growth. The results of this study illustrate the interannual transgenerational trophic plasticity of a spawning stock and its linkages to growth potential of their offsprings in the GOM. 

Abstract We investigated size-fractioned biomass, isotopes and grazing of mesozooplankton communities in the larval habitat of Atlantic bluefin tuna (ABT) in the oceanic Gulf of Mexico (GoM) during the peak spawning month of May. Euphotic-zone biomass ranged from 101 to 513 mg C m−2 during the day and 216 to 798 mg C m−2 at night. Grazing varied from 0.1 to 1.0 mg Chla m−2 d−1, averaging 1–3% of phytoplankton Chla consumed d−1. Carnivorous taxa dominated the biomass of > 1-mm zooplankton (78% day; 60% night), while only 13% of smaller zooplankton were carnivores. δ15N enrichment between small and large sizes indicates a 0.5–0.6 trophic-step difference. Although characteristics of GoM zooplankton are generally similar to those of remote oligotrophic subtropical regions, zooplankton stocks in the ABT larval habitat are disproportionately high relative to primary production, compared with HOT and BATS averages. Growth-grazing balances for phytoplankton were resolved with a statistically insignificant residual, and trophic fluxes from local productivity were sufficient to satisfy C demand of suspension feeding mesozooplankton. While carnivore C demand was met by local processes in the central GoM, experiments closer to the coastal margin suggest the need for a lateral subsidy of zooplankton biomass to the oceanic region. 

Abstract Larval abundances of Atlantic bluefin tuna (ABT) in the Gulf of Mexico are currently utilized to inform future recruitment by providing a proxy for the spawning potential of western ABT stock. Inclusion of interannual variations in larval growth is a key advance needed to translate larval abundance to recruitment success. However, little is known about the drivers of growth variations during the first weeks of life. We sampled patches of western ABT larvae in 3–4 day Lagrangian experiments in May 2017 and 2018, and assessed age and growth rates from sagittal otoliths relative to size categories of zooplankton biomass and larval feeding behaviors from stomach contents. Growth rates were similar, on average, between patches (0.37 versus 0.39 mm d−1) but differed significantly through ontogeny and were correlated with a food limitation index, highlighting the importance of prey availability. Otolith increment widths were larger for postflexion stages in 2018, coincident with high feeding on preferred prey (mainly cladocerans) and presumably higher biomass of more favorable prey type. Faster growth reflected in the otolith microstructures may improve survival during the highly vulnerable larval stages of ABT, with direct implications for recruitment processes.