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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.