Feeding ecology and nitrogen sources of developing Atlantic Bluefin Tuna (Thunnus thynnus) larvae in the Gulf of Mexico

Improved understanding of the processes shaping the early life history of Atlantic Bluefin Tuna (BFT), a period of acute starvation vulnerability impacting recruitment, is important for successful assessment and management efforts. We investigated the food chain and nitrogen (N) sources supporting young BFT larvae in their Gulf of Mexico (GOM) nursery habitat during the peak spring spawning period in 2017 and 2018. Tuna larvae and zooplankton prey were sampled in the upper 25m of the water column and analyzed for ingested prey and in situ zooplankton abundances. Compound Specific Isotopic Analysis of Nitrogen in selected Amino Acids (AAs) was then carried out on the BFT larvae using High Pressure Liquid Chromatography followed by offline Elemental Analysis – Isotope Ratio Mass Spectrometry. In both years, we observed high contributions of cladocerans to the diet of developing larvae (4-9 mm length), with the larger flexion and post-flexion development stages positively selecting for them. Of the identifiable taxa, cladocerans made up 4 to 80 % of the ingested carbon in pre- to post-flexion stage larvae, respectively. Copepod nauplii and copepodites, predominantly calanoids, also contributed significantly to the diet (76 to 15 % in pre- to post-flexion stage larvae, respectively), while appendicularians, Corycaeidae and ciliates were less important. In post-flexion stage larvae, we measured low δ¹⁵N values of 1.6 and 0.1 ‰ in the source AAs Phe and Gly, respectively, significantly lower that the deep nitrate. Modest enrichment of 10 and 8.8 ‰ was observed in the trophic AAs Glu and Ala, respectively, suggesting a low trophic position. Our findings indicate that BFT larvae in the GOM rely on a short food chain of predominantly herbivorous zooplankton, partially supported by depleted ¹⁵N sources from nitrogen fixation (N₂) and/or newly regenerated production (NH₄⁺).