The present study uses stable isotopes of nitrogen and carbon (δ15Nandδ13C) as trophic indicators for Atlantic bluefin tuna larvae (BFT) (6–10 mm standard length) in the highly contrasting environmental conditions of the Gulf of Mexico (GOM) and the Balearic Sea (MED). These regions are differentiated by their temperature regime and relative productivity, with the GOM being significantly warmer and more productive. MED BFT larvae showed the highest δ15N signatures, implying an elevated trophic position above the underlying microzooplankton baseline. Ontogenetic dietary shifts were observed in the BFT larvae from the GOM and MED which indicates early life trophodynamics differences between these spawning habitats. Significant trophic differences between the GOM and MED larvae were observed in relation to δ15N signatures in favour of the MED larvae, which may have important implications in their growth during their early life stages.These low δ15N levels in the zooplankton from the GOM may be an indication of a shifting isotopic baseline in pelagic food webs due to diatrophic inputs by cyanobacteria. Lack of enrichment for δ15N in BFT larvae compared to zooplankton implies an alternative grazing pathway from the traditional food chain of phytoplankton—zooplankton—larval fish. Results provide insight for a comparative characterization of the trophic pathways variability of the two main spawning grounds for BFT larvae.

Summary:The use of stable isotope analysis to study animal diets requires estimates of isotopic turnover rates (half time, t 50 ) and discrimination factors (Δ) for an accurate interpretation of trophic patterns. The stable isotopes of carbon and nitrogen were analysed for eggs and reared larvae of Thunnus thynnus, as well as for the different diets supplied during the experiment. The results showed high values of δ 15 N in eggs and larvae (n=646) until 4 DAH. After this time lapse, the stable isotope values declined progressively until 12 DAH, when notochord flexion began. The δ 13 C showed an inverse trend, suggesting that maternal inheritance of the stable isotopes is evident until pre-flexion stages. This study proposes a model for estimating maternal isotopic signatures of bluefin broodstock. After notochord flexion, larvae were fed with aquaculture-bred gilthead seabream, which resulted in a rapid increase of bluefin larvae δ 15 N values together with a rapid decrease in δ 13 C values. The estimated nitrogen half-time to reach the steady state from the diet was 2.5±0.3 days and the discrimination factor was 0.4±0.3(‰). These results represent the first data set that has allowed isotopic nitrogen turnover rates and discrimination factors of the larval stages of bluefin tuna to be estimated.Keywords: bluefin tuna larvae; stable isotopes; maternal transmission; discrimination factors; nitrogen turnover ratio. Factores de discriminación y tasas de renovación isotópica de nitrógeno en larvas de atún rojo de cultivo (Thunnus thynnus): efectos de la transferencia maternaResumen: El análisis de isótopos estables para estudiar la dieta de los animales requiere estimaciones de las tasas de rotación isotópicas (tiempo medio, t 50 ) y factores de discriminación (Δ) para una correcta interpretación de los patrones tróficos. Los isótopos estables de carbono y nitrógeno se analizaron en los huevos y larvas criadas de Thunnus thynnus, así como para las diferentes dietas suministradas durante el experimento. Los resultados mostraron altos valores de δ 15 N en huevos y larvas (n=646) hasta cuatro días después de eclosionar. Después de este lapso de tiempo, los valores de los isótopos estables disminuyeron progresivamente hasta el día 12, cuando se inició la flexión de la notocorda. El δ 13 C mostró una tendencia inversa, lo que sugiere que la herencia materna de los isótopos estables es evidente en la etapa de pre-flexión. Este estudio propone un modelo para estimar las firmas isotópicas maternas de reproductores de atún rojo. Después de la flexión de la notocorda, las larvas se alimentaron con larvas recién eclosionadas de dorada, lo que dio lugar a un rápido aumento de los valores de δ 15 N en las larvas de atún rojo, junto con una rápida disminución del δ 13 C. El tiempo medio de nitrógeno para alcanzar el equilibrio con la dieta fue de 2,5±0,3 días y el factor de discriminación fue de 0,4±0,3(‰). Estos resultados representan el primer conjunto de datos que permitió la estimación de la tasa de renovación isotópica de nit...

Late larval stages of anchovy Engraulis encrasicolus ranging from 10 to 20 mm standard length were sampled in the 2009 Mediterranean Acoustic Surveys project carried out during the spawning season off the Ebro River plume (NW Mediterranean [NWM] population) and the Bay of Málaga (SW Mediterranean [SWM] population). A combined study of environmental variables, daily growth, otolith biometry and stable isotope analysis (SIA) was undertaken to differentiate the trophic influence on larval growth rates. An inter-population comparative analysis determined trophic-based differences in their growth patterns. The NWM population showed a specialized prey selectivity associated with a low productive ecosystem in contrast to the SWM population, which showed a more generalist feeding behavior associated with an ecosystem of higher food resources. Moreover, δ 13 C values were significantly different between populations, indicating the different origin of carbon sources. The intra-population analysis, differentiating between an optimum and deficient growth group as defined by a prior residual analysis, showed a direct relationship between growth potential and feeding behavior. Higher growth rates registered significantly greater δ 15 N values and thereby showed a higher trophic position, indicating a greater feeding specialization in larvae originating from less productive regions. Such was not the case in the area of higher productivity. Furthermore, both populations showed that carbon sources were decisive in defining better growth potential. Finally, otolith biometry clearly differentiated between growth rates in the optimum and deficient larval growth groups.

In this study we assessed the trophic ecology of bluefin tuna Thunnus thynnus larvae from the Gulf of Mexico, together with the co-occurring larvae of blackfin tuna T. atlanticus, bullet tuna Auxis rochei, and skipjack Katsuwonus pelamis, using both bulk-tissue stable isotope analysis (SIA bulk) and compound-specific analysis of amino acids (CSIA AA). Bulk nitrogen (δ 15 N bulk) and carbon (δ 13 C bulk) values differed significantly among species, suggesting partitioning of resources due to an adaptive process allowing these tunas to share the ecosystem's trophic resources during this early life period. K. pelamis had the largest isotopic niche width, likely due to piscivorous feeding at an earlier age compared to the other species, with an isotopic niche overlap of 17.5% with T. thynnus, 15.8% with T. atlanticus, and 31.2% with A. rochei. This trophic overlap suggests a mix of competition and trophic differentiation among these 4 species of tuna larvae. Higher nitrogen isotopic signatures in preflexion versus postflexion larvae of T. thynnus measured using both SIA bulk and CSIA AA indicate maternal isotopic transmission, as well as 'capital breeder'-like characteristics. In contrast, the nitrogen isotopic ratios of the other 3 species were similar between ontogenetic stages. These observations suggest different breeding strategies within the study area for T. atlanticus, K. pelamis, and A. rochei compared to T. thynnus. No significant differences were observed among the 4 species' trophic positions (TPs) estimated by CSIA AA , whereas a higher TP was observed for T. thynnus by SIA bulk. These differences in TP estimation may be attributed to discrepancies in baseline estimates.

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