Differentiated stable isotopes signatures between pre- and post-flexion larvae of Atlantic bluefin tuna (Thunnus thynnus) and of its associated tuna species of the Balearic Sea (NW Mediterranean)

The trophic ecology of bluefin tuna larvae (Thunnus thynnus) from the Balearic Sea, together with its co-existing tuna species such as albacore (T. alalung a), bullet (Auxis rachei) and little tunny (Euthynnus alletteratus) were examined by nitrogen and carbon stable isotope analyses. A total of 286 larvae were analyzed for this study, of which 72 larvae corresponded to bluefin, 57 to albacore, 81 to bullet tuna and 76 to little tunny. Tuna larvae were separated into the pre-flexion and post-flexion developmental stages. Within the size 3-9 mm standard length (SL), the stable isotope of nitrogen (delta N-15) showed significant differences between species where bluefin tuna larvae ranked highest. Pre-flexion bluefin tuna and little tunny larvae showed significantly higher delta N-15 signatures than the post-flexion larvae. This effect is attributed to a biochemical trace of maternal delta N-15 signatures. However, neither albacore nor bullet tuna larvae showed this pattern in delta N-15 signatures, possibly owing to a compensation effect between lower maternal delta N-15 values transmitted to pre-flexion larvae and the early increase of delta N-15 values in post-flexion stages. One way ANOVA showed significant differences between species in the stable isotope ratio of carbon (delta C-13) values, which suggests specific differences of carbon sources. Furthermore, a similar significant ontogenic effect between delta C-13 signatures of pre-flexion and post-flexion larvae is also evidenced in all four species. At pre-flexion stages, all species except bullet tuna larvae showed significant negative relationships between delta N-15 and larval standard length. At post-flexion stages, a significant linear relationship with larval size was only observed in albacore and bullet tuna larvae indicating a possible trophic shift towards early piscivory. With respect to delta C-13 values with larval size, all four species showed significant linear decreases. It may be explained by the metabolism of growth of somatic mass subject to modification of the relative carbon isotopic sources. In conclusion, the species' signatures of delta N-15 and delta C-13 indicate differentiated early life trophic niches. In addition, it is worth remarking the potential use of transgenerational isotopic transmission in future research applications.