Abstract
The bluefin tuna, Thunnus thynnus, is a highly migratory and long-living fish at the top of the pelagic food web. As top predator, it plays a key role in the stability of marine food webs by exerting top-down control on its prey. The diet composition of bluefin tuna varies in relation to its growth, seasons and migratory patterns, making it difficult to evaluate spatial and temporal effects. This latter aspect is further complicated to be determined during the first months of life, when T. thynnus specimens have a rapid growth rate leading to changes in the trophic status. In this study, the potential collagen-related effects on δ15N and δ13C values were evaluated on the whole spine of adult tuna specimens collected in the central Mediterranean Sea. Obtained results showed non-significant differences between extracted and non-extracted collagen samples for δ15N in whole spine, allowing adopting the isotopic analysis both for annuli in the spine section of adults and for younger specimens, whose spine size does not permit the collagen extraction. Specifically, isotopic analysis of whole spine of the young of the year specimens, showed a rapid change in δ15N values with length, following an exponential model. For older specimens, δ15N values were higher and varied around a plateau, likely due to a higher specificity in the choice of prey and/or to change in the geographical location. Such variability was also mirrored in annuli of spines sections of adult tunas. As far as δ13C values are concerned, a strong collagen-related effect was evidenced, likely highlighting the influence of lipids. Consequently, δ13C analysis may be used only on adult specimens where collagen extraction is possible. This research also showed how isotopic analysis of both whole sample and sequence of annuli in the cross-section of dorsal spine might produce isotopic profiles useful to detect specific trophic dynamics along the bluefin tuna growth.
Highlights
In recent years, stable nitrogen and carbon isotopes analyses (δ15N and δ13C), performed on less metabolically active tissues, such as scale, otoliths, spines and bones, allowed to investigate several aspects of animal’s feeding habits, habitat use and migration (e.g. 1–6)
Stable nitrogen and carbon isotopes (δ15N and δ13C) values and elemental nitrogen (%N) and carbon (%C) composition were obtained from: (1) extracted and non-extracted collagen samples obtained from the same spines of bluefin tuna adults, in order to evaluate the possible differences in the isotopic values between the two approaches; (2) whole spine of both young of the year (YOY) and adults tuna, to highlight differences in feeding behavior between the two macro-size classes (YOY and Juveniles/Adults); (3) untreated micromilled powder obtained from individual growth layers of spine section, to provide a more detailed trophic ecology in each layer representing a different period of animal’s stage
We compared the results of isotopic analysis on dorsal spines of Bluefin tuna in order to support the adoption of isotopic analysis when the amount of sample material is not enough to perform the traditional collagen extraction
Summary
Stable nitrogen and carbon isotopes analyses (δ15N and δ13C), performed on less metabolically active tissues, such as scale, otoliths, spines and bones, allowed to investigate several aspects of animal’s feeding habits, habitat use and migration (e.g. 1–6). Stable nitrogen and carbon isotopes (δ15N and δ13C) values and elemental nitrogen (%N) and carbon (%C) composition were obtained from: (1) extracted and non-extracted collagen samples obtained from the same spines of bluefin tuna adults, in order to evaluate the possible differences in the isotopic values between the two approaches; (2) whole spine of both YOY and adults tuna (without collagen extraction), to highlight differences in feeding behavior between the two macro-size classes (YOY and Juveniles/Adults); (3) untreated micromilled powder obtained from individual growth layers of spine section, to provide a more detailed trophic ecology in each layer representing a different period of animal’s stage
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