Interspecific, ontogenetic and temporal variations in stable isotopes of small tuna species in the northeast Atlantic Ocean

ABSTRACT

In order to study the trophic level of small tuna species and their contribution to the carbon flow in pelagic food webs, an analysis of carbon and nitrogen stable isotopes was carried out. The investigation was focused on four small tuna species (Auxis rochei, Auxis thazard, Euthynnus alletteratus and Sarda sarda) commonly harvested in the northeast Atlantic Ocean. The isotope analysis showed how the results for S. sarda are different from the rest of the species analysed, with a higher trophic level, similar to other major tuna species. The greatest niche overlap in δ¹³C and δ¹⁵N occurs among A. rochei, A. thazard and E. alletteratus. Auxis rochei and E. alletteratus showed a size-dependent variability in δ¹⁵N, and in δ¹³C for S. sarda. The small tuna S. sarda exhibits the highest migration rates among various geographical areas in comparison to other small pelagic tunas, and the seasonal variability of isotope values in the area studied can be attributed to the incorporation of larger individuals with a higher lipid content. The results of this work provide new information on the ecological role played by small tuna in food webs, which is more complex and varied than currently thought. This knowledge is essential for a more effective management of fisheries.

KEYWORDS:

• Body-size
• carbon-13
• food web
• isotope ecology
• nitrogen-15
• Scombridae fish family
• trophic discrimination factor
• trophic overlap

Acknowledgments

Biological sampling for this study was conducted by the Portuguese Institute for the Ocean and Atmosphere (IPMA), within the scope of the EU Data Collection Framework (DCF/PNAB – Programa Nacional de Amostragem Biológica). The authors are grateful to the crew of Tunipex and IPMA technicians that contributed to the data collection. We express our gratitude to two anonymous referees whose valuable comments significantly enhanced the quality of our paper, and to Jon Nesbit for his revision of the English text.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Author contributions

Research idea and study design: RML, PL, EGO; data acquisition: RML, PL; data generation: EGO; data analysis/interpretation and validation: RML, EGO; supervision: EGO; manuscript drafting: RML, EGO with input from all the co-authors.