Ecological links between the yellowfin tuna and an isolated archipelago in the Atlantic Ocean revealed by otolith chemistry

The Saint Peter and Saint Paul Archipelago (SPSPA) is a small, remote group of islands on the Mid-Atlantic Ridge that is home to many large-bodied migratory pelagic fishes. Here, we used the yellowfin tuna (Thunnus albacares) as a model species to test the hypothesis that the SPSPA hosts subpopulations originating from other regions of the Atlantic based on the otolith chemistry. Juvenile yellowfin tunas were caught in the SPSPA over a one-year cycle, and their otoliths were analyzed using LA-ICPMS. The chemical composition of the natal origin in the otoliths was investigated and subsequently compared to that of the capture site. A K-means clustering algorithm identified two clusters for the natal origin, which differed mainly in terms of the elements Mg and Cu, indicating that two subpopulations of the yellowfin tuna occur in the SPSPA. A strong, peculiar signature was found for the SPSPA capture site, with the highest concentrations of Zn, Ba and Sr. No overlap was found between the natal origins and capture site, revealing that the SPSPA was not a spawning ground for the individuals. Zinc was the most influential element in separating groups, which may be an indicator of the passage of fish by the SPSPA. Herein, we suggest that the natal origins corresponded to the Gulf of Guinea and Southern Cape Verde Islands and that the SPSPA may serve as a stopover site during Atlantic migratory routes of subpopulations of the yellowfin tuna. Mark-recapture data from ICCAT support the suggested natal origins and migratory routes addressed here. These findings demonstrate the pivotal role of the SPSPA in the lifecycle of subpopulations of the yellowfin tuna crossing the Atlantic Ocean.