Foraging dichotomy in loggerhead sea turtles Caretta caretta off northwestern Africa

A foraging dichotomy among sexually mature females has been reported for several populations of loggerhead sea turtles, where large adult females forage primarily in neritic habitats and smaller adult females forage primarily in oceanic habitats. The prevalence of neritic foragers has been considered a consequence of much higher food availability in neritic foraging grounds than in oceanic habitats. However, previous satellite tracking suggested that oceanic foraging is prevalent among the adult females in Cape Verde. We used stable isotopes to assess the actual proportion of neritic and oceanic females in this population and used carapace length, clutch size and egg volume to assess differences in their fitness. Stable isotope ratios confirm that the adult female population in Cape Verde is dominated by oceanic foragers that avoid the oligotrophic region west of the archipelago. The proportion of oceanic and neritic foragers did not depart significantly from that expected if turtles settled opportunistically between the archipelago and mainland Africa at the end of their developmental migration, without any preference for the continental shelf. However, adult neritic foragers had a higher fitness, as revealed by larger carapace length and clutch size. Furthermore, they were older than adult oceanic foragers, thus indicating that some animals shifted from oceanic to neritic habitats with age, most likely due to a higher accumulated probability of detecting the African shelf over time. In conclusion, most of the females nesting in Cape Verde do not select the best available foraging grounds, but settle opportunistically in the highly productive area between the archipelago and Africa when they return from their developmental migration.