PREVALENCE OF OCEANIC FORAGGERS AMONG ADULT FEMALE LOGGERHEAD SEA TURTLES CARETTA CARETTA NESTING IN CAPE VERDE (NORTHWESTERN AFRICA)

EDER, E. B; A. CEBALLOS; S. MARTINS; H. PÉREZ-GARCÍA; I. MARÍN; A. MARCO; L. CARDONA

Baltimore

Congreso; 33rd Annual Symposium on Sea Turtle Biology and Conservation; 2013

International Sea Turtle Society

Neritic foragers prevail in most of the populations of loggerhead sea turtles studied to date, probably because of the much higher food availability in neritic foraging grounds than in oceanic habitats. However, previous satellite tracking of a few individuals (n=10) suggested that oceanic foraging is prevalent among the adult females in Cape Verde, probably because of the very high oceanic productivity fuelled by upwelling. We used stable isotopes in bone to assess the actual proportion of neritic and oceanic females in this population, using the ratios of stable isotopes of marine mammals with well known habitat preferences as a benchmark. Carapace length, clutch size and egg volume were used to assess differences in turtle fitness and skeletochronology to assess age. Stable isotope ratios confirmed that the adult female population in Cape Verde is dominated by oceanic foragers that avoided the oligotrophic region west of the archipelago, as 63 of the analyzed females grouped with oceanic minke whales (Balaenoptera acutorostrata) in a cluster analysis based on stable isotope ratios, whereas the remaining nine females grouped with coastal monk seals (Monachus monachus). The resulting proportion of oceanic (87.5%) and neritic foragers (12.5%) in the sample did not depart significantly from that expected if turtles settled opportunistically between the archipelago and mainland Africa at the end of their developmental migration (Chi-square= 0.23, df =1, p = 0.630), without any preference for the continental shelf. However, settlement on oceanic feeding grounds had a cost for females, as adult neritic foragers had a higher fitness, as revealed by larger curved carapace length (neritic range 87 - 100 cm vs. oceanic range 74 - 101.5 cm, U9,59 = 15.0, p < 0.001) and clutch volume (4188.9 ± 475.2 cm3 for neritic turtles vs. 2906.9 ± 502.4 cm3 for oceanic turtles, U9,59 = 15.5, p< 0.001). Furthermore, neritic turtles were older than adult oceanic foragers, (14 to 62 lines of arrested growth for neritic turtles vs. 7 to 31 lines of arrested growth for oceanic turtles; U9,38 = 91.5, p = 0.032), thus indicating that some animals shifted from oceanic to neritic habitats with age, probably because a higher accumulated probability of detecting the African shelf over time. These results are consistent with the hypothesis that immature loggerhead sea turtles are probably picking suitable foraging grounds to settle from the sites they have encountered previously during their developmental migration and this settlement sites may not be optimal simply because turtles do not have knowledge of other better areas that they have not encountered before during their developmental migration. In the particular case of loggerhead sea turtles from Cape Verde this is because juveniles drift westward and the most profitable, neritic feeding grounds are found eastward.