Title
Ascension Island hawksbills: Where do they hail from?
Author(s)
Abreu Grobois, F. Alberto; Broderick, Annette C.; Bruford, Michael W.; Ciofi, Claudio; Formia, Angela; Godley, Brendan J.; Putman, Nathan F.; Stroud, Stedson; Pelembe, Tara
Published
2023
Abstract
The existence of neritic juvenile hawksbills in Ascension Island (AI) habitats has remained
enigmatic since they do not nest there and the nearest nesting sites are in Brazil (2,300 km to the West) and coastal islands off West Africa (2,500 km to the East). In order to identify potential origins and possible avenues for contributions from breeding populations from both sides of the Atlantic basin and from the Indian Ocean, we used two approaches: molecular analyses and particle drift modeling. Tissue samples from 18 hawksbill juveniles (35-64 cm CCL) opportunistically caught by sport divers were processed with standard molecular techniques. The 740bp mtDNA control region haplotypes exhibited sequences characteristic of both Western (14 EiA01, 1 EiA62; 83% of total) and Eastern Atlantic (2 EiA70/EiA48, 1 EiA49; 17%) hawksbill lineages that are highly divergent as reflected in high the π values obtained (0.237). Preliminary mixed stock analyses comparing these results with available haplotype profiles of Wider Caribbean, Brazilian, and Principe rookeries and considering relative population abundances suggested Barbados (59%) and Brazil (11%) as primary sources, with insignificant contribution from Principe (1%). To test these findings and generate alternative hypotheses of transport routes to AI foraging grounds, we modeled the dispersal of pelagic juvenile hawksbills through the Atlantic and Indian Ocean basins using the surface current output from the Global Hybrid Coordinate Ocean Model (HYCOM). The program ICHTHYOP (v.2) was used to release thousands of “virtual turtles” around Ascension Island and then track their backwards trajectory using a numerical scheme to determine the turtles’ previous location based on surface current velocity the day preceding. In this way, the source location of turtles around AI could be inferred along with the daily registers of age and temperature at each position en route. In contradiction to the initial genetic estimates no turtles followed routes to Ascension from the Caribbean Sea. Instead, the simulations suggest that turtles would predominantly arrive at AI from the west coast of Sub-Saharan Africa, by way of the Benguela Current. This current is fed by the South Atlantic Current, from the western South Atlantic basin, and the Agulhas Current, from the Indian Ocean. These findings suggest that the sole source of Western Atlantic haplotypes is likely to be Brazil. This is consistent with the fact that haplotype EiA62 has only been reported in Brazil. Moreover, the strong contribution of turtles from coastal regions of Sub-Saharan Africa implicates the Principe rookery as major source of hawksbills in AI waters. The presence of hawksbill turtles originating in extremely distant regions indicates that the site might represent a geographic and genetic trans-oceanic meeting point for this endangered species, linking hawksbill turtle populations of South America, West Africa and, perhaps, western Indian Ocean.
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