Numerical dispersal simulations and genetics help explain the origin of hawksbill sea turtles in Ascension Island

Long-distance dispersal and ontogenetic shifts in habitat use are characteristic of numerous marine species and 27 have important ecological, evolutionary, and management implications. These processes, however, are often 28 challenging to study due to the vast areas involved. We used genetic markers and simulations of physical 29 transportwithin an ocean circulationmodel to gain understanding into the origin of juvenile hawksbill sea turtles 30 (Eretmochelys imbricata) found at Ascension Island, a foraging ground that is thousands of kilometers from 31 known nesting beaches. Regional origin of genetic markers suggests that turtles are from Western Atlantic 32 (86%) and Eastern Atlantic (14%) rookeries. In contrast, numerical simulations of transport by ocean currents 33 suggest that passive dispersal from the western hemisphere would be negligible and instead would primarily 34 be from the East, involving rookeries along Western Africa (i.e., Principe Island) and, potentially, from as far as 35 the Indian Ocean (e.g., Mayotte and the Seychelles). Given that genetic analysis identified the presence of a 36 haplotype endemic to Brazilian hawksbill rookeries at Ascension, we examined the possible role of swimming 37 behavior by juvenile hawksbills from NE Brazil on their current-borne transport to Ascension Island by 38 performing numerical experiments in which swimming behavior was simulated for virtual particles (simulated 39 turtles).We found that oriented swimming substantially influenced the distribution of particles, greatly altering 40 the proportion of particles dispersing into the North Atlantic and South Atlantic. Assigning location-dependent 41 orientation behavior to particles allowed them to reach Ascension Island, remain in favorable temperatures, 42 encounter productive foraging areas, and return to the vicinity of their natal site. The age at first arrival to 43 Ascension (4.5-5.5 years) of these particles corresponded well to estimates of hawksbill age based on their 44 size. Our findings suggest that ocean currents and swimming behavior play an important role in the oceanic 45 ecology of sea turtles and other marine animals.