Swim, baby, swim: the active dispersal scenario of juvenile North Pacific loggerhead turtles revealed by historical satellite tracking data and novel operational oceanography products.

Abstract

Background: How juvenile sea turtles disperse during their first years at sea, known as the "lost years", remains enigmatic. The oceanic circulation is known to play a major role, but the impact of the swimming activity is poorly understood, largely because juvenile tracking experiments rarely cover a significant fraction of the lost years' period. In addition, errors in commonly used ocean current estimates make it difficult to properly separate, in tracking data, the effect of the swimming activity from that of the drift velocity. In this paper, we re-analyze the largest extant tracking data set concerning juvenile North Pacific (NP) loggerhead turtles (Caretta caretta), attempting to more precisely characterize their lost years' swimming activity.

Methods: Juvenile loggerhead trajectories are jointly analyzed with surface drifter trajectories from the Global Drifter Program and novel operational oceanography products from the Copernicus Marine service. Combining these data sets, we present a new method to reliably separate, at least on the large scale, the turtles swimming velocity from the drift velocity which includes the impact of the current, the wind and the waves.

Results: Results reveal that the smallest juveniles perform large seasonal north-south migrations while drifting eastwards with ocean currents. As they grow larger, many individuals are observed to change behavior. While keeping their meridional seasonal migrations, they initiate their homing journey swimming vigorously westwards towards their natal area (Japan), against prevailing currents. The juvenile NP loggerheads' swimming activity is thus best described as a series of Drifting then Homing Seasonal Migrations. High interindividual synchronicity is observed during these migrations, especially around the fall equinox when individuals start swimming southwards.

Conclusion: While open-ocean dispersal of juvenile sea turtles is known to be largely governed by ocean currents, our results demonstrate that juvenile loggerheads' dispersal in the NP is also largely shaped by their well-organized large-scale swimming activity which involves ample seasonal migrations and vigorous homeward movements against adverse currents. Such an active swimming strategy comes with high energy expenditure probably balanced by increased foraging success. Analysis of forthcoming juvenile tracking experiments with our new data processing method should help reveal if juveniles from other sea turtle populations or species have evolved similar swimming strategies.