Gaspar Philippe, Temple-Boyer Julien, Briscoe Dana K, Kurita Masanori, Parker Denise M, Polovina Jeffrey J, Rice Marc R, Saito Tomomi, Balazs George H
Mercator Ocean International, Toulouse, France.
Doerr School of Sustainability, Woods Institute for the Environment, Stanford University, Stanford, CA, USA.
Mov Ecol. 2025 Jun 10;13(1):39. doi: 10.1186/s40462-025-00562-5.
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.
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 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.
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.
幼年海龟在海上的最初几年（即所谓的“迷失岁月”）是如何扩散的，仍然是个谜。已知海洋环流起着主要作用，但游泳活动的影响却知之甚少，这主要是因为幼年海龟追踪实验很少涵盖“迷失岁月”的大部分时间。此外，常用洋流估计中的误差使得在追踪数据中很难准确区分游泳活动的影响和漂移速度的影响。在本文中，我们重新分析了现存最大的关于北太平洋（NP）幼年蠵龟（蠵龟）的追踪数据集，试图更精确地描述它们“迷失岁月”的游泳活动。
将幼年蠵龟的轨迹与全球漂流计划的表面漂流轨迹以及哥白尼海洋服务的新型海洋学业务产品联合分析。结合这些数据集，我们提出了一种新方法，至少在大尺度上可靠地将海龟的游泳速度与包括洋流、风和波浪影响的漂移速度区分开来。
结果表明，最小的幼年海龟在随洋流向东漂移的同时进行大规模的季节性南北迁移。随着它们长大，观察到许多个体行为发生变化。在保持经向季节性迁移的同时，它们开始向家乡游动，逆着主流向向西大力游向它们的出生地（日本）。因此，北太平洋幼年蠵龟的游泳活动最好描述为一系列先漂流然后回家的季节性迁移。在这些迁移过程中观察到高度的个体间同步性尤其在秋分前后个体开始向南游动时。
虽然已知幼年海龟在公海的扩散很大程度上受洋流控制，但我们的结果表明，北太平洋幼年蠵龟的扩散在很大程度上也受到它们组织良好的大规模游泳活动的影响，这种活动包括大量的季节性迁移和逆着不利洋流的有力回家运动。这种积极的游泳策略伴随着高能量消耗，可能通过增加觅食成功率来平衡。用我们的新数据处理方法分析即将进行的幼年海龟追踪实验，应该有助于揭示其他海龟种群或物种的幼年海龟是否进化出了类似的游泳策略。
