Direct evidence of swimming demonstrates active dispersal in the sea turtle "lost years"

Abstract

Although oceanic dispersal in larval and juvenile marine animals is widely studied, the relative contributions of swimming behavior and ocean currents to movements and distribution are poorly understood [1-4]. The sea turtle "lost years" [5] (often referred to as the surface-pelagic [6] or oceanic [7] stage) are a classic example. Upon hatching, young turtles migrate offshore and are rarely observed until they return to coastal waters as larger juveniles [5]. Sightings of small turtles downcurrent of nesting beaches and in association with drifting organisms (e.g., Sargassum algae) led to this stage being described as a "passive migration" during which turtles' movements are dictated by ocean currents [5-10]. However, laboratory and modeling studies suggest that dispersal trajectories might also be shaped by oriented swimming [11-15]. Here, we use an experimental approach designed to directly test the passive-migration hypothesis by deploying pairs of surface drifters alongside small green (Chelonia mydas) and Kemp's ridley (Lepidochelys kempii) wild-caught turtles, tracking their movements via satellite telemetry. We conclusively demonstrate that these turtles do not behave as passive drifters. In nearly all cases, drifter trajectories were uncharacteristic of turtle trajectories. Species-specific and location-dependent oriented swimming behavior, inferred by subtracting track velocity from modeled ocean velocity, contributed substantially to individual movement and distribution. These findings highlight the importance of in situ observations for depicting the dispersal of weakly swimming animals. Such observations, paired with information on the mechanisms of orientation, will likely allow for more accurate predictions of the ecological and evolutionary processes shaped by animal movement.