Active dispersal in loggerhead sea turtles ( Caretta caretta ) during the ‘lost years’

Abstract

Highly migratory marine species can travel long distances and across entire ocean basins to reach foraging and breeding grounds, yet gaps persist in our knowledge of oceanic dispersal and habitat use. This is especially true for sea turtles, whose complex life history and lengthy pelagic stage present unique conservation challenges. Few studies have explored how these young at-sea turtles navigate their environment, but advancements in satellite technology and numerical models have shown that active and passive movements are used in relation to open ocean features. Here, we provide the first study, to the best of our knowledge, to simultaneously combine a high-resolution physical forcing ocean circulation model with long-term multi-year tracking data of young, trans-oceanic North Pacific loggerhead sea turtles during their 'lost years' at sea. From 2010 to 2014, we compare simulated trajectories of passive transport with empirical data of 1-3 year old turtles released off Japan (29.7-37.5 straight carapace length cm). After several years, the at-sea distribution of simulated current-driven trajectories significantly differed from that of the observed turtle tracks. These results underscore current theories on active dispersal by young oceanic-stage sea turtles and give further weight to hypotheses of juvenile foraging strategies for this species. Such information can also provide critical geographical information for spatially explicit conservation approaches to this endangered population.

Comparing Acoustic Tag Attachments Designed for Mobile Tracking of Hatchling Sea Turtles

“…The "lost years" paradigm begins upon denatant dispersal of hatchlings in a neritic-to-oceanic migration to unknown or unclearly defined nursery habitats. Combining miniaturized tag technology and physical modelling efforts enables much-needed characterization of movement, habitat utilization, behaviour, and life strategies of young sea turtles throughout these cryptic years (Briscoe et al, 2016). As habitats are drastically changed by anthropogenic forces, migrations of many species may shrink or shift (Brower and Malcolm, 1991;Wilcove and Wikelski, 2008).…”

“…Due to a lack of information on active dispersal capacity, modelling efforts rely heavily on classifying young turtles as "passive drifters, " with little influence on their environment (Hays et al, 2010;Gaspar et al, 2012;Shillinger G. L. et al, 2012;Putman and Mansfield, 2015). Biophysical models can be strengthened and verified by incorporating behavioural data, such as swim speed and orientation (Putman et al, 2012(Putman et al, , 2013Kobayashi et al, 2014;Briscoe et al, 2016), as both swim behaviour and ocean currents control young sea turtles' directionality and influence dispersal outcomes Putman and Mansfield, 2015;Briscoe et al, 2016). Behavioural data can be collected by deploying instruments to track turtle movements (Putman et al, 2012;Thums et al, 2013;Mansfield et al, 2014;Scott et al, 2014).…”

Passive drift or active swimming in marine organisms?

“…[13]. They used satellite telemetry to examine the movements of juvenile sea turtles, a taxa at the centre of a decades-old controversy as to whether they behave as ‘passive migrants’ in their early years during which transoceanic movements occur [14,15].…”

“…More recently, Briscoe et al . [13] compared the movements and distribution of captive-reared loggerhead sea turtles ( Caretta caretta ), released off the coast of Japan, to ocean currents from a global ocean circulation model. They concluded that turtle velocities and distribution could not be the result of ocean currents alone, indicating the importance of directional swimming during the oceanic migration of juveniles.…”

“…[13] can substantially influence transport predictions. While it might ultimately prove true that Japanese loggerhead turtles engage in ‘active dispersal’, our analyses reveal that the methods employed by Briscoe et al .…”