Nano-tags for neonates and ocean-mediated swimming behaviours linked to rapid dispersal of hatchling sea turtles

Abstract

Dispersal during juvenile life stages drives the life-history evolution and dynamics of many marine vertebrate populations. However, the movements of juvenile organisms, too small to track using conventional satellite telemetry devices, remain enigmatic. For sea turtles, this led to the paradigm of the 'lost years' since hatchlings disperse widely with ocean currents. Recently, advances in the miniaturization of tracking technology have permitted the application of nano-tags to track cryptic organisms. Here, the novel use of acoustic nano-tags on neonate loggerhead turtle hatchlings enabled us to witness first-hand their dispersal and behaviour during their first day at sea. We tracked hatchlings distances of up to 15 km and documented their rapid transport (up to 60 m min 21 ) with surface current flows passing their natal areas. Tracking was complemented with laboratory observations to monitor swimming behaviours over longer periods which highlighted (i) a positive correlation between swimming activity levels and body size and (ii) population-specific swimming behaviours (e.g. nocturnal inactivity) suggesting local oceanic conditions drive the evolution of innate swimming behaviours. Knowledge of the swimming behaviours of small organisms is crucial to improve the accuracy of ocean model simulations used to predict the fate of these organisms and determine resultant population-level implications into adulthood.

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

“…Information about sea turtle dispersal and behaviour during the "lost years" has been gained through modelling approaches (Hays et al, 2010;Shillinger G. L. et al, 2012;Putman et al, 2013;Casale and Mariani, 2014), telemetry (Nagelkerken et al, 2003;Witherington et al, 2012;Mansfield et al, 2014;Scott et al, 2014), and other emerging technologies, such as stable isotopes (Bowen and Karl, 2007;Reich et al, 2007;Snover et al, 2010;López-Castro et al, 2014). 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).…”

“…This oceanic period of sea turtle life history has been increasingly studied in recent years as sea turtle conservation efforts expand beyond terrestrial zones. These investigations of at-sea movements during early life stages have been made possible by advances in biologging technology (e.g., Mansfield et al, 2014;Scott et al, 2014;Thums et al, 2016). Threatened and endangered species with unknown life history patterns, including highly vulnerable sea turtles (Lascelles et al, 2014), have been a research focus to advance conservation and management strategies (Hamann et al, 2010).…”

“…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).…”

“…Impacts on turtle movements and behaviour are presumed to be negligible when below the colloquial 3% tag-to-body-weight threshold. Hatchling sea turtles tagged with miniature acoustic tags generally meet this requirement (Thums et al, 2013(Thums et al, , 2016Scott et al, 2014), but the influence of tag attachments on animal behaviour should still be carefully considered prior to field studies on threatened and endangered species (Vandenabeele et al, 2012). An examination of movement metrics, such as speed and diving depth, should be undertaken prior to deploying transmitters on wild turtles to ensure that the tracking process is unlikely to decrease fitness or survival, whilst providing biologically representative information (Casper, 2009;Mansfield et al, 2012;Jones et al, 2013).…”

“…An examination of movement metrics, such as speed and diving depth, should be undertaken prior to deploying transmitters on wild turtles to ensure that the tracking process is unlikely to decrease fitness or survival, whilst providing biologically representative information (Casper, 2009;Mansfield et al, 2012;Jones et al, 2013). A direct attachment method on the plastron has been developed for flatback (Natator depressus), green (Chelonia mydas), and loggerhead (Caretta caretta) hatchlings (Thums et al, 2013(Thums et al, , 2016Scott et al, 2014). However, we sought a design that would be suitable for leatherback turtles (Dermochelys coriacea), which have a unique oily skin, and that would detach easily during recovery to ensure the tags were guaranteed to be recovered and removed from a critically endangered sea turtle population in field studies.…”